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NASA
CP 2 138pt. 1
-NASA ConferencePublication
c.1
2138
Heterodyne Systems and Technology Part I
Proceedingsof an internationalconference held at Williamsburg,Virginia March 25-27, 1980
.... ,
NASA
a,
0067349
NASA Conference Publication 2138
HeterodyneSystemsand Part I
Proceedings of an international conference held at Williamsburg,Virginia March 25-27, 1980
National Aeronautics and Space Administration Scientific and Technical Information Branch 1980
Technology
PREFACE The proceedings of the International Conference on Heterodyne Systems and Technology held in Williamsburg, Virginia March 25-27, 1380, are reported in this NASA Conference Publication. This conference was sponsored by the National Aeronautics and Space Administration. The conference was the first of itskind, bringing together technologists, systems engineers, and applications scientiststo exchange technical information on all aspects ofoptical heterodyning. Topics covered various aspects of heterodyning throughout the electromagnetic spectrum including detectors, local oscillators, tunable diode lasers, astronomical systems, and environmental sensors, with both active and passive systems represented. The conference organization consistedof a General Program Chairman, Robert H. Kingston of M.I.T. Lincoln Laboratory, and a Program Committee whose members are listed for reference elsewhere in these proceedings.The conference organizers were StephenJ. Katzberg and James M. Hoell; the conference coordinator was Patricia Hurt. Conference support was led by Jane T. Everett with assistance from VivianParrous and Sheila A. Armstrong, all from Langley Research Center. Use of trade namesor names of manufacturers in this report does not constitute an official endorsement of such products or manufacturers, either expressed or implied, by NASA.
Stephen J. Katzberg James M. Hoell Conference Cochairmen
iii
II
" "
-
Robert H. Kingston Lincoln Laboratory, Massachusetts Instituteof Technology Lexington, Massachusetts 02173 heterodyne
-
(hetero
superheterodyne
-
+
dyne)
-
separate force
(super(sonic)
+
heterodyne)
To introduce the proceedings of this conference, it is interesting to review the origin of the above two wordsand trace the early history of heterodyne systems up to the first optical heterodyne experiment of 1955. The story begins in 1902 with the issue of a patentto Fessenden with the simple title, Wireless Signaling (1). The invention was the concept of transmitting two separate radio frequencies, receivingthe same on two separate antennas and mixing the two to produce an audio frequency. The "mixer" in this case was an iron-core coil driving a telephone diaphragm, and the deflection was proportional to electrical power, thus yielding acoustic output at the difference frequency. A curious and ironic phrase in the patent reads: "Bythe term 'electromagnetic waves' as used herein is meant electromagnetic waves long in period compared with that are called 'heat-waves' or 'radiant heat'." It is not clear when the term "heterodyne"was coined, but we find a paper by Hogan (2) in 1913 with the title "The heterodyne receiving system, and notes on the recent Arlington-Salem tests." One section of this paper is particularly significant as it reads, "It is evident that an economy may be brought about by transmitting only onewave and generating the second frequency at the receiving station." And thus came the birth of the local oscillator; "The heterodyne is seen to consist of a standard receiving set associated with a local generating circuitby means of an inductive coupler. The generator G may be an alternator...which is a 2 K.W. machine capable of generating frequencies up to 100,000 cycles per second." This is probably a record value for local oscillator power, radio or optical. Unfortunately, this first heterodyne system was adequate for C.W. or Morse code signals, but seriously distorted voice communications.
...
The next step in the development of the heterodyne technique was taken by Armstrong, later the inventor and developer of€.m. broadcasting. During World War I, as a member of the U.S. Army Signal Corps, he devised a technique for receiving and separatingsignals over a wide range of radio frequencies using a modification of the heterodyne principle. To quote from his 1921 paper (3): "This expedient consists in reducing the frequency of the incoming signal to some predetermined super-audible frequency which can be readily amplified, passing this current through an amplifier, and then detecting or rectifying the amplified current." Thus came the superheterodyne, supersonic being the prejet age version of ultrasonic. In a technical discussion still valid today, Armstrong points out the difficulties of r.f. amplification (poor vacuum tube amplifiers) and the difficulties ofaudio amplification (poor local oscillator stability and high tube noise at audio frequencies). Armstrong's invention was V
in response to the problem which he proposed at the beginning of his paper: "The problem may be summed up in the following words to construct a receiver for undamped, modulated continuous, and damped oscillations which is substan5 0 to 6 0 0 meters, tially equally sensitive over a range of wavelengths from which is capable of rapid adjustment from one wave to another, and which not distortor lose any characteristic note or tone inherent in the transor vacuum tubes and, mitter." By this time, the mixers were diode rectifiers since the early thirties, the superheterodyne has been the standard method radio wave reception.
-
The next two steps which led to the developnent of heterodyne syste 1 9 5 5 and 1 9 6 0 . First, Forrester, the submillimeter to the visible occurred in Gudmundsen, and Johnson( 4 ) demonstrated photoelectric mixing of the Zeeman co ponents of a mercury emission line thus establishing the feasibility of o heterodyning. Then, in 1 9 6 0 , Maiman ( 5 ) gave the first demonstration of optical maser or laser action, thus giving promise of a local oscillator and mixer in Armstrong's words could "reduce the frequency of the incomingtosignal some predetermined super-audible ( 1 ) frequency which can be readily amplified."
The papers in these proceedings take up the story from there. The appl cations are of course much broader than simple signal reception and incl spectroscopy, radiometry, communications, and radar. The critical technologies are the design and fabrication of local oscillators and detectors. We are all indebted to the staff of NASA Langley Research Center for initiating and nizing the conference and for the preparation of these proceedings, the fi such work devoted exclusively to the exciting field of heterodyne systems really short wavelengths.
REFERENCES 1 . U. S. Patent No,. 7 0 6 7, 4 0 , 2 . J.
August 1 2 1, 9 0 2 .
L. Hogan, Jr., Proc. I .R.E.,
1, 75,(1
91 3 )
3 . E. H. Armstrong, Proc. I.R.E.,9, 3,(1921
)
.
.
4. Forrester, Gudmundsen, and Johnson, Phys. Rev., 99, 1 6 9 1 (, 1 9 5 5 ) . 5 . T, A,.
Maiman, Nature,187, 4 9 3 ,( 19 6 0 )
vi
.
I-
CONTENTS Part 1
iii
PREFACE................................. J?oREWoRD
................................
V
-
SESSION I PLENARY SESSION Chairman: Robert Menzies 1. THE PHYSICS OF HETERODYNE DETECTION IN THE FAR-INFRARED: TRANSITION E’ROM ELECTRIC-FIELD TO PHOTON-ABSORPTION DETECTION IN A SIMPLE SYSTEM Malvin Carl Teich
...................
1
............
11
2. INFRARED HETERODYNE SPECTROSCOPY IN ASTRONOMY
Albert
Betz
-
SESSION I1 LOCAL OSCILLATORS A Chairman: James Hutchby
3. NEW PbSnTe HETEROJUNCTION LASER DIODE STRUCTURES WITH IMPROVED PERFORMANCE C. G. Fonstad, D. Kasemset, H. H. Hsieh, and S. Rotter
.......................
4. ADVANCES IN TUNABLE DIODE LASER TECHONOLOGY Wayne Lo
.............
33
......
45
5. LONG WAVELENGTH PbSnTe LASERS WITH CW OPERATION ABOVE 77K Koji Shinohara, Mitsuo Yoshikawa, Michiharu Ito, and Ryuiti Ueda 6. WIDELY TUNABLE (PbSn)Te LASERS USING ETCHED CAVITIES FOR MASSPRODUCTION. Matthew D. Miller
.........................
7. RELIABILITY IMPROVEMENTS IN TUNABLE Pbl,xSnxSe DIODE LASERS OPERATING IN THE 8.5-30 MICROMETER SPECTRAL REGION Kurt J. Linden, Jack F. Butler, Kenneth W. Nill, and Robert E. Reeder
........
8. LONG WAVELENGTB Pbl,xSn,Se LOCAL OSCILLATORS E. R. Washwell
AND Pbl,xSnxTe
55
63
DIODE LASERS AS
.........................
vi i
23
79
9. DIAGNOSTICS AND CONTROL OF WAVENUMBER STABILITY AND PURITY OF TUNABLE DIODE LASERS RELEVANT TO THEIR USE As LOCAL OSCILLATORS IN HETERODYNE SYSTEMS S. Poultney, D. Chen, G. Steinberg, F. Wu, A. Pires, M. Miller, and M. McNally
.................
97
...........
129
. FINE WAVELENGTH ID FOR TUNABLE LASER LCXAL OSCILLATORS . . . . . . . M. G. Savage and R. C. Augeri
1 43
...
153
13. ATMOSPHERIC STUDIES AND APPLICATIONS WITH INFRARED HETERODYNE DETECTION TECHNIQUES Robert T. Menzies
..................
171
14. INFRARED HETERODYNE RADIOMETER FOR AIRBORNE ATMOSPHERIC TRANSMITTANCE MEASUREMENTS J. M. Wolczok, R. A. Lange, and A. J. DiNardo
181
10. EXCESS NOISE IN Pbl,xSn,Se SEMICONDUCTOR LASERS Charles N. Harward and Barry D. Sidney
11
12. A REVIEW OF THE NASA/OAST CRYOGENIC COOLERS TECHNOUXY PROGRAM J. G. Lundholm, Jr. and Allan Sherman
-
SESSION I11 INCOHERENT APPLICATIONS Chairman: Albert Betz
....................
1 5. DEVELOPMENT AND PERFORMANCE OF A LASER HETERODYNE SPECTROMETER USING TUNEABLE SEMICONDUCTOR LASERS AS LOCAL OSCILLATORS D. Glenar, T. Kostiuk, D. E. Jennings, and M. J. Mumma
.......................
16. ATMOSPHERIC SOLAR ABSORPTION MEASUREMENTS IN THE 9-11 MICRON REGION USING A DIODE LASER HETERODYNE SPECTROMETER Charles N. Harward and James M. Hoe11, Jr.
...........................
199
209
17. SENSITIVITY STUDIES AND LABORATORY MEASUREMENTS FOR THE LASER HETERODYNE SPECTROMETER EXPERIMENT Frank Allario, Stephen J. Katzberg, and Jack C. Larsen
221
1 8. A 1 63 MICRON LASER HETERODYNE RADIOMETER FOR OH: PROGRESS REPORT. H. M. Pickett and T. L. Boyd
241
...........
.........................
19. HETERODYNE DETECTION OF THE 752.033-GHz H20 ROTATIONAL ABSORPTION LINE G. F. Dionne, J. F. Fitzgerald, T-S. Chang, M. M. Litvak, and H. R. Fetterman
....................
viii
249
Part 2*
-
SESSION IV DETECTORS Chairman: David Spears 20. n-p (Hg,Cd)Te PHOTODIODES FOR 8-14 MICROMETER HETERODYNE APPLICATIONS J. F. Shanley and C. T. Flanagan 21. CONCEPTUAL DESIGN AND APPLICATIONS OF HgCdTe INFRARED PHOTODIODES FOR HETERODYNE SYSTEMS Michel B. Sirieix and Henry Bofheimer
........................... ................
22. COMPARATIVE PERFORMANCE OF HgCdTe PHOTODIODES FOR HETERODYNE APPLICATION Herbert R. Kowitz
......................
23. EXTENDING THE OPERATING TEMPERATURE, WAVELENGTH AND FREQUENCY RESPONSE OF HgCdTe HETERODYNE DETECTORS D. L. Spears
.........
24. INFRARED HETERODYNE RECEIVERS WITH IF RESPONSES APPROACHING 5 GHz J. M. Wollczok and B. J. Peyton
.........................
25. FAR INFRARED HETERODYNE SYSTEMS P. E. Tannenwald
263 281
297
309
327
...................
341
......
353
26. BULK SUBMILLIMETER-WAVE MIXERS: STRAIN M. M. Litvak and H. M. Pickett
AND SUPERLATTICES
27. OPTICAL CONSIDERATIONS IN INFRARED RETERODYNE SPECTROMETER DESIGN T. Kostiuk, M. J. Mumma, and D. Zipoy
........................
28. RJ? SPECTROMETERS FOR HETERODYNE RECEIVERS D. Buhl and M. J. Mumma
365
..............
373
...........
385
29. ACOUSTO-OPTIC SPECTROMETER FOR RADIO ASTRONOMY Gordon Chin, David Buhl, and Jose M. Florez
-
SESSION V LOCAL OSCILLATORS B Chairman: A r m Mooradian 30. MICROWAVE TUNABLE LASER SOURCE: A STABLE, PRECISION TUNABLE HETERODYNE iXX2AL OSCILLATOR
................
Glen W. Sachse *Papers 20 to39 are presented under separate cover.
ix
399
3 1 . SUBMILLIMGTER LOCAL OSCILLATORS FOR HETWODYNE SPECTROSCOPY
Edward J. Danielewicz, Jr.
.....
417
....
449
3 2 . SUB MM LASER LOCAL OSCILLATORS: DESIGN CRITERIA AND RESULTS
Gerhard A. Koepf 33. DESIGN
CONSIDERATIONS AREVIEW John J. Degnan
FOR
OPTICAL
HETERODYNE
RECEIVERS:
.............................
461
SESSION VI - COHERFNT APPLICATIONS Chairman: Frank Goodwin
...............
487
....
495
34. MIXING EFFICIENCY IN HETERODYNE SYSTEMS
David
Fink
35. SPATIAL E'REQUENCY RESPONSE OFAN OPTICAL HETERODYNE RECEIVER
Carl L. Fales and Don M. Robinson 36. HIGH
PRESSURE SENSING A. Javan
GAS
LASER
TECHNOLOGY
FOR
ATMOSPHERIC REMOTE
..............................
51 1
......................
529
3 7 . AN AIRBORNE DOPPLER LIDAR
Charles A. DiMarzio and James W. Bilbro 38. SURFACE
RELIEF STRUCTURES FOR MULTIPLE LO BEAM GENERATION WITH HETERODYNING DETECTOR ARRAYS Wilfrid B. Veldkamp
.................
541
39. DIAL
WITH HETERODYNE DETECTION INCLUDING SPECKLE NOISE: AIRCWT/SHUTTLE MEASUREMENTS OF0 3 , H20, and NH3 WITH PULSED TUNABLE C02 LASERS Philip Brockman, RobertV. Hess, Leo D. Staton, and Clayton H . Bair
..................
557
CORRESPONDENCE HETERODYNE SIGNAL-TO-NOISE ING EXPERIMENTS. William R. Cochran PROGRAMCOMMITTEE.
RATIOS
IN
ACOUSTIC
MODE
SCATTER-
.........................
569
...........................
573
X
"
THE PHYSICS OF HETERODYNE
DETECTION I N THE FAR-INFRARED: TRANSITION
FROM ELECTRIC-FIELD TO PHOTON-ABSORPTION DETECTION
I N A SIMPLE SYSTEM*
Malvin Carl T e i c h Columbia Radiation Laboratory Departmentof Electrical E n g i n e e r i n g Columbia University New York, New York 10027
SUMMARY
Afterbrieflyreviewingthehistoryofheterodynedetectionfromthe r a d i o w a v et ot h eo p t i c a lr e g i o n so ft h ee l e c t r o m a g n e t i cs p e c t r u m , w e f o c u so n t h e submillimeter/far-infrared b y i n v e s t i g a t i n g t h e t r a n s i t i o n f r o m electricf i e l dt op h o t o n - a b s o r p t i o nd e t e c t i o ni n a simplesystem. The r e s p o n s eo fa n i s o l a t e dt w o - l e v e ld e t e c t o rt o a c o h e r e n ts o u r c eo fi n c i d e n tr a d i a t i o n is c a l c u l a t e df o rb o t hh e t e r o d y n ea n dv i d e od e t e c t i o n . When t h ep r o c e s s e so f photonabsorptionandphotonemissioncannotbedistinguished,therelative d e t e c t e d power a t double-andsum-frequencies is foundtobemultipliedby a c o e f f i c i e n t ,w h i c h i s less t h a no re q u a lt ou n i t y ,a n dw h i c hd e p e n d so nt h e i n c i d e n tp h o t o ne n e r g ya n do nt h ee f f e c t i v et e m p e r a t u r eo ft h es y s t e m . INTRODUCTION
Heterodynedetectionhas a longandaugusthistoryintheannalsof e l e c t r i c a le n g i n e e r i n g ,r e a c h i n gb a c kt ot h e e a r l i e s t y e a r so ft h ec e n t u r y . The term h a s i t s r o o t si nt h eG r e e k w o r d s" h e t e r o s "( o t h e r )a n d "dynamis" (force). I n 1 9 0 2 , ReginaldFessenden w a s awarded a p a t e n t (1) " r e l a t i n g t o c e r t a i n improvements i ns y s t e m sw h e r et h es i g n a l i s t r a n s m i t t e db y[ r a d i o l w a v e s differinginperiod,andtothegenerationofbeats by t h e waves a n d t h e employmentof s u i t a b l er e c e i v i n ga p p a r a t u sr e s p o n s i v eo n l yt ot h ec o m b i n e d a c t i o no f wavescorresponding i np e r i o dt ot h o s eg e n e r a t e d .I1 The subsequentrealizationthatoneofthese waves c o u l d b e l o c a l l y g e n e r a t e d ( t h ed e v e l o p m e n to ft h el o c a lo s c i l l a t o r )p r o v i d e d a s u b s t a n t i a li m p r o v e m e n t i ns y s t e mp e r f o r m a n c e .P r a c t i c a ld e m o n s t r a t i o n so ft h eu s e f u l n e s so ft h e t e c h n i q u e were c a r r i e d o u t b e t w e e n t h e " F e s s e n d e n s t a t i o n s " o f t h e U.S. Navy a t A r l i n g t o n( V i r g i n i a )a n dt h eS c o u tC r u i s e r S a l e m , b e t w e e nt h e S a l e m and t h e Birmingham(1910) , and a t t h e N a t i o n a l E l e c t r i c S i g n a l i n g Company. I n
...
.. .
*
Work s u p p o r t e d b y t h e J o i n t Services E l e c t r o n i c s P r o g r a m (U.S. Army, U.S. Navy,and U.S. A i r F o r c e )u n d e rC o n t r a c t No. DAAG29-79-C-0079. 1
1913,John Hogan p r o v i d e d a t h o r o u g h l ye n j o y a b l ea c c o u n to ft h ed e v e l o p m e n t , use,andperformanceoftheFessendenheterodynesignalingsysteminthefirst v o l u m eo ft h eP r o c e e d i n g so ft h e IRE (2). It w a s notlongthereafter,in1917,that Edwin H. A r m s t r o n go ft h e D e p a r t m e n to fE l e c t r i c a lE n g i n e e r i n g a t Columbia U n i v e r s i t y c a r r i e d o u t a thorough investigation of the heterodyne phenomenon o c c u r r i n g i n t h e o s c i l l a t i n g s t a t e of t h e' ' r e g e n e r a t i v ee l e c t r o nr e l a y 'I (3). A m a j o rb r e a k t h r o u g hi nt h e f i e l d ,t h ed e v e l o p m e n to ft h es u p e r h e t e r o d y n er e c e i v e r , w a s a c h i e v e d by Armstrong i n 1 9 2 1 ( 4 ) , a n d t h i s famous i n v e n t i o n i s now used i n s y s t e m s a s d i v e r s e as household AM r a d i o r e c e i v e r s andmicrowaveDopplerradars. The prefix"super"refersto"thesuper-audiblefrequencythatcouldbereadily amp1i f i e d "
.
Inthesucceedingyears,theapplicationofheterodyneandsuperheterodyne p r i n c i p l e sf o l l o w e dt h ei n c e s s a n t m a r c ht o w a r dh i g h e rf r e q u e n c i e st h a t c u l m i n a t e di nt h er e m a r k a b l ed e v e l o p m e n t si nm i c r o w a v ee l e c t r o n i c sa b o u tt h e t i m e of World War 11. The m a r r i a g e o f h e t e r o d y n i n g a n d t h e o p t i c a l r e g i o n t o o k p l a c e i n 1 9 5 5 . I n a now c l a s s i c e x p e r i m e n t ,F o r r e s t e r , Gudmundsen,and Johnson(5)observed t h em i x i n go f two Zeeman componentsof a visible(incoherent)spectralline i n a s p e c i a l l yc o n s t r u c t e dp h o t o m u l t i p l i e rt u b e .W i t ht h ed e v e l o p m e n to ft h e l a s e r , o p t i c a l h e t e r o d y n i n g became c o n s i d e r a b l y easier t o o b s e r v e and w a s s t u d i e di n1 9 6 2 b yJ a v a n ,B a l l i k ,a n d Bond (6) a t 1 . 1 5 u m u s i n g a He-Ne l a s e r , andbyMcMurtryandSiegman ( 7 ) a t 6943 1 u s i n g a ruby laser. The developmentof new t r a n s m i t t i n g a n d r e c e i v i n g c o m p o n e n t s i n t h e m i d d l ei n f r a r e dr e g i o no ft h ee l e c t r o m a g n e t i cs p e c t r u ml e dT e i c h ,K e y e s ,a n d Kingston (8) i n 1 9 6 6 t o p e r f o r m a h e t e r o d y n e e x p e r i m e n tu s i n g a C02 l a s e r a t 10.6 u m i n c o n j u n c t i o n w i t h a copper-dopedgermaniumphotoconductivedetector o p e r a t e d a t 4OK. S u b s e q u e n te x p e r i m e n t sw i t hl e a d - t i ns e l e n i d ep h o t o v o l t a i c d e t e c t o r sc o n f i r m e dt h eo p t i m a ln a t u r eo ft h ed e t e c t i o np r o c e s s( 9 ) - ( 1 1 ) . The s t a t e of t h e a r t i n a p p l y i n g t h e s e r e s u l t s t o c o h e r e n t i n f r a r e d r a d a r was reviewed by K i n g s t o ni n 1977(12). Elbaum andTeich ( 1 3 ) h a v er e c e n t l y shown t h e i m p o r t a n c e o f p r o p e r l y c h o o s i n g t h e p e r f o r m a n c e m e a s u r e f o r a h e t e r o d y n es y s t e m .T h i sc a ns o m e t i m e sb ec r i t i c a lt oa v o i df a u l t y estimates o fs i g n a la n dn o i s el e v e l sl e a d i n gt oi n c o n s i s t e n to ri n c o r r e c tr e s u l t s , as p o i n t e do u t by a numberof authors (8), ( 1 4 ) . The f o r e g o i n gd i s c u s s i o nh a sd e a l tw i t hm o r e - o r - l e s si d e a l i z e ds y s t e m s ; i t mustbekeptin mind t h a t t h e r e a r e a v a r i e t y o f e f f e c t s t h a t c a n alter heterodyneperformanceinimportantways. Two examples were r e a d i l yp r o v i d e d at t h i sc o n f e r e n c e :C h a r l e s Townes d i s c u s s e da t m o s p h e r i cp e r t u r b a t i o n so fp h a s e c o h e r e n c ei nl a r g ed i a m e t e rh e t e r o d y n er e c e i v e r s , and M. J . Mumma and T . K o s t i u k discussedtheeffects of v a r i o u s s o u r c e s of n o i s e on systemperformance. Finally, we point out that a number o f s y s t e m c o n f i g u r a t i o n s e m p l o y i n g differentformsofnonlinearheterodynedetectionhavebeenproposedfor v a r i o u sa p p l i c a t i o n s( 1 5 ) ,( 1 6 ) .T h e s e make u s e of m u l t i p l ef r e q u e n c i e s , 2
n o n l i n e a rd e t e c t o r s ,a n d / o rc o r r e l a t i o ns c h e m e s . A l l are aimed a t i n c r e a s i n g the signal-to-noise ratio in situations for which usual operating conditions are r e l a x e d i n p a r t i c u l a r ways. One o ft h e s es c h e m e s( 1 7 ) ,( 1 5 ) ,u s i n g a twof r e q u e n c yt r a n s m i t t e r , i s c u r r e n t l yu n d e rc o n s i d e r a t i o nf o rp o s s i b l ea p p l i c a t i o n a light-choppingmechanism inover-waterpilotrescuemissionswheretheuseof i s i n t e r d i c t e d b e c a u s e of i t s i n t e r r u p t i v e n a t u r e ( p r i v a t e c o m m u n i c a t i o n w i t h W. Tanaka, Naval Weapons C e n t e r ) . Inthefollowingsection, w e relax t h e a s s u m p t i o n t h a t h e t e r o d y n e d e t e c t i o n t a k e s p l a c e bymeans o fp h o t o na b s o r p t i o n , as i t d o e s i n t h e m i d d l e i n f r a r e d a n do p t i c a lr e g i o n s( 1 8 ) ,( 1 9 ) ,o r bymeans o fe l e c t r i c - f i e l dd e t e c t i o n , as i t d o e si nt h er a d i o w a v ea n dm i c r o w a v er e g i o n s .T h i s w i l l e n a b l e us toexamine the heterodyne detection process i n t h e submillimeter/far-infrared r e g i o n o f t h es p e c t r u mi n a s i m p l e way. The r e a d e r i s c a u t i o n e d t h a t t h e t r e a t m e n t is heuristicinnature,and i s o n l yi n t e n d e dt op r o v i d e a qualitativedescription of t h e u n d e r l y i n g d e t e c t i o n p r o c e s s .
TRANSITION FROM ELECTRIC-FIELD TO PHOTON-ABSORPTION DETECTION I N A
SIMPLE SYSTEM
The i n c r e a s i n g i m p o r t a n c e of h e t e r o d v n i m a n d e l e c t r i c - f i e l d d e t e c t i o n i n us toexamine t h es u b m i l l i m e t e r ,i n f r a r e d , and o p t i c a l( 2 0 1 ,( 2 1 )h a se n c o u r a g e d theoperationofsuchsystemsinrelationtothe moreconventionalphotona b s o r p t i o nd e t e c t o r( 2 2 ) - ( 2 4 ) . F o rh e t e r o d y n em i x i n gw i t hc o h e r e n ts i g n a l si n a t w o - l e v e ld e t e c t o r , a s i m p l ea r g u m e n it n d i c a t e st h a dt o u b l e -a n ds u m - f r e q u e n c yt e r m si nt h e d e t e c t e d power a r e m u l t i p l i e d b y t h e f a c t o r s e c h ( h v / 2 k T e ) , w h i c h v a r i e s s m o t h l y fromunityintheelectric-fielddetectionregimetozerointhephotona b s o r p t i o nd e t e c t i o nr e g i m e .T h i sf a c t o rd e p e n d s o n b o t ht h ei n c i d e n tp h o t o n energy hvand on t h e e f f e c t i v e e x c i t a t i o n e n e r g y o f t h e ( t w o - l e v e l ) d e t e c t o r , kTe. I t i s o b s e r v e dt h a tt h e s e terms o n l ya p p e a r when i t c a n n o tb ed e t e r m i n e d w h e t h e rp h o t o na b s o r p t i o no rp h o t o ne m i s s i o nh a st a k e np l a c e .D i f f e r e n c e f r e q u e n c ys i g n a l s ,o nt h eo t h e rh a n d , arise f r o mo u ri n a b i l i t yt od e t e r m i n e fromwhich beam a photon i s a b s o r b e d i n a h e t e r o d y n e e x p e r i m e n t ( 1 8 ) , ( 1 9 ) . We c o n s i d e r a s i m p l e h y p o t h e t i c a l t w o - l e v e l s y s t e m w i t h a n e f f e c t i v e t e m p e r a t u r e Te. I t i s assumed t h a tt h es y s t e mr e s p o n d sl i n e a r l yt ot h ei n c i d e n t radiationintensity,andthat its interactionwiththefield is sufficiently weak s u c h t h a t t h e s t a t e of t h e f i e l d i s n o t p e r t u r b e d b y t h e p r e s e n c e o f t h e detector.
IL?}
We l a b e l t h e i n i t i a l and f i n a l s t a t e s of t h es y s t e m as la) and and o ft h er a d i a t i o nf i e l d as li) and I f ) , r e s p e c t i v e l y .F o ra ne l e c t r i c - d i p o l e t r a n s i t i o n ,t h et r a n s i t i o np r o b a b i l i t y W f i (which i s r e l a t e d t o t h e d e t e c t e d power) i s givenapproximatelyby
3
where e is t h e e l e c t r o n i c c h a r g e , q is t h e d e t e c t o r c o o r d i n a t e , a n d E+ and Eare t h e p o s i t i v e - a n d n e g a t i v e - p o r t i o n s of t h e e l e c t r i c - f i e l d o p e r a t o r , r e s p e c t i v e l y .S i n c e E+ c o r r e s p o n d st op h o t o na b s o r p t i o no ra n n i h i l a t i o n ,a n d E- c o r r e s p o n d s t o p h o t o n e m i s s i o n o r c r e a t i o n , t h e t r a n s i t i o n p r o b a b i l i t y may b e w r i t t e n as Wfi
+/ a i ) +
= ( I w h i c hr e p r e s e n t st h eq u a n t u me f f i c i e n c y K, may b ef a c t o r e do u t ofEq. ( 2 ) . We t h e n sum o v e r t h e f i n a l s t a t e s o ft h ef i e l d( 2 3 ) ,w h i c h are n o to b s e r v e d ,t o obtain
+ (i I aR*aUE-E- + aRaU*E+E+ I i)
-
The n o r m a l l y o r d e r e d f i r s t term c o r r e s p o n d s t o s t i m u l a t e d a b s o r p t i o n , t h e a n t i n o r m a l l yo r d e r e ds e c o n d term c o r r e s p o n d st op h o t o ne m i s s i o n( 2 5 ) ,a n dt h e t h i r d i s a n i n t e r f e r e n c e term.
We now a s s u m e t h a t b e f o r e t h e i n t e r a c t i o n , t h e p r o b a b i l i t y a m p l i t u d e s o f t h e two p o s s i b l e s t a t e s , a g and a,, were r e l a t e d by t h e B o l t z m a n n f a c t o r , w i t h l o w e ra n du p p e rl e v e le n e r g i e sr e p r e s e n t e db y ER and E,, r e s p e c t i v e l y ,a n d withexcitationenergy kTe d e f i n i n g t h e e f f e c t i v e t e m p e r a t u r e o f t h e d e t e c t o r . Thus ,
yielding
a
R = (1
+
-1/2 e-X>
e
i@
and
with x hv/kTe and eie, ei@ r e p r e s e n t i n gp h a s ef a c t o r s . Then, g e n e r a l i z i n g to an arbitrary radiation field represented by t h e d e n s i t y o p e r a t o r p, we cavalierlyobtaintheexpression
4
2,
I
Considering ideal heterodyne detection, i.e. ,two p a r a l l e l , m o n o c h r o m a t i c , a n d c o h e r e n t waves o f f r e q u e n c i e s VI a n dv 2i m p i n g i n gn o r m a l l yo nt h ed e t e c t o r , so t h a t t h e a n t i n o r m a l l y o r d e r e d a n d t h e a n dn e g l e c t i n gs p o n t a n e o u se m i s s i o n n o r m a l l yo r d e r e d terms a r e e q u a l i n m a g n i t u d e ( 2 5 ) , t h e f i r s t two terms above g e n e r a t ed ca n dd i f f e r e n c e - f r e q u e n c ys i g n a l s ,w h i l et h et h i r d t e r m contributes double-andsum-frequencysignals.This may b e c l e a r l y s e e n b y e x p l i c i t l y r e w r i t i n g Eq. ( 7 ) a s
i- [sech(hv/2kT )]
e
(IE,~0
2
cos(4.rrvt-2a-y) 1
+I ~ 1 0
2
cos(4.rrv2t-2~-y)
ET =I
where E ? \ eia r e p r e s e n t s t h e complex e l e c t r i c - f i e l d a m p l i t u d e o f t h e c o n s t i t u e n tf i e l dw i t hf r e q u e n c y VI andphase a. Double-andsum-frequency terms i n t h e h e t e r o d y n e s i g n a l a r e t h e r e f o r e m u l t i p l i e d by t h e f a c t o r sech(hv/2kTe). Forhv/kTe + 0, thisfactorapproaches fieldheterodynesignalobtains
1 andtheclassicalelectric-
+ m, sech(hv/2kTe) -+ 0 and t h e p h o t o n - a b s o r p t i o n ( o p t i c a l ) Forhv/kTe h e t e r o d y n es i g n a lo b t a i n s (18), (19),
A g r a p h i c a lp r e s e n t a t i o no ft h ef u n c t i o ns e c h ( h v / 2 k T e ) v s . i n F i g . 1.
(hV/kTe) i s provided
5
The r e s u l t f o r t h e h e t e r o d y n e case i s e a s i l y r e d u c e d t o t h e d i r e c t d e t e c t i o n( v i d e o ) case f o r a c o h e r e n t s i g n a l by s e t t i n g / E ? ] = 0 , w h i c h y i e l d s
d ‘ ir
a
I E ~ ~+’ -[ sP e c h ( h v / 2 k T e ) ] [ c o s ( 4 a v 1t 0 7
2a
-
y)]f.
(11)
T h u s ,d o u b l e - f r e q u e n c yi n t e n s i t yf l u c t u a t i o n s are d i s c e r n e df o re l e c t r i c fielddirectdetectors,whilethey are suppressedforphoton-absorptiondirect detectorswhichrespondsimply as
/.E?/
’.
CONCLUSION The p h o t o n - a b s o r p t i o nd e t e c t o r is, b y d e f i n i t i o n , i n i t i a l l y i n i t s ground s t a t e andfunctionsbytheannihilationof a s i n g l e ( i n g e n e r a l nonmonoc h r o m a t i c )p h o t o n( 1 8 ) . The p r e s e n c eo ft h ed i f f e r e n c e - f r e q u e n c ys i g n a l is u n d e r s t o o dt o a r i s e f r o mo u ri n a b i l i t yt od e t e r m i n ef r o mw h i c ho ft h e two c o n s t i t u e n t beams t h es i n g l ep h o t o n i s absorbed ( 1 9 ) . The two-level e l e c t r i c f i e l d d e t e c t o r , on t h e o t h e r h a n d , h a s e q u a l p r o b a b i l i t y of b e i n g i n t h e l o w e r and i n t h e u p p e r s t a t e , so t h a t t h e p u r e p r o c e s s e s of p h o t o n a n n i h i l a t i o n a n dp h o t o na b s o r p t i o no c c u rw i t he q u a ll i k e l i h o o d ,a n d w e m u s ta d dt h ee f f e c t s of b o t h . When w e a r e u n a b l et od e t e r m i n ew h i c ho ft h e s ep r o c e s s e s i s occurring, w e mustaddamplitudesratherthansquaresofamplitudes,therebyallowing i n t e r f e r e n c et o , o c c u r . Any a t t e m p t , i n t h i s case, t od e t e r m i n ew h e t h e rp h o t o n e m i s s i o no rp h o t o na b s o r p t i o nt a k e sp l a c ew o u l dr a n d o m i z et h ep h a s e y , and t h e r e b yw a s ho u tt h e sum- anddouble-frequencycomponents.Ingeneral,then, a p h o t o ni n c i d e n to n a v i d e od e t e c t o ri n d u c e s upwardand downward t r a n s i t i o n s w i t hd i f f e r e n tp r o b a b i l i t i e s .T h i s ,i nt u r n , creates a quantum-mechanical p r o b a b i l i t yd e n s i t y( a n dc h a r g ed i s t r i b u t i o n )t h a tv a r i e si n time, producing a c u r r e n t a t t h ei n c i d e n tr a d i a t i o nf r e q u e n c y . Thepower a b s o r b e dt h e n c o n t a i n s a d o u b l e - f r e q u e n c ys i g n a l .F o rh e t e r o d y n ed e t e c t i o n ,i nt h eg e n e r a l c a s e ,s u m - f r e q u e n c ys i g n a l s a r e observed as w e l l . The f o r e g o i n g h e u r i s t i c model y i e l d s a s i m p l e r e s u l t f o r a n i d e a l i z e d t w o - l e v e ls y s t e m .R e p l a c i n gt h eo p e r a t o re qb yt h en o n - r e l a t i v i s t i cH a m i l t o n i a n (6 - &)’/2m, whereand fl r e p r e s e n tt h e momentum a n dv e c t o r - p o t e n t i a l o p e r a t o r s ,r e s p e c t i v e l y ,w o u l da l l o wt r a n s i t i o n s more g e n e r a l t h a n e l e c t r i c d i p o l e ,a n da b s o r p t i o n s of‘more thanonequantum,tooccur. A rigorous t r e a t m e n ts h o u l dc o n s i d e r a c o l l e c t i o no fs u c hs y s t e m s( a s a model f o r a b u l k p h o t o d e t e c t o r o r metal a n t e n n a ) , i n t h e p r e s e n c e o f a s u r r o u n d i n gr e s e r v o i r ,a n d s h o u l db ec a r r i e do u tu s i n gt h ed e n s i t ym a t r i xf o r m a l i s m . I t i s expected thattheeffectsdescribedhere a r e i m p o r t a n t i n a broadrangeofsystems, i n c l u d i n gt h eJ o s e p h s o nd e t e c t o r (26), ( 2 7 ) . T u c k e r( 2 8 )h a sr e c e n t l yc a r r i e do u t a r i g o r o u s a n a l y s i s o f quantuml i m i t e dd e t e c t i o ni nt u n n e lj u n c t i o nm i x e r s . H e d e m o n s t r a t e st h a tn o n l i n e a r t u n n e l i n g d e v i c e s a r e p r e d i c t e dt ou n d e r g o a transition.fromenergydetectors tophotoncounters a t f r e q u e n c i e sw h e r et h ep h o t o ne n e r g yb e c o m e sc o m p a r a b l e t ot h ev o l t a g e s c a l e o ft h ed cn o n l i n e a r i t y . I t i s a p p a r e n tt h a tt h e characterofthisresult i s c l o s e l yr e l a t e dt ot h ed i s c u s s i o np r e s e n t e dh e r e . 6
REFERENCES (1)Fessenden, 12,1902.
(2)
R. A.:
Wireless S i g n a l i n g . U. S . P a t e n t No. 706,740,-August
andNotesontheRecent Hogan, J . L . : TheHeterodyneReceivingSystem, Arlington-Salem Tests. Proc. I R E , v o l . 1, pp. 75-102, J u l y1 9 1 3 .
(3)Armstrong, E . H. : A S t u d yo fH e t e r o d y n eA m p l i f i c a t i o nb yt h eE l e c t r o n Relay.Proc. I R E , vol.5,pp.145-168,April1917. (4)Armstrong, E. H. v o l .9 , pp.3-27,
: A New S y s t e mo fS h o r t
Wave A m p l i f i c a t i o n .P r o c .
IRE,
February1921.
A. T . ; Gudmundsen, R . A . ; andJohnson, ( 5 )F o r r e s t e r , MixingofIncoherentLight.Phys.Rev.,vol.99,pp.1691-1700, September1955.
P.O.:
Photoelectric
(6)Javan, A; B a l l i k , E. A . ; and Bond, W . L . : F r e q u e n c yC h a r a c t e r i s t i c s of a Continuous-Wave H e - N e O p t i c a l Maser. J. Opt. S O C . Amer., v o l .5 2 , pp.96-98,January1962. (7)
McMurtry, B. J . ; andSiegman, A. E . : PhotomixingExperimentswith Ruby O p t i c a l Maser a n dT r a v e l i n g Wave MicrowavePhototubes.Appl. O p t . ,v o l . 1, pp.51-53, January1962.
( 8 )T e i c h , M. C . ; Keyes, R. J . ; andKingston, D e t e c t i o n a t 10.6 Um i nP h o t o c o n d u c t i v e v o l .9 , pp.357-360, 1 5 November 1966.
R. H.: Ge:Cu.
Optimum Heterodyne Appl.Phys. Letters,
( 9T ) eich, M. C . : I n f r a r e d H e t e r o d y n eD e t e c t i o nP. r o cI.E E E , v o l5. 6 , pp.37-46, J a n u a r y1 9 6 8[ r e p r i n t e di nI n f r a r e dD e t e c t o r s ,H u d s o n , andHudson, J . W . Eds. (Dowden, Hutchinson,andRoss,Stroudsburg, 1975) 3 .
R.D.,Jr.,
(10) Teich, M. C . : Homodyne D e t e c t i o no fI n f r a r e dR a d i a t i o nf r o m a Moving D i f f u s eT a r g e t P . roc. I E E E , v o l5 . 7 p, p . 786-792, May 1969. (11) T e i c h , M. C. : C o h e r e n tD e t e c t i o ni nt h eI n f r a r e d . 1.nSemiconductors and S e m i m e t a l s , W i l l a r d s o n , R. K . , and Beer, A. C . Eds.(Academic, New Y o r k ,1 9 7 0 ) ,v o l .5 ,I n f r a r e dD e t e c t o r s ,c h .9 ,p p . 361-407. (12)Kingston, R. H.: C o h e r e n tO p t i c a lR a d a r .O p t i c s pp. 27-31, Summer 1977.
N e w s , v o l .3 ,
no. 3,
(13)Elbaum, M; andTeich, M. C . : H e t e r o d y n eD e t e c t i o no f Random G a u s s i a n SignalsintheOptical alld I n f r a r e d : 0 p t i m i z a . t i o n O f P u l s e D u r a t i o n . O p t Commun., V O ~ .27,pp.257-261, November 1978.
7
,
(14)Brockman, P . ; Hess , R. V. ; S t a t o n L. D.; and Bair, C. H.: DIAL With Heterodyne Detection I n c l u d i n g S p e c k l e N o i s e : A i r c r a f t / S h u t t l e M e a s u r e ments of 0 3 , H20, NH3 WithPulsedTunable C02 Lasers. HeterodyneSystems andTechnology, NASA CP-2138, 1980.(Paper 39 of t h i s c o m p i l a t i o n . )
(15) T e i c h , M. C . : N o n l i n e a rH e t e r o d y n eD e t e c t i o n .I n Keyes, R. J . Ed. ( S p r i n g e r - V e r l a g ,B e r l i d H e i d e l b e r g ,1 9 7 7 ) ,v o l .1 9 , O p t i c a la n dI n f r a r e dD e t e c t o r s ,c h .7 ,p p . 229-300. (16)Teich, M. C.: H e t e r o d y n eC o r r e l a t i o nR a d i o m e t r y .O p t .E n g i n e e r i n g , v o l .1 7 , pp.170-175,March1978. (17)Teich, M . C . : T h r e e - F r e q u e n c yH e t e r o d y n eS y s t e mf o rA c q u i s i t i o na n d TrackingofRadarandCommunicationsSignals.Appl.Phys. Letters, v o l .1 5 ,p p . 420-423,December 1969. (18)Teich, M. C . : F i e l d - T h e o r e t i c a lT r e a t m e n t of Photomixing.Appl.Phys. L e t t e r s , v o l .1 4 ,p p . 201-203,March 1969.
(19) Teich, M. C . : Quantum TheoryofHeterodyneDetection. P h o t o c o n d u c t i v i t y Conf pp.1-5.
., P e l l ,
E . M. Ed.
In P r o c .T h i r d (Pergamon, New York,1971),
(20)Hocker, L . 0 . ; S o k o l o f f , D . R . ; Daneu, V . ; SzBke, A . ; FrequencyMixing i n t h e I n f r a r e d a n d F a r - I n f r a r e d U s i n g L e t t e r s , v o l .1 2 ,p p . pointContactDiode.Appl.Phys. 1 968.
andJavan, A.: a Metal-to-fletal 401-402, J u n e
(21) Abrams, R . L . ; andGandrud, W . B . : HeterodyneDetectionof10.6-LI R a d i a t i o nb yM e t a l - t o - M e t a lP o i n tC o n t a c tD i o d e s .A p p l .P h y s . Letters, V O ~ . 17, pp.150-152,August1970. L.: F l u c t u a t i o n so fP h o t o n B e a m s : The D i s t r i b u t i o n o f t h e (22)Mandel, Photo-Electrons.Proc.Phys. S O C . (London),vol.74,pp.233-243,1959.
(23)Glauber, R . J . : TheQuantum TheoryofOpticalCoherence.Phys. Rev., v o l . 130, pp.2529-2539, J u n e1 9 6 3 ;C o h e r e n ta n dI n c o h e r e n t States of t h eR a d i a t i o nF i e l d .P h y s . Rev., v o l .1 3 1 ,p p . 2766-2788,September1963. (24)Kelley, P. L . ; a n dK l e i n e r , W . H . : T h e o r yo fE l e c t r o m a g n e t i cF i e l d MeasurementandPhotoelectronCounting.Phys. Rev., vol.136,pp. A334, October1964. (25)Mandel, L: A n t i n o r m a l l yO r d e r e dC o r r e l a t i o n sa n d Quantum Counters. Phys.Rev.,vol.152,pp.438-451, December 1966. (26) McDonald, D . G . ; R i s l e y , A. S . ; Cupp, J . D . ; Evenson, K. M . ; and Ashley, J . R.: Four-Hundredth-OrderHarmonicMixingofMicrowaveand I n f r a r e d Laser R a d i a t i o n U s i n g a J o s e p h s o n J u n c t i o n a n d a Maser. Appl.Phys. L e t t e r s , v o l . 20,pp.296-299, A p r i l1 9 7 2 .
8
A316-
(27) Zirmnerman, J . E.: H e t e r o d y n eD e t e c t i o nw i t hS u p e r c o n d u c t i n gP o i n tC o n t a c t s andEnhancedHeterodyneSignalsfromTightlyCoupledContacts. J . Appl. Phys.,vol.41,pp.1589-1593, March1970. J. R . : Quantum L i m i t e d D e t e c t i o n i n T u n n e l J u n c t i o n Mixers. (28)Tucker, I E E E J. Quantum E l e c t r o n . , v o l . QE-15, pp.1234-1258, November 1979.
9
I.o
0.8
c/)
0.2
0
0.1
0.2
.0.5 1.0 2.0
5.0
IO
20
hv/kT Figure 1.- Factor sech (hv/2kT) vs. hv/kT. This quantity appears as a coefficient in the absorption/emission interference term.
10
INFRARED
HETERODYNE
SPECTROSCOPY IN ASTRONOMY*
Albert Betz Department of Physics University of California, Berkeley SUMMARY
A heterodyne spectrometer has been constructed and applied to problems in infrared astronomical spectroscopy. The instrument offers distinct observational advantages for the detection and analysis of individual spectral lines at Doppler-limited resolution. Observations of carbon dioxide in planetary atmospheres and ammonia in circumstellar environments demonstrate the substantial role that infrared heterodyne techniques will play in the astronomical spectroscopy of the future. INTRODUCTION For astronomical observations which require Doppler-limited resolution and high absolute frequency accuracy, laser heterodyne spectroscopy offers to the infrared the technical advantages long enjoyed at radio frequencies. The vibrational-rotational spectra of planetary and circumstellar molecules can now be studied at the same high resolution commonly used for the pure rotational and other low-energy transitions falling in the microwave domain. In the infrared, the Doppler-limited resolutionof heterodyne spectroscopy has made possible new types of observations. Examples are: studies of exact lineshapes of CO2 absorption profiles in the atmospheres of Mars and Venus to derive the vertical pressure-temperature structure of the atmosphere, accurate measurements of wind velocities in the stratosphere and mesosphere of Venus from the Doppler shifts of narrow CO lines, and investigationsof the gas dynamics of circumstellar 2 10 pm absorption lines of NH3. molecules by the detection of the
In heterodyne spectroscopy, whether radio or infrared, the source radiation collected through a telescope is mixed with a stable local oscillator signal in a nonlinear element (usually a device with a square-law response to the electric field), and the resulting difference frequencies are analyzeda in bank of radio frequency (RF) filters and power detectors. In a properly designed system, the frequency calibration and stability of the local oscillator (such as a klystron in the radio or a laser in the infrared) determines the absolu frequency calibration and ultimate frequency resolution capability of the entire spectrometer. In practice, the frequency widths of the individual RF filter channels are usually chosen to resolve adequately a Doppler-limited profile. The total number of available channels, and hence the total spectral range visible at one time, may be dictated either by economics or the intermediate *This
work
supported
in
part
by
NASA
Grant
NGR
05-003-452. 11
f r e q u e n c y( I F )b a n d w i d t ha v a i l a b l ef r o mt h em i x e r .D g p p l e r - l i m i t e ds p e c t r o s , and s o whereaschannel copyusuallyrequiresresolvingpowersbetterthan10 widthsof 300 kHzmay b e a d e q u a t e i n a 1 mm w a v e l e n g t h r e c e i v e r , c h a n n e l s as wide as 30 MHz w o u l db em o r ep r a c t i c a lf o r a 10 pm s p e c t r o m e t e r . The f o l l o w i n g a heterodynespectrometer sectionsdiscusstheimplementationanduseofsuch f o r t h e 1 0 pm b a n d , w i t h i n i t i a l e m p h a s i s o n t h e l o c a l o s c i l l a t o r a n d mixer c h a r a c t e r i s t i c so ft h ei n s t r u m e n t .
INSTRUMENT
F i g u r e 1 shows a s i m p l i f i e ds c h e m a t i c of t h es p e c t r o m e t e r . The converging beam f r o m t h e t e l e s c o p e e n t e r s on t h e l e f t , comes t o a f o c u s , a n d t h e n p a s s e s through a c o a t e d N a C l b e a m s p l i t t e r .( T h et e l e s c o p eu s e df o r a l l t h e s eo b s e r v a t i o n s is e i t h e r t h e 1 . 5 o r 0 . 7 5 meter a p e r t u r e s o f t h e McMath S o l a rT e l e s c o p e a t K i t t P e a kN a t i o n a lO b s e r v a t o r yi nT u c s o n . )T h ev e r t i c a l l y - p o l a r i z e do u t p u t from a C02 l a s e r i s f i r s t n a t c h e d t o t h e d i v e r g e n c e o f t h e s i g n a l beam andthen p a r t i a l l y r e f l e c t e d by t h e b e a m s p l i t t e r . By expandingtheGaussian l a s e r beam beforethebeamsplitterandthenselectingonlythecenter, a n e a r l yu n i f o r m intensitydistributioncanbeobtainedtomatchwiththenearlyuniformtransv e r s ei n t e n s i t yd i s t r i b u t i o n of t h et e l e s c o p e beam. I np r a c t i c e ,t h e" l e n s " at theoutputofthe laser i s a p a i r o f small m i r r o r s w h i c h p e r f o r m t h e e q u i v a l e n t beam-matching f u n c t i o n i l l u s t r a t e d i n t h e d i a g r a m . Thesecondgermanium lens justbeforethedetector i s as shown,however,and i s used a t a p p r o x i m a t e l yf / 3 inordertoachieve a small s p o t s i z e ( ' ~ 6 0 vm) a t anoptimum p o i n t o n t h e p h o t o d e t e c t o r (%150 pm d i a m e t e r ) .O n l ya b o u t 0.5 t o 0 . 8 mW of l a s e r power i s a c t u a l l y i n c i d e n t on t h e d e t e c t o r , a n HgCdTe photodiodedevelopedby DavidSpearsofLincolnLaboratory.With a low n o i s ep r e a m p l i f i e rf o l l o w i n g t h em i x e r ,u s e f u lb a n d w i d t h st o 2 GHz c a n e a s i l y b e o b t a i n e d w i t h t h e s e p h o t o detectors,althoughtheparticularones w e have a r e n o r m a l l y r a t e d t o %1.5 GHz. The q u e s t i o n o f d e f i n i n g I F b a n d w i d t h i n a r e a l a p p l i c a t i o n w i l l b ed i s c u s s e d l a t e r . The a m p l i f i e d I F o u t p u t i s t h e n d i r e c t e d i n t o a second RF mixerwhich c o n v e r t s a s e l e c t e d1 2 8 0 MHz widesegmentinto a 64x20 MHz f i l t e r bank.The u n d e s i r e di m a g er e s p o n s eo ft h e 2nd mixer i s r e j e c t e d by f i l t e r i n g b e f o r e t h e mixer. The d i r e c t RF t o I F i s o l a t i o n of t h i sm i x e r i s measured t o exceed 26 dB o v e rt h eo u t p u tb a n d .( F u t u r ed e s i g n so ft h i ss u b s y s t e m w i l l employ a more c o m p l i c a t e db u tv e r s a t i l ec o m b i n a t i o no f 2nd a n d3 r dl o c a lo s c i l l a t o r s . ) The 2nd LO i s a l s o u s e d t o t r a c k o u t c h a n g e s o f s o u r c e D o p p l e r v e l o c i t y ( s u c h as r o t a t i o no ft h eE a r t h% 3 MHz/hr) d u r i n gt h ec o u r s eo fo b s e r v a t i o n s .F o r p l a n e t a r y o b s e r v a t i o n s a d i f f e r e n t f i l t e r bankof40x5 MHz f i l t e r s (200 MHz t o t a l ) i s used t op r o v i d ef i n e rr e s o l u t i o n . The d e t e c t e do u t p u t sf r o mt h e f i l t e r - b a n k c h a n n e l s are s i m u l t a n e o u s l y i n t e g r a t e d i n a f o l l o w i n g set o fa n a l o g i n t e g r a t o r sa n dt h en e to u t p u t sp e r i o d i c a l l ys a m p l e d by a computer. On weak s o u r c e s ,r e p e a t e di n t e g r a t i o n s of 8 min i n t e r s p a c e d b y 2 min c a l i b r a t i o n s on a blackbodysource may a c c u m u l a t ef o rs e v e r a lh o u r s . Notshown i nt h i sf i g u r e are s k y - c h o p p e rf o r e o p t i c s a t t h et e l e s c o p ef o c u sw h i c ha l t e r n a t et h ef i e l d - o f v i e wo ft h ed e t e c t o r o na n do f ft h es o u r c e a t 150 Hz.The c h o p p e ds i g n a l sf r o m t h e f i l t e r bank a r e synchronouslydemodulated at thisfrequencyjustbefore integration.
12
FRONT END COMPONENTS
Laser A l l o u r o b s e r v a t i o n a l work t o d a t e h a s b e e n a c c o m p l i s h e d w i t h f i x e d frequency C 0 2 lasers, which are s t e p - t u n a b l e t o d i s c r e t e v i b r a t i o n a l - r o t a t i o n a l t r a n s i t i o n so fv a r i o u s CO i s o t o p e so v e r t h e 9 t o 1 2 pm band.The particular 2 model now i n u s e i s b a s e d o n t h e L i n c o l n L a b o r a t o r y d e s i g n o f C h a r l e s F r e e d i n u s i n g a s e m i - s e a l e dd i s c h a r g et u b e ,i n v a rs t a b i l i z i n gr o d s ,a n dg r a t i n gt u n i n g c o n t r o lw i t hz e r o - o r d e ro u t p u tc o u p l i n g T . h i sd e s i g nm a x i m i z e st h e Q o ft h e 1% m laser cavityandpermits laser o s c i l l a t i o n on J-values up t o %60on t h e s t r o n g e ri s o t o p e so f CO Outputpowers up t o s e v e r a l watts are s t i l l o b t a i n e d on t h e s t r o n g e r l i n e s . 2 A s i d e f r o m c o n t i n u o u s t u n a b i l i t y , t h i s laser o f f e r s a l l t h ef e a t u r e sd e s i r e di n a l o c a lo s c i l l a t o r : more t h a na d e q u a t ep o w e ri n a c l e a n s i n g l e mode, freedomfrom AM and FM p e r t u r b a t i o n s , r e a s o n a b l e p h y s i c a l s i z e a n de f f i c i e n c y ,a n da c c u r a t ea b s o l u t ef r e q u e n c yc a l i b r a t i o nf o r a l l disc r e t eo s c i l l a t i o nf r e q u e n c i e s .I nf a c t , t h el a c k of c o n t i n u o u st u n a b i l i t ya n d henceuncertaintyintheoscillationfrequency may beviewed as a p o s i t i v e f e a t u r e f o r many s p e c t r o s c o p i c a p p l i c a t i o n s , i n t h a t t h e l a s e r frequency i s auto-calibratedeithertobetterthan 1 p a r t i n l o 7 a c c u r a c yw i t ht h ed i s c h a r g e impedancesensingtechnique,3ortobetterthan 1 p a r t i n lo9 a b s o l u t e a c c u r a c y w i t ht h es a t u r a t e dr e s o n a n c ef l u o r e s c e n c et e c h n i q u e C . onsequently, a separate i n v o l v e d c a l i b r a t i o n mechanism i s n o t n e c e s s a r y , as would b e t h e c a s e w i t h t u n a b l ed i o d e lasers, forexample. T h ec o m p l e t e n e s so fs p e c t r a lc o v e r a g ei nt h e 9 t o 12 pm r e g i o n c a n b e e s t i m a t e d by c o u n t i n g t h e t o t a l numberof laser l i n e s a v a i l a b l e from i s o t o p i c CO and l 4 N 2 I 6 O l a s e r s i nt h es e q u e n c ea n d" h o t "b a n d s 2 as w e l l as t h e more c o n v e n t i o n a l l a s e r bands. The a v e r a g es p a c i n gb e t w e e nl i n e s i s %3 GHz, which i s a l s o c l o s e t o t h e I F o u t p u tb a n d w i d t ha v a i l a b l ef r o m stateo f - t h e - a r ti n f r a r e dp h o t o m i x e r s .T h i s i s n o tt o s a y t h a t a l a s e r l i n e i s always a v a i l a b l et oc o n v e r t a t a r g e tf r e q u e n c yi n t ot h ec e n t e r of t h ea v a i l a b l e I F band, b u t t h a t , more o f t e n t h a n n o t , a l a s e r l i n ec a nb ef o u n di nt h e 9 to 1 2 pm range s o t h a t p r a c t i c a l o b s e r v a t i o n s may b ea t t e m p t e d . An a d d e di n f l u e n c i n gf a c t o ra i d i n gt h eu s eo f a fixed-frequency LO i s t h a t t h e D o p p l e r f r e q u e n c yo ft h et a r g e t line w i l l varywiththeorbitalmotion of t h e E a r t h , a n d s h i f t s as l a r g e as '3 GHz('30 km/s)canbeexpected a t 10 urn, dependingonthe d i r e c t i o nt ot h es o u r c e . Even f o r t h e l e a s t f a v o r a b l yp l a c e d s t e l l a r s o u r c e s , d u r i n g a t l e a s t onemonthof t h e y e a r a s p e c i f i e d l i n e w i l l u s u a l l yb eo b s e r v a b l ew i t h a CO o r N 0 laser. Themore f a v o r a b l ep l a n e t a r ys o u r c e s ,t h e 2 2 p l a n e t s Mars andVenus w i t h r i c h CO atmospheres, a r e t r i v i a l l y o b s e r v a b l e f r o m 2 a Dopplerviewpoint, as t h e D o p p l e r s h i f t s f r o m m o t i o n w i t h r e s p e c t t o t h e E a r t h do n o te x c e e d 1.5 GHz (15 k m / s ) .I nt h i s l a t t e r a p p l i c a t i o n ,w h e r e CO i t s e l f i s t h et a r g e tm o l e c u l e ,t h e CO l a s e r i s o fc o u r s ei d e a l l ys u i t e d as $he 2 l o c a lo s c i l l a t o r . In t h o s e o t h e r cases w h e r e t h e t a r g e t l i n e i s n o t a laser t r a n s i t i o n ,s u c h as t h e NH l i n e s s e e n i n s t a r s , t h e nt h eo f f s e tf r e q u e n c y betweenthetargetlineandthe laser m u s t b e d e t e r m i n e d b y l a b o r a t o r y s p e c t r o s c o p y .A s i d ef r o mt h eh e t e r o d y n em e a s u r e m e n t sd o n ei no u r own l a b o r a t o r y on specificlinesof NH , OCS, C2H4, andSiH4,bothheterodyneand a numberof otherlaser-related2echniqueshavebeenappliedtothe 10 vm s p e c t r o s c o p y o f astrophysically important molecules with %3 MHz accuracy. 5-11
.
13
Detector/Preamplifier
-2m The photomixer is a reversed-biased HgCdTe photodiode with % 2 ~ 1an 0-~c active area and an effective AC quantum efficiency of %35% 10.6 at pm. l 2 The LO power level required for quantum-noise (shot-noise) limited operation depen of course, on the noise temperature of the first IF amplifier stage. With K noise temperature over the conventional bipolar amplifiers featuring an %250 100 to 1500MHz band, laser power levels of about 0.5 0.8tomW are required on the detector to raise the laser-induced shot noise from the photomixer t LO power levels much above 1 mW on several times the amplifier noise level. the detector risk inducing saturation and a concomitant decrease in effective quantum efficiency. An operational definition of the detector bandwidth for quantum-noise-limited performance can be stated as IF thefrequency where the LO induced shot noise and amplifier noise contributions are equal. Operation bandwidths exceeding the detector's- 3 dB bandwidth are thus possible. For the MHz Lincoln Laboratory photomixers that we use, the output bandwidth %1500 is temperature. for operation with a bipolar preamplifier with aK noise 250 Obviously, a reduction in amplifier noise at high intermediate frequencies extend the usable bandwidth of the photodetector. Any drop in IF system gai with detector rolloff can be compensated in later stages of amplification the noise temperature factor is not critical. The current availability of low-noise GaAs field effect transistors permits an optimized preamplifier to constructed to match the photomixer response characteristics. An amplifier based on designs of the Radio Astronomy Laboratory at Berkeley is now i the spectrometer. It features a reactively-tuned 2-stage design with about 20 dB gain and good input/output match (VSWR s2.0) from 500 to 2000 MHz. The to fall at %1600 MHz so as to provide the noise temperature minimum is adjusted best performance ata frequency where the photomixer response is already fall off. When operated at room temperature, the amplifier has a noise temperature of 60 K at 1600 MHz, 80 K at 2000 MHz, and90 K at 1000 MHz. Below 1 GHz, the noise temperature rises rapidly to %250 K at 500 MHz. When the preamplifier is 1, the noise temperatures fall to about cooled to 77 K, as depicted in Figure half of.their respective room temperature values. Even for a warm amplifier, however, the detector/preamp system operates in the quantum noise limit to beyond 2 GHz, the limit of our measurement system. Although this relatively slight increase of 30% in operating bandwidth may appear small and not worth the additional effort required, in practice it significantly improves the spectrometer's usefulness, in that wide spectral lines can be centered at higher intermediate frequency and thus avoid the lineshape interpretation problems associated with side-band foldover about the local oscillator freqcency. In addition, several lines of circumstellar NH have been detected 3 with the 2 GHz system which are unreachable with only a 1.5 GHz IF. For astronomical observations, more benefits will come from increased output b widths in photomixers than from improvements in quantum efficiency, althoug 3 GHz of course, both are desired. But, even as improved detectors with FET preamps will bandwidths ( - 3 dB) are fabricated in the near future, cooled still be desired to further extend operational bandwidths. Currently, a cool preamp with an0.1 to 4.0 GHz response is under development at Berkeley. This amplifier will be integrated with the photodiode package to eliminate inter vening cabling and the attendant problems associated VSWR with variations. 14
A n o t h e ro b v i o u s . a d v a n t a g eo f a low n o i s e preamp i s t h e r e d u c t i o n i n LO d r i v e l e v e l r e q u i r e d t o raise t h e l a s e r - i n d u c e d s h o t n o i s e a b o v e t h e p r e a m p l i f i e r n o i s ec o n t r i b u t i o n .C o o l e d FET p r e a m p ss h o u l da l l o wr e c e i v e r su s i n gt u n a b l e d i o d e lasers o fm o d e s ts i n g l e mode power (%lo0 pW) t oa c h i e v eq u a n t u m - n o i s e l i m i t e dp e r f o r m a n c eo v e rI Fb a n d w i d t h sa p p r o a c h i n gt h ed e t e c t o r ' s 73dB response.
OPERATIONAL CHARACTERISTICS
The s e n s i t i v i t y o f t h e s p e c t r o m e t e r on t h e t e l e s c o p e may b e computed d i r e c t l y from t h et h r o u g h p u t so ft h ev a r i o u sc o m p o n e n t s . A t a wavelengthof 10.5 pm, 92% o f t h e i n c i d e n t s i g n a l f l u x i s t r a n s m i t t e dt h r o u g ht h e terrestrial atmosphere, 77% t h r o u g ht h e 5 m i r r o r s o f t h e s o l a r t e l e s c o p e , 50%average t h r o u g ht h es k y - c h o p p e r ,a n d7 5 %t h r o u g ht h et a b l eo p t i c s . Of t h er e m a i n i n g f l u x , 35% p r o d u c e sa ne f f e c t i v e I F s i g n a lc u r r e n ti nt h ep h o t o m i x e r .F u r t h e r m o r e ,o n l ys i g n a lr a d i a t i o ni nt h ep o l a r i z a t i o no ft h e l a s e r i s d e t e c t e d ,w h i c h as a na d d i t i o n a l 50% l o s s f o ru n p o l a r i z e ds o u r c e s . The n e t may b er e g a r d e d s y s t e md e t e c t i o ne f f i c i e n c yo fo n l y 5%, however, i s s t i l l a d e q u a t e f o r u s e f u l a s t r o n o m i c a lw o r k , as w i l l b e s e e n i n t h e n e x t s e c t i o n . S p e c t r a are g e n e r a t e d b y i n t e g r a t i n g f o r 4 m i n u t e sw i t ht h es o u r c ei nt h e p o s i t i v e beam o f t h e s k y - c h o p p e r a n d t h e n f o r 4 minuteswiththesourceinthe n e g a t i v e beam. The d i f f e r e n c eb e t w e e nt h e s ei n t e g r a t i o n s is t h et o t a ls i g n a l w i t hi n t e g r a t o ro f f s e t sc a n c e l l i n g .L a b o r a t o r yb l a c k b o d i e s a t ambientandan 0 e l e v a t e dt e m p e r a t u r e ( 2 2 8 C) a r e t h e n i n t e r p o s e d i n t h e twobeams forthe c a l i b r a t i o n c y c l e and t h e p r o c e s s r e p e a t e d , e x c e p t t h a t e a c h i n t e g r a t i o n is f o r 1 minute. The f i n a l s p e c t r a i s produced by n o r m a l i z i n gt h es i g n a l by t h e b l a c k b o d yi n t e g r a t i o n .P r e v i o u sc a l i b r a t i o n s of t e l e s c o p ea n da t m o s p h e r i c t r a n s m i s s i o na l l o wa na b s o l u t es i g n a ll e v e lt ob ei n f e r r e d . It i s i m p o r t a n t t h a t t h e l a s e r poweron t h ep h o t o d e t e c t o rr e m a i nc o n s t a n tu n t i lt h ec a l i b r a t i o n c y c l e i s completed.Otherwise,changesinthephotodiodeimpedance w i l l not a l l o wg a i nv a r i a t i o n sf r o ms t a n d i n gw a v e sb e t w e e nm i x e ra n d preamp t o b e coma d i s t o r t e d s p e c t r a may p l e t e l y n o r m a l i z e d by t h e c a l i b r a t i o n c y c l e , a n d result. The i n f r a r e d t r a n s m i s s i o n o f t h e t e r r e s t r i a l atmospherecanbedetermined as a f r o ms p e c t r ao ft h e Moon o rt h ep l a n e tM e r c u r y . The Sun may a l s ob eu s e d b l a c k b o d yf o rs p e c t r a lc a l i b r a t i o no ft h e t e r r e s t r i a l atmosphere i f a c c u r a c i e s no b e t t e r t h a n %l% are r e q u i r e d . Our r e p e a t e do b s e r v a t i o n so ft h ei n t r i n s i c s o l a r s p e c t r u m show n u m e r o u s u n i d e n t i f i e d a b s o r p t i o n l i n e s t h r o u g h o u t t h e 10 u m spectrum. The l i n e s a r e on t h eo r d e ro f 1%deepand 300 t o 400 MHz wide,and v a r yi ns t r e n g t h and d e t a i l from place t op l a c e on t h e Sun. P o s s i b l ec o n t r i butorsincludethefundamentalvibrational-rotationalbandsof FeO a n d o t h e r m e t a l l i co x i d e s ;h o w e v e r ,t h ec u r r e n t s t a t e o fl a b o r a t o r ys p e c t r o s c o p y on t h e s e molecules i s r e l a t i v e l y s o p o o r t h a t p o s i t i v e i d e n t i f i c a t i o n s may n o t r e a d i l y b ea c h i e v e d .
15
ASTRONOMICALAPPLICATIONS
Planets
was t o
The f i r s t u s e o f i n f r a r e d h e t e r o d y n e f o r a s t r o n o m i c a l s p e c t r o s c o p y
l i n e si nt h ea t m o s p h e r e of Mars. l 3 2 F i g u r e 2 shows a s u b s e q u e n t o b s e r v a t i o n w i t h a n i m p r o v e d v e r s i o n o f t h a t e a r l y
m e a s u r et h el i n e s h a p e so fi n d i v i d u a l
13C160
i nt h e1 0 ym laser band i s s e e n i n 2 a b s o r p t i o ni nt h eM a r t i a na t m o s p h e r e . Each c h a n n e l i s 5 MHz wideandoverlappingsegmentsof 40 c h a n n e l s were combined t o r e c o n s t r u c t t h e w i n g of h a l f t h e line. The e q u i v a l e n ti n t e g r a t i o n t i m e f o rt h es p e c t r u m i s 40 minutes. The similar o b s e r v a t i o n so fo t h e r CO l i n e s w a s t o model t h e i n t e n to ft h i sa n d 2 v e r t i c a lp r e s s u r e - t e m p e r a t u r es t r u c t u r e of t h ea t m o s p h e r eo f Mars. l 4 The d a t a were f i t t e d w i t h a s i m p l e 4 p a r a m e t e rm o d e l ,a n dt h es o l i dc u r v e s show t h e e f f e c t so fd i f f e r e n ts u r f a c ep r e s s u r e estimates t o t h e f i t of t h ed a t a . The a b i l i t y of h e t e r o d y n e s p e c t r o s c o p y t o r e s o l v e t h e l i n e s h a p e p e r m i t s m e a s u r e m e n t so fa t m o s p h e r i cs u r f a c ep r e s s u r et ob e t t e rt h a n0 . 5m i l l i b a r (%0.4 t o r r ) accuracy. Two a d d i t i o n a lu n e x p e c t e df e a t u r e s( a tt h et i m e ) a r e a l s oS e e ni n t h i sp r o f i l e . The small a r r o wo nt h er i g h tp o i n t st ot h ea b s o r p t i o nl i n e of t h e spectrometer.
TheP(16)
l i n e of l 2 C l 6 0
P ( 2 3 )t r a n s i t i o n
of 12C160180
c e n t r a lc o r eo ft h e
l2Cl6O
ym laser band.
i nt h e1 0
On t h e l e f t , t h e
a b s o r p t i o nl i n e i s s e e nt ob ei ne m i s s i o n . The 2 w i d t ho ft h i sG a u s s i a nr e - e m i s s i o nc o m p o n e n t i s o n l y %35 MHz (FWHM), which i m p l i e s a g a sk i n e t i ct e m p e r a t u r e ofonly%170 K a t t h e a l t i t u d e of l i n e formation. The i n t e n s i t yo ft h ee m i s s i o n ,h o w e v e r , i s much s t r o n g e rt h a nt h a t expectedfrom C 0 2 i n l o c a l thermodynamicequilibrium a t t h i s t e m p e r a t u r e a n d s u g g e s t s a n o n t h e r m a le x c i t a t i o n mechanism f o r t h e phenomenon.The detection of similar a n ds t r o n g e rr e - e m i s s i o nc o r e si n
a b s o r p t i o nl i n e s on Venus 2 s t r e n g t h e n st h ei n t e r p r e t a t i o nt h a tt h en o n t h e r m a le x c i t a t i o no f CO comesfrom d i r e c ts o l a r pumpingof s h o r tw a v e l e n g t h C02 bands. 9 l 6 The e m i t g i n g C02 l i e s h i g hi nt h em e s o s p h e r e s of t h e p l a n e t s ( 7 5 km on Mars, 120 km onVenus) so t h a tc o l l i s i o n a ld e - e x c i t a t i o nc a n n o tq u e n c ht h es u b s e q u e n t 10 ym 11 f l u o r e s c e n c e " .F i g u r e 3 i l l u s t r a t e st h ea p p e a r a n c e of t h er e - e m i s s i o nc o r eo f t h eP ( 1 6 )l i n ef o rt h r e ed i f f e r e n tp o s i t i o n s on Venus. S u n l i g h ti l l u m i n a t e s t h ep l a n e tf r o mt h el e f ti nt h i sd i a g r a ma n d i s s t r o n g e s t a t p o s i t i o n 1, c l o s e t ot h el o c a l noonon t h ep l a n e t . The s t r e n g t h of t h e 10 ym e m i s s i o n l i n e i s c l e a r l ys e e nt ob ep r o p o r t i o n a lt ot h ei n c i d e n ts o l a ri n t e n s i t y . No emission l i n e i s s e e nf r o mt h ed a r kh a l fo ft h ep l a n e t at theright;onlytherelatively 12 1 6 flatresidualcontinuumradiation a t t h ec e n t e ro ft h ev e r yb r o a d O2 a b s q r p t i o nl i n e i s d e t e c t e d . On Venus, t h e column d e n s i t yo f 12C160, is s o l2Cl6O
L
h i g ht h a tt h ea b s o r p t i o nl i n e s widerthanourphotomixerbandwidth. 13C160
16
2
of l 2 C l 6 O
are " s a t u r a t i o n - b r o a d e n e d "t ob e
2 The c o m p l e t ep r o f i l e so fw e a k e rl i n e s
c a nb er e a d i l ys e e ni na b s o r p t i o n ,h o w e v e r .F o rV e n u s ,t h e
much of
Doppler-shifts of both the mesospheric l2Cl60, emission components (120 km) L
and the stratospheric 13C160 absorption lines(80 km altitude) have been 2 monitored for several years in order to determine the average wind circulation patterns at these altitudes. The signal-to-noise ratio and absolute laser frequency calibration of the heterodyne spectra permit emission and absorption line shifts tobe measured to %2 and 15 m/sec accuracy, respectively. The results indicate an average retrograde circulation of 90 m/sec at stratospheric altitudes and a symmetric subsolar to anti-solar flow as fast as 130 m/sec in the mesosphere. (A complete report on four yearsof observations will be submitted for publication following planned observations in late Spring, 1980.) Stars In 1978, the first application of the spectrometer to stellar 10 pm spectroscopy was the detection of several absorption lines in thev 2 band of ammonia in the gas cloud surrounding a supergiant star called IRC + 10216. Since then, circumstellar ammonia has been detected around a number of supergiant stars of various types, some of which also emit peculiar microwave maser emission from circumstellar OH, H 0 , and Si0 molecules.'9,20 2 Ammonia is relatively abundant in these sources and is an excellent indicator of the gas dynamics throughout the circumstellar region. The absolute frequency accuracy and resolution of the infrared heterodyne spectrometer permits direct comparisons with similarly accurate microwave data available from radio astronomy on these maser stars. For the non-maser star +IRC 10216, Figure 4 shows an absorption line of 14NH
in the ground (0,O) rotational state,a 3 ( 100°K) o p e r a t i o no fP b - s a l t lasers, n o to n l y must t h e l o w t e m p e r a t u r e t h r e s h o l d c u r r e n t d e n s i t y a n d t o t a l c u r r e n t b e d e c r e a s e d as h a s b e e n d i s c u s s e d , b u t t h e rate o fi n c r e a s ei nt h r e s h o l dw i t hi n c r e a s i n g t e m p e r a t u r e m u s t a l s ob er e d u c e d .T h i s i s a no b v i o u ss t a t e m e n tb u t it i s w o r t hm a k i n gb e c a u s et h eh i g ht e m p e r a t u r eb e h a v i o ro fP b - s a l t lasers i s domi n a t e d by a f a c t o r w h i c h h a s b e e n i g n o r e d i n t h e p r e s e n t d i s c u s s i o n t h u s f a r ,
26
\
andwhich i s o f t e nn e g l e c t e d , i . e . t h e a c t i v e l a y e rd o p i n g level. The a c t i v e layermustbeveryheavilydoped i n P b - s a l t lasers [ 4 c ] , o n t h e o r d e r o f 1018 cm-3, b e c a u s e t h e h i g h t e m p e r a t u r e t h r e s h o l d is d i r e c t l y p r o p o r t i o n a l t o t h e m i n o r i t y carrier d e n s i t y n e c e s s a r y t o a c h i e v e p o p u l a t i o n i n v e r s i o n i n t h e acti v e r e g i o n [ l o ] , a n dt h i sd e n s i t yd e c r e a s e sw i t hi n c r e a s e dd o p i n g levels [4c]. A s t h e d o p i n g l e v e l 2 s i n c r e a s e d ,h o w e v e r , a t some p o i n t the m i n o r i t y c a r r i e r l i f e t i m em u s ts u r e l yd e c r e a s e ,a n dt h ef r e e carrier a b s o r p t i o nl o s s e sm u s ti n crease; b o t h e f f e c t s w i l l work t o i n c r e a s e t h e t h r e s h o l d . Theoptimum active regiondopi'ng l e v e l i s , i n f a c t , unknown, and t h i s i s p e r h a p so n eo ft h em o s t i m p o r t a n tq u e s t i o n sr e m a i n i n gt ob ea n s w e r e d . It shouldbegivenimmediate a t t e n t 2 o n . A t t h e same t i m e t h ec o n t r o lo fd o p i n g l e v e l s a n do fd o p a n t si nt h e grown e p i - l a y e r s m u s t a l s o b e s t u d i e d much more. It i s n o t s u f f i c i e n t t o know w h a t t h e optimumdoping l e v e l i s b e c a u s e o n e m u s t a l s o b e a b l e t o p r o d u c e layershavingtherequireddopingprofile. As regardstheburied mesa d i o d e sm e n t i o n e da b o v e ,t h e a c t i v e r e g i o n dopi n g l e v e l w a s r e l a t i v e l yl o w ,a n dt h e r a t e o f increase o f t h e i r t h r e s h o l d w i t h t e m p e r a t u r e w a s c o n s i d e r a b l y more r a p i d t h a n h a s b e e n s e e n i n b r o a d area dev i c e s w i t h more h e a v i l yd o p e d a c t i v e r e g i o n s , a n d t h e r e i s c l e a r l y room f o r i m provement.
SINGLE MODE OUTPUT A laser c a v i t y l i k e t h a t f o u n d i n a d i o d e l a s e r c a nh a v et h r e et y p e so f f a m i l i e s ofmodes:normaltransversemodes, modes r e l a t e dt ot h ep o r t i o no f t h ec a v i t yn o r m a lt ot h ej u n c t i o np l a n e ; l a t e r a l t r a n s v e r s e modes r e l a t e d t o thecavityinthejunctionplanebutperpendicular t o t h e d i r e c t i o n of propagat i o n ;a n dl o n g i t u d i n a lo ra x i a l modes a r i s i n gf r o mt h el o n gd i m e n s i o no fc a v i t yi nt h ej u n c t i o np l a n e . The n o r m a lt r a n s v e r s e modes a r e c o n t r o l l e d i n t h e d o u b l eh e t e r o j u n c t i o n l a s e r by making t h e a c t i v e r e g i o ns u f f i c i e n t l yt h i n . The l a t e r a l t r a n s v e r s e modes c a n b e c o n t r o l l e d by u s i n g a s u i t a b l e s t r i p e s t r u c t u r e . The l o n g 2 t u d i n a l modes a r e d e t e r m i n e dp r i m a r i l y by t h e s p a c i n g o f t h e e n d mirr o r s i n a Fabry-Perot l a s e r , o r t h e g r a t i n g p e r i o d i n a d i s t r i b u t e df e e d b a c k laser. W e w i l l assume t h ea c t i v er e g i o nc a nr e a d i l yb e made s u f f i c i e n t l y t h i n s o t h a t t h e l a s e r o p e r a t e so n l yi nt h el o w e s to r d e rn o r m a l l a t e r a l mode, and w e w i l l thenlookfirst a t c o n t r o l l i n gt h et r a n s v e r s e l a t e r a l m o d e s ,a n dt h e nt h e l o n g i t u d i n a l modes.
C o n t r o l o f T r a n s v e r s e Modes Mode c o n t r o l i n t h e p l a n e of t h e j u n c t i o n t r a n s v e r s e t o the d i r e c t i o n of p r o p a g a t t o nc a nb ea c h i e v e d byprovidi'ng a refractive index step in the plane e i t h e r by p a s s i v e ,b u i l t - i n means o r by a c t i v e , inducedmeans.Inthe latter, t h e r e i s transverse g u i d i n g o r c o n f i n e m e n t o n l y i n t h e p r e s e n c e o f a n excitat i o nc u r r e n t , i . e . g a i ng u i d i n g .T h er e s u l t i n gi n d e xs t e p i s h a r dt oc o n t r o l , however,and vari-es w i t h t h e o p e r a t i n g c o n d i t i o n s . A built-inindexstep is
*
-1.
Inanoxidedefinedstripecontact transverseconfihementmechanism.
l a s e r , g a i ng u i d i n gw o u l db et h ep r i m a r y 27
more d e s i r a b l e a n d i n f a c t s h o u l d b e l a r g e e n o u g h t o d o m i n a t e o v e r a n y g a i n i n ducedstepthat may p o t e n t i a l l y e x i s t . On t h e o t h e r h a n d , t h e r e i s a d i s t i n c t a d v a n t a g e i n k e e p i n gt h ei n d e xs t e p as small as p o s s i b l e . The smaller t h ei n d e xs t e p ,a n dt h u s ,t h ew e a k e rt h e g u i d i n g ,t h ew i d e rt h eg u i d ec a nb ea n d s t i l l s u p p o r to n l yt h el o w e s to r d e r mode. If t h ei n d e xs t e p i s l a r g e , as i s t h e case i n a n e t c h e d mesa s t r u c t u r e , t h e s t r i p e mustbeextremelynarrowtoensureoperationonlyinthelowest o r d e rt r a n s v e r s e mode. I na d d i t i o nt ob e i n g e a s i e r t o f a b r i c a t e , a wider l a s e r w i l l have a much h i g h e r o u t p u t power t h a n a very narrow one. The t r a n s v e r s e i n d e x s t e p is greatlyreducedintheburied mesa laser s t r u c t u r e d e s c r i b e d e a r l i e r a n dt h o s e lasers d o i n d e e d o p e r a t e i n t h e l o w e s t o r d e rt r a n s v e r s e mode up t o f o u r times t h r e s h o l d [ 2 ] . Thishasbeenconfirmed o f t h ee m i s s i o n ,w h i c h showed onesymmetriby n e a r a n d f a r f i e l d m e a s u r e m e n t s c a l l ys h a p e de m i s s i o np e a k . Above f o u r times t h r e s h o l de v i d e n c e of a second, h i g h e ro r d e rt r a n s v e r s e mode f a m i l y i s s e e na l t h o u g ht h ee m i s s i o nr e m a i n ss u b s t a n t i a l l y s i n g l e mode t o s i x times t h r e s h o l d . The p r e s e n t b u r i e d mesa l a s e r s have a c t i v e r e g i o n s 1 . 5 ym t h i c k , a n d mesas 5 Um w i d e .I ft h e a c t i v e r e g i o n i s r e d u c e dt o 0.5 um t h i c k , s t i l l wider mesas c a nb eu s e d .N o n e t h e l e s s ,t h et r a n s v e r s ei n d e xs t e p i s s t i l l l a r g e rt h a n n e c e s s a r yt od o m i n a t eo v e rg a i ng u i d i n ga n de v e nw i d e r s t r i p e s couldbeused if t h eg u i d i n g was weaker. A p o s s i b l e s o l u t i o n may b et h ei n c o r p o r a t i o no fa n a d d i t i o n a l , smaller i n d e x s t e p w i t h i n t h e b u r i e d mesa, p o s s i b l y i n t h e s p i r i t o ft h ec h a n n e l e ds u b s t r a t ep l a n a r (CSP) geometry lasers [ 1 6 ] . Withsuch a comb i n a t i o no u rc a l c u l a t i o n si n d i c a t et h a t s t r i p e w i d t h s of 2 5 t o 30 microns a r e p r a c t i c a l .F a b r i c a t i n gs u c h a d e v i c e w i l l r e q u i r ea d d i t i o n a ld e v e l o p m e n to f t h e LPE g r o w t ht e c h n o l o g y ,h o w e v e r ,a n do t h e rs o l u t i o n s a r e a l s o b e i n g explored. C o n t r o l of L o n g i t u d i n a l Modes a. Role of Transverseand
L a t e r a l Modes
The i m p o r t a n c eo ft r a n s v e r s e mode c o n t r o l t o t h e a c h i e v e m e n t o f s i n g l e mode o p e r a t i o n i n l a s e r d i o d e sh a so n l yr e c e n t l yb e e nu n d e r s t o o d ,b u t i t i s now g e n e r a l l ya c c e p t e dt h a t a d e v i c et h a t i s c o n s t r a i n e d t o o p e r a t e o n l y i n o n e t r a n s v e r s e mode ( t y p i c a l l y t h i s i s t h e l o w e s t o r d e r mode) w i l l o s c i l l a t e o v e r a b r o a dr a n g eo fd r i v ec u r r e n t si no n l yo n e( l o n g i t u d i n a l ) mode. W e can e a s i lyobtainlowestordernormaltransverse mode o p e r a t i o n of a DH l a s e r b u t unless a s u i t a b l e s t r i p e i s u s e d , t h e r e w i l l be many l a t e r a l t r a n s v e r s em o d e s , a n dt h e r e i s a f a m i l yo fl o n g i t u d i n a l( F a b r y - P e r o t ) modes a s s o c i a t e d w i t h e a c h t r a n s v e r s e mode. T h u s ,w h i l e i n a homogeneouslybroadened l a s e r l i k e a d i o d e l a s e r , o n e l o n g i t u d i n a l mode i n e a c h f a m i l y w i l l dominate when o p e r a t i n g w e l l a b o v et h r e s h o l d ;t h e r e w i l l s t i l l be many modes i n t h e o u t p u t b e c a u s e t h e r e are many t r a n s v e r s em o d e s .E l i m i n a t i n g a l l b u to n et r a n s v e r s e mode e l i m i n a t e s a l l b u to n ef a m i l y of F a b r y - P e r o tm o d e s ,a n dl e a d st os i n g l e mode o u t p u t . Conseq u e n t l y ,t h ek e yt oo b t a i n i n g a s i n g l e l o n g i t u d i n a l mode o u t p u t , i . e . t r u l y s i n g l e mode o u t p u t , l i e s i n a c h i e v i n g o p e r a t i o n i n o n l y t h e l o w e s t o r d e r t r a n s v e r s e mode.
28
b .D i s t r i b u t e dF e e d b a c k A s was j u s t d i s c u s s e d , o n e l o n g i t u d i n a l mode i n e a c h t r a n s v e r s e mode fami l y w i l l d o m i n a t ea n dt h a t mode i s t h e o n e nearest t h e p e a k o f the g a i n c u r v e . A s t h ej u n c t i o nt e m p e r a t u r eo ft h e laser i s changed, i . e . , as i t i s t u n e d ,t h e gaincurveshiftsandthusthepreferredlongitudinal mode may changeand mode hopping may o c c u r .T h i sh o p p i n g w i l l b e f r e q u e n t i n t h e case of a Fabry-Perot modes a r e c l o s e l y s p a c e d The s p a c i n go f (.FP) l a s e r w h e r e t h e l o n g i t u d i n a l t h e c a v i t y modes c a n b e i n c r e a s e d , a n d t h u s t h e i n c i d e n c e o f mode hoppingdec r e a s e d ,b ys h o r t e n i n gt h e l a s e r , b u tt h i sa l s oi n c r e a s e st h et h r e s h o l d( w h i c h varies as 1/L)and i s t h u s a clear compromisesolution.
."
A n o t h e rs o l u t i o n i s t o r e p l a c e t h e F a b r y - P e r o t e n d - m i r r o r c a v i t y b y a distributedcavityformed by making a weak p e r i o d i c v a r i a t i o n i n r e f r a c t i v e i n d e x withintheactiveregioninthedirectionofthepropagationofthelight. Such a d i s t r i b u t e d f e e d b a c k (DFB) c a v i t y a l s o h a s a f a m i l yo fc l o s e l ys p a c e d modes b u t t h e modes away f r o m t h e two nearest t h e B r a g g f r e q u e n c y o f t h e p e r i o d i cv a r i a t i o no r" g r a t i n g "h a v e much h i g h e r t h r e s h o l d s a n d t h e r e is strong d i s c r i m i n a t i o na g a i n s tt h e m .T h u s , a DFB l a s e r w i l l l a s e i n a s i n g l e mode, t h e near-Bragg mode n e a r e s t t h e g a i n p e a k , o v e r a much w i d e r t u n i n g r a n g e t h a n an FP l a s e r w i l l b e f o r e mode hoppingoccurs. On t h e o t h e r h a n d , t h e DFB l a s e r w i l l o n l y l a s e a t t h o s et e m p e r a t u r e sf o rw h i c ht h eg a i nc u r v eo v e r l a p st h eB r a g g f r e q u e n c y ,w h e r e a st h e r e a r e always modes t h a t o v e r l a p t h e g a i n c u r v e i n an FP laser. W e havesuccessfullyfabricated PbSnTe DFB l a s e r s by l i q u i d p h a s e e p i t a x y a n do b s e r v e du n u s u a l l yc l e a n ,s i n g l e mode e m i s s i o n s p e c t r a c o n t i n u o u s l y t u n a b l e o v e r a r a n g ei ne x c e s so f 20 cm-1 (0.33 pm) c e n t e r e da b o u t 780 cm-1 ( 1 2 . 8 um) a t a n a v e r a g e r a t e of 1 . 2 c m - l / " K from 9°K t o 26°K.
The DFB l a s e r s were produced by LPE growthof PbSnTe (1.5 Um t h i c k ) a n d P b T e (0.5 um) l a y e r s d i r e c t l y o n a 0.79 um p e r i o d c o r r u g a t i o n i o n m i l l e d i n t o a PbTe s u b s t r a t e , a n d h a d a 25 um w i d e i n s u l a t o r d e f i n e d s t r i p e c o n t a c t e x t e n d i n g over 400 pm o ft h e i ro v e r a l ll e n g t ho f 710 pm. Fabry-Perot modes were t h u s e l i m i n a t e d by p r o v i d i n ga n unpumped a b s o r b i n gr e g i o no no n ee n do ft h e device; t h ee n do ft h ea b s o r b i n gr e g i o n was a l s os a w - c u t a t a na n g l eo fa p p r o x i m a t e l y
30" t o t h e s t r i p e a x i s t o f u r t h e r r e d u c e
FP r e f l e c t i o n s .
These DFB lasers are u n i q u e i n t h a t t h e y a r e f a b r i c a t e d by l i q u i d p h a s e e p i t a x ya n di nt h a tt h ef e e d b a c kc o r r u g a t i o n is placedwithintheactiveregion o ft h ed e v i c ea n d i s p r o d u c e db e f o r e ,r a t h e rt h a na f t e r ,t h eg r o w t ho ft h e a c t i v e l a y e r . The t u n i n g r a t e a n dr a n g e ,a n dt h ec l e a n l i n e s so ft h e mode spect r u ma n dt h ed e g r e eo fs i n g l e mode o p e r a t i o n of t h e s e d e v i c e s a r e s u p e r i o r t o p r e v i o u s l yr e p o r t e d PbSnTe DFB lasers [17]. The f u l lo p e r a t i n gr a n g eo ft h e s e d e v i c e s i s i n f a c t s t i l l unknown b e c a u s et h e y were a l r e a d y o p e r a t i n g a t t h e lowesttemperatureandevenbroadertuningranges may b e p o s s i b l e . These DFB l a s e r s h a dm o d e r a t et h r e s h o l d -1.
2
3 . 6 kA/cm2 ( p u l s e d ) .W h i l et h e
The mode s p a c i n g i s A /nL,where A i s t h el a s i n gw a v e l e n g t h , l e n g t h ,a n d n t h e i n d e x o f r e f r a c t i o n .
L thecavity
29
output power vs. current relationship is not completely free kinks, ofsubstan- '\ :t tially single mode operation can be obtained over the entire operating in the specand to over10 times threshold. Weaker modes can at times be seen trum, however, which indicates that the transverse modes are not adequat suppressed by the stripe contact.
While these DFB lasers clearly demonstrate the merit of this struct they were not fabricated in the lattice matched system described earlier was the mesa stripe employed and thus there remains much room for improve On the other hand, the technology for fabricating micron period corrugatio and the stripe mesa technology are both sufficiently new that it is felt that on fully developing those technologies, the immediate emphasis should be placed after which their combination to produce optimal DFB lasers will be relat straightforward. CONCLUSIONS AND FUTURE PRIORITIES
The recent developments described above demonstrate the value of usin of Pb-salt laser sophisticated device geometries to improve the performance diodes. It should be clear, furthermore, thato n l y through the use of heterostructures and stripe geometry devices with built-in index steps can the g of high temperature CW operation and single mode, low noise emission be ducibly achieved. To fabricate such structures requires the use of epitaxial techniques and of more complex processing sequences than are commonly use Pb-salt devices at present, but the gains to be realized are tremendous.
With each new solution to yesterday's problems, and with each device finement, device performance is improved but new limitations are encounte which, if overcome, promise still better performance. With Pb-salt lasers, t most important immediate goal remains the perfection of the stripe geometr single mode devicein alattice-matched system. While nothing has been said in this presentation about the impact of this on emission noise, it is expec that such deviceswill have much lower noise, and determining if this is i the most important the case will be the next question to answer. Beyond that basic issues before us appear to be controlling and optimizing doping pr in devices; determining and minimizing the role of defects and impurities limiting the internal radiative recombination efficiency; understanding the limitations on laser output power at high pumping levels; and understandin of these lasers. sources of, and solutions for, noise in the output
30
I
REFERENCES
1. Kasemset, D.; and Fonstad,C. G.: Lattice-Matched Pbl-xSnxTe/PbTel-ySey DH Laser Diodes Operating to 166K. Proceedings of the 1979 Internatlona1 Electron Devices Meeting, IEEE, 1979. pp. 130-132. 2.
Kasemset, D.; Rotter, S.; and Fonstad, C. G.: Pbl-xSnxTe/PbTel-ySey Lattice-Matched Buried Heterostructure Lasers with CW Single Mode Output. IEEE Electron Device Letters, vol. EDL-1, May 1980, pp. 75-78.
3. Hsieh, H. H.; and Fonstad, C. G.: LPE-Grown PbSnTe Distributed Feedback Laser Diodes with Broad Continuous Tuning Range in Single Mode Output. Proceedings of the 1979 International Electron Devices Meeting, IEEE, 1979. pp. 126-129. 4a. Tomasetta, L. R.; and Fonstad, C. G.: Liquid Phase Epitaxial Growth of Laser Heterostructures in Pbl-xSnxTe. Appl. Phys. Lett., vol. 24, 1974, pp. 567-570.
4b. Tomasetta, L. R.; and Fonstad,C. G.: Threshold Reduction in Pbl-xSnxTe Laser Diodes through the use of Double Heterojunction Geometries. Appl. Phys. Lett., vol. 24, 1974, pp. 440-442. 4c. Tomasetta, L. R.; and Fonstad, C. G.: Lead-Tin Telluride Double Heterojunction Laser Diodes: Theory and Experiment. IEEE J. Quant. Electron., V O ~ .QE-11, 1975, pp. 384-390. 5a. Walpole, J. N.; Calawa, A. R.; Ralston, R.; Harmon, T.; and McVittie,J.: Single Heterojunction Pbl-,Sn,Te Diode Lasers. Appl. Phys. Lett., vol. 23, 1973, pp. 620-622. 5b. Groves, S. H.; Nill, K. W.; and Strauss, A. J.: Double Heterostructure Pbl-xSnxTe-PbTe Lasers with CW Operation at 77K. Appl. Phys. Lett., V O ~ .25, 1974,pp. 331-333.
6. Armiento, C. A.; and Fonstad, C. G.: An Analysis of the Effects of Interface Recombination on the Transient Response of Double Heterojunction Devices. IEEE J. Quant. Electron., vol. QE-13, 1977, pp. 783-791. 7.
Fonstad, C. G.; and Armiento, C. A.: Approximating the Transient Response of Double Heterojunction Devices. J. Appl. Phys., vol. 49, 1978, pp. 2435-2438.
8. Kasemset, D.; and Fonstad, C. G.: Measurement of Interface Recombination Velocity in (Pb,Sn)Te/PbTe Double Heterostructure Laser Diodes. Appl. Phys. Lett., vol. 34, 1979,pp. 432-434. 9.
Kasemset, D.; and Fonstad,C. G.: Reduction of Interface Recombination Velocity with Decreasing Lattice Parameter Mismatch in PbSnTe Heterojunctions. J. Appl. Phys., vol. 50, 1979, pp. 5028-5029. 31
10. Kasemset, D.; and Fonstad, C. G . : Minority Carrier Lifetimes and Lasing Thresholds of PbSnTe Heterostructure Lasers. IEEE J. Quant. Electron., V O ~ .QE-15, 1979, pp. 1266-1270.
11. Walpole, J. N.; Groves, S. H.; Calawa, A. R.; and Harman, T. C.: Lattice Misfit Dislocationsin Heteroepitaxial Pbl-xSnxTe. Solid State Research Report, M.I.T. Lincoln Laboratory,No. 4, 1974, p. 13. Walpole, J. N.:
12.
private communication.
13. Ralston, R. W.; Melngailis, I.; Calawa, A. R.; and Lindley, W. T.: Stripe Geometry Pbl-,S%Te Diode Lasers. IEEE J . Quant. Electron., vol. QE-9, 1973, pp. 350-356.
14. Walpole, J. N.; Calawa, A. R.; Harman, T. C.; and Groves, S. H.: Double Heterostructure PbSnTe Lasers Grown by Molecular Beam Epitaxy with Operation up to 114K. Appl. Phys. Lett., vol. 28, 1976, pp. 552-554. 15a. Tsukuda, T.: GaAs-Gal-,Al,As Buried Heterostructure Injection Lasers. J . Appl. Phys., V O ~ .45, 1974,pp. 4899-4906. 15b. Kishino, K.; Suematsu, Y.; Takahashi, Y.; Tanbun-ek, T.; and Itaya,Y.: Fabrication and Lasing Properties of Xesa Substrate Buried HeterostrucJ . Quant. Electron., ture GaInAsP/InP Lasers at 1.3 um Wavelength. IEEE V O ~ .QE-16, 1980, pp. 160-164.
16. Aiki, K.; Nakamura, M.; Kuroda, T.; Umeda,J.; Ito, R.; Chinone, N.; and Maeda, M.: Tranverse Mode Stabilized AlxGal-,As Injection Lasers with Channeled-Substrate-Planar Structure. IEEE J. Quant. Electron., vol. QE-14, 1978, pp. 89-94. 17a. Walpole, J . N.; Calawa, A. R.; Chinn, S. R.; Groves, S. H.; and Harman, T. C.: Distributed Feedback Pbl-xSnxTe Double Heterostructure Lasers. Appl. Phys. Lett., vol. 29, 1976, pp. 307-309. 17b. Walpole, N. J.; Calawa, A. R.; Chinn, S. R.; Groves, S. H.; and Harman, T. C.: CW Operation of Distributed Feedback Pb Sn Te Lasers. Appl. Phys. l-x x Lett., vol. 30, 1977, pp. 524-526. -~
+
y) double 1-Y Y 1-Y Y heterostructure. To prepare this structure LPE, by the growth temperature must be below suppress diffusion of the tin during the epitaxial growth.
600°C to
In this paper, the mechanism by which the heterojunction boundaries become irregular is cleared and a new LPE method which prevents the irregularity is explained. The lasers prepared from the wafers grown by the new method have demonstratedCW operation at wavelengths longer than 10 urn above liquid nitrogen temperature. INTRODUCTION Lead tin telluride diode lasers are well known as excellent radiation sources in the wavelengths longer than6 pm"). The diode lasers should be of double heterostructures to reduce the threshold current densities or to operate at temperatures above77K(2). Lasers with wavelengths from 6 to 10 ~ I Uoperate easily above 77K and they inare practical use. On the otherhand, it is very difficult for the lasers to operateCW on with wavelengths over10 urn above liquid nitrogen temperature. The reason for this is the difficulty of preparing flat heterojunctions for the lasers with wavelengths over 10 pm. The heterojunction boundaries become irregular if the junctions are made by the usual LPE method. A new LPE method has been developed irregular heterojunction boundaries.
which
overcomes
By the new method, lasers capable of CW operation wavelengths longer than10 pm at temperatures above 77K.
the have
problem been
of
made
45
at
EXPERIMENTAL
Highly reliable
lasers w i t h mesa strata s t r u c t u r e h a v e b e e n
developed (3) ( 4 ) . The s t r u c t u r e o ft h e laser is shown i n F i g u r e 1. The heterojunctionshavebeenpreparedbytheliquidphaseepitaxialgrowth method u s i n gg r a p h i t es l i d i n gb o a t . To c o n f i n et h ep h o t o n sa n d carriers i n t h e a c t i v el a y e r ,t h ep r o p o r t i o no f SnTe which i s denoted a s -X i nt h e a c t i v e l a y e r i s h i g h e rt h a n Y which i s t h ep r o p o r t i o n of two c o n f i n i n gl a y e r s . The insulating films have been
made by t h e a n o d i c o x i d a t i o n
(4)
.
The w a v e l e n g t ho ft h e lasers i s determinedbythecompositionofthe a c t i v er e g i o na n dt h et e m p e r a t u r e . To make t h ew a v e l e n g t hl o n g e rt h a n a t 77K, t h ev a l u eo f X mustbe more t h a n 0 . 2 , w h i l ef o rw a v e l e n g t h so f 6 t o 1 0 pm X is less t h a n 0.2.
1 0 um
When t h e d o u b l e h e t e r o j u n c t i o n s are f o r m e d b y t h e u s u a l LPE t e c h n i q u e , theshapesoftheheterojunctionboundariesvaryobviouslywhetherthe comp o s i t i o n of t h e a c t i v e l a y e r i s o v e r 0.2 o r n o t .
Two p h o t o m i c r o g r a p h s o f e t c h e d c r o s s s e c t i o n s o f d o u b l e h e t e r o w a f e r s are shown i nF i g u r e 2 . Both are grownby t h e same LPE method. One i s anexample o ft h ew a f e r sf o rt h e 6 t o 10 v m lasers, a n d t h e o t h e r s i s an example of t h e 10 pm. w a f e r sf o ro v e r H e t e r o j u n c t i o n s c o n t a i n many d i s l o c a t i o n s , w h i c h i s due t o t h e m i s f i t of the lattice constant. The m i s f i td i s l o c a t i o n sb e t w e e nt h es u b s t r a t ea n dt h e f i r s tl a y e rs p r e a di nt h ed i r e c t i o n of t h ec r y s t a lg r o w t h .T h i s i s d u et ot h e
of Pb and Sn(5). The g r o w t ht e m p e r a t u r eotfh e m u t u a sl e l fd i f f u s i o n s e c o n da n dt h et h i r dl a y e r s mustbelowenough tosuppressmutualdiffusion. T h e r e f o r e ,t h ea c t i v er e g i o n i s grown a t a t e m p e r a t u r eo f 600°C. A p h o t o m i c r o g r a p hf o rt h el o n gw a v e l e n g t h l a s e r shows t h a t t h e h e t e r o junctionboundariesbetweenthefirst a n ds e c o n dl a y e r sa n db e t w e e nt h es e c o n d and t h i r d l a y e r s are i r r e g u l a r . When t h eh e t e r o j u n c t i o nb o u n d a r i e s are i r r e g u l a r ,t h es c a t t e r i n gl o s st h a tr e s u l t s becomes s o s e r i o u s t h a t lasers made fromwafers shown i n F i g u r e 2 A do n o to p e r a t ee v e n a t 20K. But l a s e r s withwavelengthsfrom 6 t o 10 v m , p r e p a r e df r o mt h ew a f e rw i t hf l a th e t e r o j u n c t i o n s shown i n F i g u r e 2 B , g i v e good CW o p e r a t i o n a b o v e 77K. To c o n f i r m t h e c a u s e o f i r r e g u l a r i t y i n w a f e r s f o r l o n g w a v e l e n g t h lasers, a n o t h e re x p e r i m e n t w a s performed. The r e s u l t i s shown i nF i g u r e 3 . The f i r s t l a y e r of Y = 0.13 was grown a l l o v e rt h es u r f a c e of t h e PbTe s u b s t r a t e . A f t e rr e m o v i n gt h es o l u t i o nf o rt h ef i r s tl a y e r ,t h es o l u t i o nf o rt h es e c o n d l a y e r was k e p t o n j u s t h a l f of t h e f i r s t e p i t a x i a l l a y e r f o r 1 minute i s o t h e r m a l l y a t 6OO0C. Then t h es o l u t i o n was removed. The s u r f a c ew h e r et h e s e c o n ds o l u t i o n w a s k e p t on i s u n e v e n ,w h i l et h er e m a i n i n gh a l f of t h e s u r f a c e i s s m o o t h .T h ec r o s ss e c t i o n a lv i e w shows t h a t a t some p l a c e s ,t h e m e l t i n gb a c ko c c u r s as was e x p e c t e d ,a n dm o r e o v e r ,e p i t a x i a lg r o w t ho c c u r s n e a rt h em e l t e db a c k area.
From the measurement of the lattice constantbythe X-ray d i f f r a c t i o n t e c h n i q u e ,t h ec o m p o s i t i o n of t h e p r e c i p i t a t e d l a y e r w a s determinedtobe x = 0.25. From t h i s e x p e r i m e n t , t h e c a u s e o f i r r e g u l a r i t y i s found t o b e d u e n o t o n l yt ot h em e l t i n gb a c kb u ta l s ot h ep r e c i p i t a t i o n . Of c o u r s e ,t h es a t u r a t e d s o l u t i o n s were u s e d f o r t h e e p i t a x i a l g r o w t h , b u t even w i t h t h e s e s a t u r a t e d solutions, both melting back and precipitation occurred. The mechanismof themeltingback and p r e c i p i t a t i o n u s i n g s a t u r a t e d s o l u t i o n sc a nb ee x p l a i n e db yt h ep h a s ed i a g r a m . The ternaryphasediagramofPb,
Sn
and
Te
w a s i n v e s t i g a t e di nd e t a i l
by Harris, e t a l . (6) The t e m p e r a t u r e o f t h e e p i t a x i a l g r o w t h o f t h e s e c o n d a n d t h i r d l a y e r s i s below 600"C, so t h e r e g i o n b e l o w t h e 600°C i s o t h e r m a l l i q u i d u s l i n e m u s t b ec o n s i d e r e d .N o t et h a tt h ei s o c o m p o s i t i o n a ls o l i d u sl i n e sc u r v es h a r p l y wherethecomposition i s o v e r x = 0.2. First, the irregularity a n ds e c o n de p i t a x i a ll a y e r s
o ft h eh e t e r o j u n c t i o nb o u n d a r yb e t w e e nt h ef i r s t is considered.
The t e r n a r yp h a s ed i a g r a mw h i c h is relatedtothefirstandsecondlayers 4 . When t h e f i r s t l a y e r i s a l r e a d y grown a n dt h es o l u t i o n f o rt h es e c o n dl a y e r makes c o n t a c t w i t h t h e f i r s t l a y e r , t h e f i r s t l a y e r is in e q u i l i b r i u mw i t ht h es o l u t i o n whosecomposition i s d e n o t e db yt h ep o i n t B where Y = 0 . 1 3s o l i d u si n t e r c e p t st h e 600°C l i q u i d u s . So, t h ef i r s tl a y e r i s n o ti ne q u i l i b r i u mw i t ht h es o l u t i o n a t t h ep o i n t A where x = 0.25 s o l i d u si n t e r c e p t st h e 600°C l i q u i d u s ,w h i c h i s t h ec o m p o s i t i o no ft h es e c o n d are s o d i f f e r e n t s o l u t i o n . The f i r s tc o m p o s i t i o na n dt h i ss e c o n dc o m p o s i t i o n t h a t a d i s e q u i l i b r i u me m e r g e si n s t a n t a n e o u s l y when t h e s e c o n d s o l u t i o n is h e l do nt h ef i r s te p i t a x i a ll a y e r . To r e s t o r ee q u i l i b r i u mb e t w e e nt h es o l i d a n dt h el i q u i d ,t h eC o m p o s i t i o n of t h es e c o n ds o l u t i o nc h a n g e sf r o mt h ep o i n t A t o w a r dt h ep o i n t B a l o n gt h e 600°C i s o t h e r m a l i q u i d u sl i n e . i s shown i n F i g u r e
I no r d e rt oc h a n g et h ec o m p o s i t i o no ft h i ss o l u t i o n ,t h em o l e c u l a r c h a n g eb e t w e e nt h es o l i da n dt h el i q u i do fd i f f e r e n tc o m p o s i t i o n
interis required.
By t h e l o c a l m e l t i n g b a c k o f t h e f i r s t l a y e r o f Y = 0 . 1 3 ,t h ec o m p o s i t i o n o ft h es o l u t i o n moves f r o mt h ep o i n t A t o w a r dt h ep o i n t C which i s t h e c o m p o s i t i o no ft h ef i r s tl a y e r .T h i sc o m p o s i t i o n a lc h a n g e i s i n d i c a t e d by t h e v e c t o r a. On t h eo t h e rh a n d ,b yt h ep r e c i p i t a t i o no f x = 0.25, t h e composit i o n of t h es o l u t i o n moves f r o mt h ep o i n t A a g a i n s tt h ep o i n t D which i s t h ec o m p o s i t i o no ft h es e c o n dl a y e r .T h i sc o m p o s i t i o n a lc h a n g ed u et ot h e p r e c i p i t a t i o n i s i n d i c a t e db yt h ev e c t o rb . When t h e m e l t i n g b a c k and p r e c i p i t a t i o n o c c u r s i m u l t a n e o u s l y , t h e m o t i v e f o r c e of t h ec o m p o s i t i o n a lc h a n g ea l o n gt h e 600°C i s o t h e r m a ll i q u i d u se m e r g e s . T h i s i s t h e r e a s o n why t h e i n t e r f a c e o ft h e f i r s t andsecondlayersbecomes irregular. 47
Themechanismof
the irregularity between the second and the third layers reverse m e c h a n i s m o f t h e a f o r e m e n t i o n e d i r r e g u l a r i t y at the interf a c eo ft h ef i r s ta n ds e c o n dl a y e r ;t h a t i s , t h ec o m p o s i t i o no ft h et h i r d s o l u t i o n moves t o w a r d t h e c o m p o s i t i o n o f t h e s e c o n d s o l u t i o n a l o n g t h e 6OOOC liquidusline.
is j u s t t h e
A l l t h i s e x p l a i n s why t h e h e t e r o j u n c t i o n s become i r r e g u l a r when d o u b l e h e t e r o w a f e r s are p r o d u c e d b y t h e u s u a l LPE method. However, a way h a s b e e n f o u n d t o p r o d u c e f l a t h e t e r o j u n c t i o n b o u n d a r i e s on d o u b l eh e t e r o w a f e r sf o rt h e 6 t o 1 0 pm lasers. The r e a s o nt h i sc a nb ed o n ec a nb eu n d e r s t o o df r o mt h ep h a s ed i a g r a m . The i s o c o m p o s i t i o n a ll i n e s are s t r a i g h tb e l o w x = 0.20. So, t h ec o m p o s i t i o n a l d i f f e r e n c eo ft h es o l u t i o n sb e t w e e nt h ef i r s tl a y e ra n dt h es e c o n dl a y e r is not s o l a r g e ,t h er e s u l tb e i n gt h a ts e r i o u sd i s e q u i l i b r i u md o e sn o to c c u r . Of c o u r s e ,t h es o l u t i o nc o m p o s i t i o nf o rt h es e c o n dl a y e r moves t o w a r dt h a to ft h e firstsolution,butthemotiveforce i s s o small t h a t t h e b o u n d a r i e s do n o t become i r r e g u l a r . i t is s e e nt h a tt oo b t a i nf l a th e t e r o From t h e a f o r e m e n t i o n e d a n a l y s i s , j u n c t i o nb o u n d a r i e sf o rt h el o n gw a v e l e n g t h l a s e r , i t i s n e c e s s a r yt op r o d u c e So, supercooled p r e c i p i t a t i o no n l y ,w h i l es u p p r e s s i n gt h em e l t i n gb a c k . s o l u t i o n sf o rt h eg r o w t ho ft h es e c o n da n dt h i r dl a y e r s were used i n o r d e r t o a l l o w no t i m e f o r c o m p o s i t i o n a l c h a n g e .
The t e m p e r a t u r e p r o f i l e f o r t h e u s u a l LPE growthmethod is shown i n F i g u r e 5. The f i r s t l a y e r is grown s l o w l yt oa s s u r et h eq u a l i t yo ft h ee p i t a x i a ll a y e r ,t h e nt h es e c o n da n dt h i r dl a y e r s are grown c o n t i n u o u s l y a t t h e c o o l i n g r a t e of 1 ° C p e r m i n u t e . The m o d i f i e d t e m p e r a t u r e p r o f i l e i n o r d e r t o a c h i e v e t h e s u p e r c o o l e d c o n d i t i o n is shown i n F i g u r e 6 . The b e g i n n i n g h a l f o f t h e f i r s t l a y e r is grown u n d e r t h e u s u a l c o n d i t i o n s , b u t i n t h e m i d d l e p o i n t of t h e f i r s t l a y e r g r o w t h ,t h ec o o l i n g r a t e i s changed t o as much as 3°C p e r minute. While t h e l a s t h a l f of t h e f i r s t l a y e r i s g r o w i n g ,t h es o l u t i o n sf o rt h es e c o n da n dt h i r d l a y e r s are s u f f i c i e n t l y s u p e r c o o l e d p r i o r t o making c o n t a c t w i t h t h e s u b s t r a t e . Then t h e s e c o n d and t h e t h i r d l a y e r s are grown c o n t i n u o u s l y . The c o o l i n g r a t e of 3OC p e rm i n u t e supercooled.
is sufficienttokeepthesolutions
A p h o t o m i c r o g r a p ho ft h ec r o s ss e c t i o n grownby t h e new LPE method i s shown i nF i g u r e 7. N o t i c e how f l a tt h eh e t e r o j u n c t i o nb o u n d a r i e s are. Lasers p r e p a r e d f r o m t h e w a f e r s l i k e t h i s e a s i l y o p e r a t e w a v e l e n g t h sl o n g e rt h a n1 0 urn above 77K.
on CW w i t h
I n F i g u r e 8, o n e e x a m p l e o f t h e l a s i n g s p e c t r u m i s shown d e m o n s t r a t i n g CW o p e r a t i o n on an 11.45 wm w a v e l e n g t hw i t hs i n g l e mode.The t e m p e r a t u r eo ft h e h e a t s i n k i s 77K.
CONCLUDING REMARKS
The mechanism by which heterojunction boundaries become i r r e g u l a r w a s shown and a new method w a s e x p l a i n e d of l i q u i d p h a s e e p i t a x i a l g r o w t h w h i c h p r e v e n t st h i si r r e g u l a r i t y . I
Lasers p r e p a r e d f r o m t h e w a f e r s grownby t h i s new LPE methodhave 10 above 77K. demonstrated CW o p e r a t i o n a t w a v e l e n g t h sl o n g e rt h a n
16
W e assume t h a t lasers w i t h e m i s s i o n w a v e l e n g t h s l o n g e r t h a n b eo p e r a t e dc o n t i n u o u s l ya b o v e 77K i f n e c e s s a r y .
urn
can also
REFERENCES
1.
D i m o c h , J. 0.; M e l n g a i l i s I, . ; a n dS t r a u s s , (1966)
2.
Groves, S. H . ; 331(1974)
N i l l , K. W.;and
A. J.:
S t r a u s s , A . J.:
Phys. Rev. 1 6 , 1193
A p p l . Phys. L e t t . 25,
3.Yoshikawa,Mitsuo;Shinohara,Koji;andUeda,Ryuiti: 699 (1977) 4.Yoshikawa,Mitsuo;Koseto,Masaru;andUeda,Ryuiti:
A p p l . Phys. L e t t . 25,
CLEOS '78,SanDiego
5.Yoshikawa,Mitsuo; I t oM , ichiharuS ; h i n o h a r aK , o j i ; a n dU e d a R , yuiti: J . C r y s t a l Growth 4 7 , 230(1979) 6.
Harris, J. S . ; Longo, J. T . ; G e r t n e r , E. R.;andClarke, Growth 28, 334 (1975)
J. E . :
J. C r y s t a l
49
P-Pb1-y Sn y Te-h p-PbTe
SUB
I I
for
for
OX1 DE FILM
In'
ACT1 VE REG10N F i g u r e 1.- S c h e m a t i c s t r u c t u r e
50
of t h e laser d i o d e .
Figure 2.-
E t c h e dc r o s ss e c t i o n s
of d o u b l eh e t e r o w a f e r s .
p 2NDSOLUTION WASKEPT ON
I
SURFACE
CROSS SECTION F i g u r e 3 . - H e t e r o i n t e r f a c em o r p h o l o g y . 51
crystal
Te
Sn \ \
F i g u r e 4.-
Ternary phase diagram of Pb-Sn-Te e x p l a i n i n g t h e heterojunctionboundary'sirregularity.
E
TIME USUAL LPE AslOpm
F i g u r e 5.- The t e m p e r a t u r e p r o f i l e
52
L
of u s u a l LPE method.
1 I6
a
z W I-
\
\
TI ME NEW LPE F i g u r e 6.-
The t e m p e r a t u r e p r o f i l e
of t h e new LPE method.
F i g u r e 7.- E t c h e d c r o s s s e c t i o n of t h e d o u b l e h e t e r o w a f e r € o r t h e longwavelengthlasers grown by t h e new LPE method.
53
1111lIlII I1
11.2
11.4
WAVELENGTH
11.6
h (pm)
Figure 8.- One example of the lasing spectrum.
54
11.8
WIDELY TUNABLE (PbSn)Te LASERS U S I N G ETCHED CAVITIES FOR MASS PRODUCTIONt
Matthew D. M i l l e r The Perkin-ElmerCorporation
SUMMARY
Lead s a l t diode l a s e r s a r e b e i n gu s e di n c r e a s i n g l y as t u n a b l es o u r c e s o fm o n o c h r o m a t i ci n f r a r e dr a d i a t i o ni n a v a r i e t yo fs p e c t r o s c o p i cs y s t e m s . T h e s ed e v i c e sa r ep a r t i c u l a r l yu s e f u l ,b o t hi nt h el a b o r a t o r y and i n t h e f i e l d , (compared t ot h e r m a ls o u r c e s )a n d because of t h e i r h i g h s p e c t r a l b r i g h t n e s s w i d es p e c t r a lc o v e r a g e( c o m p a r e dt ol i n e - t u n a b l eg a sl a s e r s ) .W h i l et h ep r i m a r yc o m m e r c i a la p p l i c a t i o no ft h e s el a s e r sh a sb e e nf o ru l t r a - h i g hr e s o l u t i o n ( l o m 4c m ” )l a b o r a t o r ys p e c t r o s c o p y ,t h e r ea r en u m e r o u ss y s t e m sa p p l i c a t i o n s , i n c l u d i n gl a s e ra b s o r b t i o np o l l u t i o nm o n i t o r s and l a s e rh e t e r o d y n er a d i o m e t e r s , f o r w h i c hd i o d el a s e r sh a v eg r e a tp o t e n t i a lu t i l i t y .T h e r ea r e ,h o w e v e r , s e v e r a lp r o b l e ma r e a sr e l a t e dt ot h ew i d e ru s eo ft h e s ec o m p o n e n t s . Among t h e s ea r et o t a lt u n i n gr a n g e , mode c o n t r o l , and h i g hf a b r i c a t i o nc o s t . A f a b r i c a t i o nt e c h n i q u ew h i c hs p e c i f i c a l l ya d d r e s s e st h ep r o b l e m so ft u n i n g range and c o s t , and which a l s o h a s p o t e n t i a l a p p l i c a t i o n f o r mode c o n t r o l , i s r e p o r t e dh e r e .
PROCEDURE
C r y s t a l Growth grown by m u l t i p l e s o u r c e moleclula r (Pb.81Sn.lg)Te homo j unctionswere b eam e p i t a x y ( M B E ) I n d e p e n d e n t l yc o n t r o l l e ds o u r c e so f PbTe and SnTe p e r m i t t e dp r e c i s ec o m p o s i t i o nc o n t r o l ,w h i l e B i 2 T e a and T 1 wereused a se x t r i n s i c n and p d o p a n t s ,r e s p e c t i v e l y . A f i f t h Knudsen s o u r c e was i n c l u d e dt op r o v i d e e x c e s sT e ,i fn e c e s s a r y ,t oc o n t r o ls t o i c h i o m e t r y . The s u b s t r a t e was BaF,, f i r s t used by Holloway ( r e f .1 ) .U n l i k ee a r l i e r workwhichused t h e( 1 1 1 ) c l e a v a g ep l a n e ,t h es u b s t r a t e sf o rt h i s workwerechemicallypolished(100) o r i e n t e dm a t e r i a l ,t oa l l o wf o rt h ep r e f e r e n c eo ft h el e a d s a l t s t o grow i n t h e( 1 0 0 )o r i e n t a t i o n .
.
C r y s t a lg r o w t ht o o kp l a c ea tt e m p e r a t u r e sf r o m 400 t o 42OoC andgrowth r a t e s offrom 2 t o 4 pm p e rh o u r . The l a s e r ’ s v e r t i c a l s t r u c t u r e was a f i v e l a y e rh o m o j u n c t i o nu t i l i z i n gc a r r i e rc o n c e n t r a t i o ng r a d i e n t sf o rb o t ho p t i c a l and e l e c t r i c a lc o n f i n e m e n t .S t r u c t u r e s similar t ot h i sh a v eb e e nr e p o r t e d by Lo ( r e f . 2 ) u s i n g b u l k d i f f u s e d m a t e r i a l andbyWalpole ( r e f . 3) using MBE-grown m a t e r i a l on a b u l ks u b s t r a t e . A t y p i c a lg r o w t hs e q u e n c ec o n s i s t e d ofgrowingann-typecontactlayer 0.5 p m t h i c k anddoped t o 10’’ cmm3, an ? R e s e a r c hp a r t i a l l ys u p p o r t e d
by NASA c o n t r a c t NAS1-14996 55
”
n - t y p eb u f f e rl a y e r 2 pm t h i c k anddoped t o 3 ~ 1 0 ’ ~ c m - ~an , undoped act i v el a y e r1 . 5 pm t h i c kw h i c h was t y p i c a l l y 1 ~ 1 0 ‘ p-type, ~ 2 pm o f 3 ~ 1 0 ‘ ~ p - t y p eb u f f e r ,a n d ,f i n a l l y ,0 . 5 pm o f l ~ l O ’ p~- t y p ec o n t a c tl a y e r .F o l l o w i n gg r o w t h ,t h ee n t i r em u l t i l a y e rf i l m ,w h i c h was t y p i c a l l y 8mm l o n g , l O m m wide, and 9 pm t h i c k , was m e t a l l i z e d and s o l d e r e d t o a c o p p e rh e a ts i n k , formi n ge l e c t r i c a lc o n t a c tt ot h ep - c o n t a c tl a y e r . The BaF2 s u b s t r a t e was t h e n s l o w l yd i s s o l v e do f fu s i n g w a r m f l o w i n gd e - i o n i z e d water. D e v i c eF a b r i c a t i o n S t a n d a r di n t e g r a t e dc i r c u i tp r o c e s s e s were usedtomass-produce mesa d i o d el a s e r s . The p r o c e s s s t e p s from c r y s t a lg r o w t ht o wire bonding a r e summ a r i z e di nf i g u r e 1. F o l l o w i n gs u b s t r a t er e m o v a l r, e c t a n g u l a rF a b r y - P e r o t c a v i t i e s were e t c h e d ,e i t h e rw e t - c h e m i c a lo rb yi o nm i l l i n g .N e x t , a BaF, i n s u l a t i n g l a y e r was d e p o s i t e d and a c o n t a c t window openedto p e r m i t e l e c t r i c a lc o n t a c tt ot h en - t y p ec o n t a c tl a y e rw i t h o u tc r e a t i n g an e l e c t r i c a l s h o r t t ot h ec o p p e rh e a ts i n k . Metal was t h e nd e p o s i t e do v e rt h ee n t i r es t r u c t u r e and s u b s e q u e n t l y removed fromonefaceofthe l a s e r . The d e v i c e was completed by w i r e - b o n d i n gt ot h et o pc o n t a c t metal, n e x tt ot h ed e v i c e ,i no r d e rt o avoid damage duetomechanical s t r e s s . F i g u r e 2 shows t h ef i n i s h e d mesa d i o d e in detail. A t y p i c a ls t r u c t u r e is 75 pm wide, 250 pm l o n g , and 9 pm h i g h . An a r r a y o ft h e s ed i o d e sp r i o rt of i n a l wire bonding i s s e e ni nf i g u r e 3 . The d i o d e s arespaced500 pm a p a r t i n xand y. The d a r ks q u a r e s are t h ep h o t o r e s i s t w h i c hh a sb e e nu s e dt op a t t e r nt h et o pc o n t a c t metal, and t h es m a l l e rr e c t a n g l e s w i t h i nt h es q u a r e s are t h e l a s e r s , whereone f a c eo ft h e l a s e r i s covered by m e t a l and o n ef a c ee x t e n d s beyond t h e metal, p e r m i t t i n g r a d i a t i o n t o e s c a p e .
LASER PERFORMANCE
D i o d e sf a b r i c a t e di nt h i s way havebeenoperated cw a t h e a t s i n k t e m p e r a t u r e sr a n g i n gf r o m 12K t o 60K, and t h e y o p e r a t e d i n 5 ps p u l s e s up t o 95K, a t w h i c ht e m p e r a t u r et h ep u l s e dt h r e s h o l dc u r r e n t was 10,00OA/cm2. Low tempera t u r e t h r e s h o l d s were between 500A/cm2 and1000A/cm2,bothpulsed and cw. T h e s ev a l u e sf o rt h r e s h o l dc u r r e n t s comparefavorablywiththoseobtained f o r d e v i c e s madeby more c o n v e n t i o n a lo n e - a t - a - t i m ep r o c e s s e sb a s e d on t h e growthofhighqualitybulksinglecrystalmaterial. The e m i s s i o nw a v e l e n g t ho fl e a ds a l tl a s e r s i s v a r i e d by c h a n g i n gt h e t e m p e r a t u r ea tt h ej u n c t i o n .T h i st u n i n g i s accomplished by v a r y i n gt h eh e a t s i n kt e m p e r a t u r e and t h ec u r r e n ti nt h el a s e r ,t o g e t h e ro rs e p a r a t e l y . A l a s e rc a no p e r a t ei ne i t h e r a s i n g l e mode or multimode,depending upon small c h a n g e si nt h eo p e r a z i n gp o i n t .F i g u r e 4 i l l u s t r a t e sb o t ht h ec u r r e n tt u n i n g and t h e mode s t r u c t u r e o f a d i o d eo p e r a t e d a t a h e a t s i n k t e m p e r a t u r e o f 20K. Sincetheaverageemissionwavelength i s d e t e r m i n e dp r i m a r i l y by thetemperat u r ed e p e n d e n c eo ft h e bandgap o ft h es e m i c o n d u c t o r ,o n ec a np r e d i c tt h e averagewavelength of e m i s s i o ng i v e nt h e I-V c h a r a c t e r i s t i c so ft h el a s e r , t h em e a s u r e dt h e r m a lr e s i s t a n c e , and t h e known temperaturedependenceofthe bandgap.
56
F i g u r e 5 s u m n a r i z e sb o t ht h et e m p e r a t u r et u n i n g and t h ec u r r e n tt u n i n g o f a laser w h i c ho p e r a t e di nt h ep u l s e d mode up t o 95K. The cw e m i s s i o n wavel e n g t h i s t a k e nt ob et h ec e n t e rf r e q u e n c yf o rc a s e so fm u l t i m o d ee m i s s i o n . Thiswavelength i s p l o t t e d as a f u n c t i o no fc u r r e n t and h e a ts i n kt e m p e r a t u r e . The s o l i d l i n e s were c a l c u l a t e d fromthe known e l e c t r i c a l and t h e r m a lp r o p e r t i e s ofthediode. B e c a u s eo fd i f f i c u l t i e si nt h et h e r m a lp a c k a g e ,t h i sd i o d ee x h i b i t e d q u i t e a h i g ht e m p e r a t u r e r i s e a tt h ej u n c t i o n ;t h u s ,t h e cw emissionof10 pm a t 1.35Aand a h e a ts i n kt e m p e r a t u r eo f 15K c o r r e s p o n d e dt o a j u n c t i o nt e m p e r a t u r eo f 95K, t h eh i g h e s tt e m p e r a t u r ef o rp u l s e do p e r a t i o n .S i g n i f i c a n t l y , w e h a v eo b t a i n e d ,i n a s i n g l ed i o d e , cw emissionfrom 10 p m t o 14 pm, a t u n i n g r a n g ec o m p a r a b l et ot h ew i d e s tr e p o r t e di nt h el i t e r a t u r e .
CONCLUSION
U s e f u ll a s e rp e r f o r m a n c eh a sb e e nd e m o n s t r a t e di nd e v i c e sf a b r i c a t e d u s i n gt h i n - f i l m and i n t e g r a t e dc i r c u i tt e c h n o l o g i e sg e a r e dt o w a r d massproduct i o n . The MBE c r y s t a lg r o w t ht e c h n i q u ep e r m i t si n d e p e n d e n t and p r e c i s e cont r o lo fc o m p o s i t i o n and c a r r i e rc o n c e n t r a t i o n . The l a s e r sd e s c r i b e di nt h i s paperwere grownona BaP2 s u b s t r a t e and e x h i b i t e da ne x t r e m e l yw i d et u n i n g r a n g e ,d e s p i t e a 1 . 4 %l a t t i c em i s m a t c h .T h i sw i d et u n i n gr a n g e i s probably a t t r i b u t a b l et oe x c e l l e n to p t i c a l and e l e c t r i c a lc o n f i n e m e n t ,a c c o m p l i s h e d w i t ht h et h i na c t i v er e g i o n and s h a r pc o n c e n t r a t i o ng r a d i e n t sa c h i e v a b l ew i t h MBE
.
On t h eo t h e rh a n d ,t h e s ed e v i c e sd i dn o t showgood performanceinterms o fs i n g l e mode ormultimodeoutputpower.Thereare two l i k e l yc a u s e sf o r t h e low o u t p u t power ( t y p i c a l l y 10 pW). There i s probablynon-optimaloutput c o u p l i n g due t oi m p e r f e c t and m i s a l i g n e de t c h e d end m i r r o r s .R e f i n e m e n t s o ft h ei o n - m i l l i n gp r o c e s sh a v el e dt o some improvements i nt h i sa r e a . More s i g n i f i c a n t l y ,t h e r e may be s e v e r e quantum e f f i c i e n c y r e d u c t i o n due t o t h e l a t t i c e m i s m a t c h .T h i se f f e c th a sb e e nd e m o n s t r a t e d by Kasemset for(PbSn)Te h e t e r o j u n c t i o n s( r e f .4 ) . F i g u r e 6 p r e s e n t st h er a n g eo fl a t t i c ec o n s t a n t sf o r some p o t e n t i a ls u b s t r a t em a t e r i a l s . The o n l yp o s s i b i l i t i e sf o rl a t t i c e - m a t c h i n gt o (PbSn)Te a r eb u l km a t e r i a lo rt h e mixed a l k a l i - h a l i d e , which i s a verypoorthermal match and n o t a v e r y good s u b s t r a t e . On t h eo t h e rh a n d , i t i s p o s s i b l et o l a t t i c e - m a t c h( P b S n l S et o( B a S r ) F q . I na d d i t i o nt op o t e n t i a lc o s tr e d u c t i o n ,t h et h i n - f i l mp r o c e s s i n gt e c h n i q u e sp r o v i d et h ep o s s i b i l i t yf o ri n t e g r a t e da r r a y so fd i o d e sd i f f e r i n gf r o m e a c ho t h e ri n a c o n t r o l l e df a s h i o n .T h i s would g r e a t l ya i di nt h ep r o b l e m o fs p e c t r a lc o v e r a g e . The p h o t o l i t h o g r a p h i cc a v i t yd e f i n i t i o na l l o w st h e u s eo fs h a p e dc a v i t i e sf o r mode c o n t r o l , and t h e m u l t i - l a y e r t h i n f i l m c r y s t a l growth p e r m i t s g r e a tf l e x i b i l i t yi nv e r t i c a ls t r u c t u r e .T h u s ,s t a n d a r d dev i c e s may b eo b t a i n e dw i t hi m p r o v e dp r o p e r t i e si n terms o fb o t hc o s t andperf ormance.
57
REFERENCES
1.
Holloway, H. e t a l . : PbTe Diodes.Appl.Phys.
2.
L o , W.: HomojunctionLead-Tin-TellurideDiodeLaserswithIncreasedFrequencyTuningRange. I E E E J o u r n a lo f Quantum E l e c t r o n i c s , QE-13, 1977.
3.
I n j e c t i o nL u m i n e s c e n c e and L a s e rA c t i o ni nE p i t a x i a l L e t t . , vol.21,no.5,1972.
p . 591,
Walpole, J . N . e t a l . : MBE Homostructure PbTe DiodeLaserswith cw Operat i o n up t o 100K. DeviceResearchConference,Ithaca, NY, June1977.
4. Kasemset, D. and F o n s t a d , C.: M i n o r i t yC a r r i e L r ifetimes and L a s i n g T h r e s h o l d s of PbSnTe H e t e r o s t r u c t u r eL a s e r s . I E E E T r a n s a c t i o n s on Elect r o nD e v i c e s , ED-26, p. 1841,1979.
58
BaF2
-
1. F I L M GROWN BYMULTIPLE SOURCEON MBE BaF2 SUBSTRATE 2.
v////////////~-(PbSn)Te
SOLDERED TO COPPERHEAT SINK
cu 3.
4.
5.
BaF D l SSOLVED WITHWARM WATER, FAB~Y-PEROTCAVITIES ETCHED BaF2 INSULATORDEPOSITED, CONTACT WINDOWS OPENED
cu
c\\\\\\\\\\+ V//////////AI
/aF2
cu
TOP METALIZATIONDEPOSITED, REMOVED FROM EMITTING FACE, CONTACT WIRE BONDED AWAY FROM ACT IVE LASER
Figure 1.- Processing steps in laser fabrication, from crystal growth to final wire-bonding.
l V l C l ML LCnUUU I
COPPER HEAT S I N K Figure 2.-
Detailed illustration of etched mesa diode laser. 59
Figure 3.- Scanning electron micrograph of an array of lasers prior to wire-bonding.
HEAT SINK TEMP INTENSITY (arbitrary units)
=
20K
I =0.95A
I
11.55 11.60 11.65 11.70 11.75 866
WAVELENGTH 862 858 855
=
0.90A
Ipm)
851
(Cm-')
Figure 4.- Output spectrum illustratingcomplex mode structure and rapid wavelength tuningwith current. 60
l3 AVERAGE EM1SSlON WAVELENGTH
12
E1
I = 0.25
\e
0.50 0.75
1.0 1.25
e 0
e
1.35 I
I
I
*e
0
I
I
I
I
I
I
1
20 40 30 HEAT SINK TEMPERATURE (K)
10
J
50
Figure 5.- Range of accessible wavelengths for diode operating cw. Wavelength to 10 Urn. Solid lines are tuned with temperature and current from 14 theoretical tuning curves derived from temperature dependence of the band gap and known electrical and thermal properties of the diode.
SnSe
PbSE
H
S rF2
BaF2
SnT KC I
KBr
I
2I 5.6 CU B IC L A l l IC E CON STANT(angstroms ) Figure 6.- Range of cubic lattice constants €or potential substrate €or epitaxial growth of lead salt laser material.
materials
61
WLIABILITY IMPROVEMENTS
-
I N TUNABLE PblxSnxSe
DIODE LASERS
OPERATING I N THE 8.5-30 MICROMETER SPECTRAL mGION*
K u r t J. Linden,Jack
F. B u t l e r , Kenneth W.
N i l 1 and Robert E.
Reeder
LaserAnalytics,Inc. SUMMARY
This paper describes recent developments i n the technology of Pb-salt d i o d e l a s e r s which have l e d t o s i g n i f i c a n t improvements i n r e l i a b i l i t y and l i f e t i m e , and t o improved operationatverylongwavelengths. A combination ofpackaging and contacting-metallurgy improvements h a s l e d t o d i o d e l a s e r s that are stable both i n terms of temperature cycling and shelf-storage time. Laserscycledover 500 times between 7 7 K and 3 0 0 K have exhibited no measurable changes i n e i t h e r e l e c t r i c a l c o n t a c t r e s i s t a n c e o r t h r e s h o l d c u r r e n t . U t i l i z i n g a newly developed metallurgical contacting process, both lasers and experimentaln-type and p-typebulkmaterials have been shown t o have e l e c t r i c a l contactresistancevaluesthatarestableforshelfstorageperiodswell in excess of one year. T h i s paper summarizes some of thepreviouslyencountered problems and reviewsseveralexperiments which have l e d t o d e v i c e s w i t h improvedperformance stability. Such stabledeviceconfigurations have been achieved for material compositions yielding lasers which operate continuously a t wavelengths as long as 30.3 m.
INTRODUCTION A number of d i f f e r e n t f a i l u r e mechanisms have been observed i n Pb-salt d i o d el a s e r s . Some of t h e s e , such a s f a i l u r e s due t o power s u p p l yt r a n s i e n t s , accidental back-biasing or excessive forward-bias currents are operator or equipment r e l a t e d and can be avoided by t h e u s e r i f m a n u f a c t u r e r ' s i n s t r u c t i o n s a r e followed.Other f a i l u r e s observedappear t o have been r e l a t e d t o e i t h e rt e m p e r a t u r ec y c l i n g( r e f . 1) o rs h e l fs t o r a g e of t h e l a s e r s ( r e f . 2). I t i s such f a i l u r e s and t h e s u c c e s s f u l e f f o r t s t o overcome them t h a t a r e d e s cribed i n t h i s paper. I n c o n t r a s t t o observations made on some of t h e 1 1 1 - V compound diode l a s e r s , l a s e r d e g r a d a t i o n underoperatingconditionshasnot been a problem w i t h t h e P b - s a l t l a s e r s .
S t a b i l i t y i n t h e o p e r a t i n g c h a r a c t e r i s t i c s ofdiode lasers requires s t a b i l i t y i n a l l p a r a m e t e r sa f f e c t i n gt h e s ec h a r a c t e r i s t i c s . Such parameters i n c l u d et h ed e v i c em e c h a n i c a lc h a r a c t e r i s t i c s( i . e . ,t e m p e r a t u r e - d e p e n d e n t
*Thework reported here has, i n p a r t , been supported by t h e NASA Langley ResearchCenter and t h e Los Alamos S c i e n t i f i cL a b o r a t o r y . Some of t h e r e s u l t s on thermal resistance s t z b i l i t y were obtained on a program supported by t h e U . S . A r m y Harry Diamond Laboratories.
63
s t r e s s e f f e c t s which a r e u s u a l l y package r e l a t e d ) , e l e c t r i c a l c h a r a c t e r i s t i c s (i.e., the stability of t h e e l e c t r i c a l c o n t a c t r e s i s t a n c e ) and thermalcharacteristics(i.e.,thestability of thedevicethermalresistance).Thispaper d e s c r i b e st h es u c c e s s f u ls t a b i l i z a t i o n of t h e f i r s t two parameters. The problem ofthermal resistance stability has been s t u d i e d a s p a r t of a s p e c i a l r e s e a r c h e f f o r t under Army c o n t r a c t DAAK21-79-C-0041, -&%rtionsof which a r e reported here. Following a d e s c r i p t i o n of present laser packaging techniques, the therSn Se d i o d el a s e r s i s described i n mal c y c l i n g s t a b i l i t y of some t y p i c a l Pb 1-X t h es e c t i o ne n t i t l e d "Thermal Cycling S t a b l l i & " . The s e c t i o ne n t i t l e d "Previously Observed Contact Resistance Degradation" discusses some t y p i c a l contact resistance degradation problems observed i n bothp-type and n-type i n p-n j u n c t i o nl a s e r s . The s e c t i o n e n t i t l e d Pbl-xSnSe materialsaswellas X "Diagnostic Measurements and S t a b i l i t y R e s u l t s on Lasers Fabricated byNewly Developed Techniques" describesexperiments aimed a t i n v e s t i g a t i n g s o u r c e s of such degradation and p r e s e n t s r e s u l t s on t h e s t a b i l i t y of bulkn-type and ptypedevices and of l a s e r s f a b r i c a t e d by newly developedtechniques. Some special results obtained for long-wavelength l a s e r s of Pbl-xSnSe are preX "Long Wavelength Laser Performance and Stasented i n t h e s e c t i o n e n t i t l e d an e f f o r t aimed a t s e p a r a t i n g t h e e f f e c t s of e l e c t r i c a l b i l i t y " .F i n a l l y , and thermal resistance has led to some i n t e r e s t i n g o b s e r v a t i o n s , a s d e s c r i b e d i n the section entitled "Thermal R e s i s t a n c e S t a b i l i t y Measurements".
SYMBOLS
R
f
'th
forward b i a s r e s i s t a n c e
of d i o d e l a s e r s
l a s e rt h r e s h o l dc u r r e n t
(mA)
(ohms)
AP
r e l a t i v e quantum e f f i c i e n c y of l a s e r (no u n i t s )
P max
r e l a t i v e maximum output power of l a s e r (no u n i t s )
RA
resistance
x
f r e e spacewavelength
W
power (mW o r pW a s i n d i c a t e d )
CW
continuous wave
I
l a s e rc u r r e n t
n
r e f r a c t i v e index of l a s e r m a t e r i a l
L
length of o p t i c a l c a v i t y
m
order of l o n g i t u d i n a l mode (an i n t e g e r )
T
temperature (K)
64
-
active area product
of diode laser
(w)
(mA)
of the diode laser
(pm)
THERMAL
CYCLING
STABILITY
to which Pb-salt lasers are subjected are among The temperature extremes the most severe in the semiconductor industry. Ranging from liquid helium (4.2K) to room temperature(300K), such temperature extremes require careful choice of crystal mounting techniques, package materials and physical configurations. The choice of materials is determined by such factors as thermal conductivity, thermal expansion coefficient and electrical properties at cryogenic temperatures.
The presently used package for Pb-salt diode lasers is shown in the scanning electron microscope (SEM) photograph of figure1. This package provides easy access to the laser face and utilizes a pressure-free design in which t laser crystal is welded between an indium-plated stud anda flexible top contact strap.
In an experiment to investigate the temperature cycling stability of this Sn Se diode lasers were cycled between room temperature and package, 3 Pb 77K a total o&-500xtimes. Each cycle consisted of a 10 minute dip in liquid nitrogen followed bya 20 minute blower-driven warm-up, with the lasers mounted in an evacuated "dip-stick". The dip-stick was designed to facilitate electrical testing by dipping into a liquid helium tank. Both the electrical contact resistance and threshold current were periodically measured. The results are summarized in tableI. The variations in threshold current are commonly observed in Pb-salt diode lasers and are believed due to changes in surface leakage current. The contact resistance is essentially the same over the entire test period, since a variationof ? .0005 ohms is well within the expected experimental error. It is thus apparent that500 temperature cycles have had no significant effect on the laser contact resistance. Subsequent room temperature storage lifetime data on these lasers are presented in the section entitled "Diagnostic Measurements and Stability Results on Lasers Fabricated by Newly Developed Techniques".
PREVIOUSLY
OBSERVED
CONTACT
RESISTANCE
DEGRADATION
The electrical contact resistance to diode lasers is a major factor in laser heating and therefore current tuning rate during operation. A common source of shelf storage degradation of Pb-salt diode lasers has been a progressing increase in this parameter. This phenomenon was even found to occur on the bulk n-type and p-typePb Sn Se samples. The contact resistance to 1-x x n-type and p-type samples of Pbo 92 Sno 74Se, measured over a period of several months, is shown in figuFe ?#e sample areas were approximately 8 x lom4 cm2 and the storage conditions were at room temperature, under ambient conditions.
2.
65
The progressive contact resistance degradation illustrated here was what erratic in that not all lasers degraded this rapidly. Lasers can, in CW conditions with contact resistance values as high general, operate under of approximately 0.1 ohms. Thus lasers with initial resistance values0.005 ohms may continue operating for long periods before the resistance inc to the point where CW laser action ceases. During this time, however, the progressive resistance increases would cause continual changes in the la tuning rates and operating frequencies at given temperature-current combin tions. The diagnostic measurements carried out to investigate the sources of this contact resistance degradation and the results of these measurements discussed in the following section. DIAGNOSTIC
MEASUREMENTS
FABRICATED
BY
AND
STABILITY
RESULTS
ON
NEWLY
DEVELOPED
TECHNIQUES
LASERS
The existence of progressive increases in electrical contact resistanc of both bulk and p-n junction devices suggested metallurgical interacti either between the contact metal layers or between the semiconductor and metal layers as possible sources of this problem. These possibilities were ( A E S ) of the semiconductor investigated by use of Auger electron spectroscopy and metal interface surfaces. Several Pb Sn Se wafers were analyzed by re1- x moving the metallization layer by a tape-pufling technique and probing t semiconductor surfaces. The results are summarized in Table11. These results of In on the suggest that failing devices were characterized by the presence 150f i below the crystal surface p-type semiconductor surface extending at most (such a distance being the resolution limit of these AES measurements). S of the bond used in mounting the laser crystals into In is a critical part laser packages, the elimination of this material was not considered. To prevent the apparent migration of In through the Au and, in some cases, b metal layer, the useof new metallic barrier layers was investigated. The pr per use of such layers can be expected to prevent In migration to the c surface. A process utilizing multiple metallization layers deposited by el tron beam evaporation techniques was developed, a andnumberof experimental bulk n-type and p-type devices were fabricated and evaluated for shelf sto stability. These devices have exhibited excellent contact resistance stability to date. As of 2/20/80, the p-type samples have an accumulated shelf storage time of 17 months, while the n-type samples have been shelf-stored for almo 15 months. The retest data for these junctionless devices are summarized in figure 3 . As the device areas were approximately 8 x cm2, the RA products of the n-type samples are approximately 4 x ohm cm2 per side while those of the p-side are approximately 1.6 x ohm cm2 . The n-contact values are among the lowest ever reported to any semiconductor material. The stability of a number of CID-type (ref. 3 ) Pbl-xSn Se lasers fabricated by the improved metallization techniques was trackeda For period of 4 which tabulates the over a year. Some of these results are shown in figure
66
contact resistance, threshold current and relative quantum efficiency as a function of time for a group 4of Pb Sn Se lasers. A total time period of 15 months is covered by these data ( k g lztest points corresponding to measurements madeon 2/20/80). The relative quantum efficiency parameter was obtained by measuring the xelative rate at which the output power increased with laser currentj u s t above threshold. In absolute terms, the external quantum efficiencies corresponding to these values are of the order of 0.5% per lase end. The comparative data are presented here to show that the output power from these lasers was relatively stable over15 the month shelf storage time period. All data points presented in figure 4 were obtained at liquid helium temperature. The emission spectra for one of these lasers (8292-2) as measured under identical test conditions at time periods approximately l year apart are shown in figure 5. While there appears to be a spectral shift towards higher energy by about 6 cm'l, the relative longitudinal mode distribution appears quite stable, and the total output power somewhat higher. Some of this discrepancy may be due to a change of power meters during this ti interval.
A second group of lasers stored and retested over a 15 month period are the 3 devices which comprised the group described in the section entitled "Thermal Cycling Stability". After being temperature recycled 500 times the devices were periodically retested €or contact resistance, threshold current, relative quantum efficiency (AP) and relative maximum output power (P) . As m in the case of relative quantum efficiency, the relative maximum output power corresponded to the current through a Ge:Cu photodetector mounted adjacent to the laser and is shown €or comparative purposes only. The actual maximum output power for these lasers was of the order of a mW per end. The value of these test parameters at various retest intervals are shown in table 111. All parameters are seen to be relatively stable andnoshow tendency for degradation over this time period. Unit 8324-10 apparently an hadinitially high threshold current (which could have been an instrument error) but thereafter appears to have settled down. The period in time at which these devices were temperature cycled is indicated in the table. A third group of lasers consisted 2of units which were fabricated from Pb 923Sn Se (corresponding to 16 urn emission at 15K). These units were 2 pec-lo ic&!?ly retested over 11 an month shelf storage period and then delivered under NASA-Langley contract NAS1-15190. The results of these measurements, shown in table IV, indicate excellent stability. The metallization process used in fabricating the devices described here was shown to be capable of withstanding air-ambient bake temperatures 65OC of for periods of 63 hours. In an experiment involving 9 lasers from several different Pb Sn Se wafers,8 of them (89%) survived this bake. Of these 1-x survivors, 6 (or 45%) remained stable thereafter. While it is not standard practice to subject Pb-salt lasers to such high temperatures, the experiment demonstrated the relative thermal stability of the presently used metallization process.
67
LONG
WAVELENGTHLASER PERFORMANCE AND STABILITY
Long wavelength lasers (?t>25pm) have been fabricated from Pb Sn Se material with compositions on both side of the zero-gap crossover p&i$t ?ref. 4). The temperature dependence of the bandgap (hence, emission frequency) on posite sides of the crossover at x 0.14 differ markedly. For x < 0.14 the band gap (emission frequency) increases with increasing temperature while f x > 0.14 the band gap decreases with increasing temperature. We report t operational results obtained from somePb 90Sn Se lasers in the 29-30 pm 10 spectral region. This composition region has tfl6 advantage of yielding the most power at the long wavelength region of operation, since this at occ a Pb Selaser thelowesttemperatures.Theemissionspectrumof operating CW at 10K and 2000mA is shownin figure 6. a to a1 measured output power of 141 pW, the mode which occurs at 338.5 cm-l (29.5 pml has a CW power of approximately 30 pw. When operated just above threshold, the laser emits at 3 3 0 cm-l (30.3 pm) in a single mode. This spectrum is shown in figure7.
-
&?Esn0 '?
The stability of four such lasers was monitored 10 over 1/2 amonth time period (as of 2/22/80). A plot of contact resistance, threshold current an 8. All operating parameters relative quantum efficiency is shown in figure are reasonably stable over this time period, although two of the lasers As in the measurements described exhibit some increase in contact resistance. previously, some of the differences in the relative quantum efficiency to the fact that these measurements were made in different sample test tures with different detector sensitivity values.
THERMAL
RESISTANCE
STABILITY
MEASUREMENTS
of Pb-salt In an effortto determine the thermal resistance stability diode lasers, several units known to have stable electrical contact resis values were checked for single mode tuning rate. This parameter was perio ically remeasured. The results indicated that while some devices are stable, some can also showslight a tendency towards increasing thermal resistance with time.
The
single
tuning mode
)+(a
rate-is a1
givenby
where m is the order of the longitudinal laser mode, typically 550 for a 1/2 mm cavity length Ldh at 10 pm. The refractive index, n, is temperat r %dependent and the term is larger than the thermal expansion - term L aT (which is known ?.oage approximately 20 x l'K for PbSe). The
-
68
Y
aT term
is proportional to the thermal resistance of the laser. Thus, the $.Ingle mode tuning rate provides a qualitative measure o f the thermal resistance. Preliminary measurements indicate that small thermal resistance increases can occur on devices that are electrically stable. This technique provides a useful tool for analyzing the thermal stability of various laser mounting techniques.
An experiment was performed to measure the temperature differential across a laser crystal under operating conditions. By use of a precalibrated miniature Si diode sensor which was mounted directly to the top contact strap of a laser, it was possible to measure the temperature at that point, while the temperature on the other side (in contact with the large heat of thesink mounting stud) was measured with a second calibrated sensor. It was found that temperature differentials 2-5K of were typical at the full laser operating current values of 2 amps. TO the best of our knowledge, this represents the first reported measurement of this parameter.
CONCLUDING REMARKS
Package design modifications and contact technology developments have led to significant improvements in the reliability and stability of Pb Sn Se diode lasers operating in the 8-30 Um spectral region. Representak5e fasers subjected to over500 temperature cycles between 77K and 300K have exhibited no significant changes in their operating parameters. Lasers stored under ambient temperature conditions have exhibited excellent stability in electrical contact resistance, threshold current and relative output power over test periods of 15 months. These improvements have been successful even for long wavelength material compositions with the demonstration of laser performance stability €or periods in excess 10 of months to date. Such lasers have emitted up to.30 V W of CW power at 29.5 pm. Recently fabricated lasers have been shown capable of withstanding 65OC bake temperatures with high survival and good subsequent stability yields. Indirect measurements of thermal resistance have indicated that it is, in some cases, possible to measure small increases in thermal resistance on devices whose electrical contact resistance is stable. Such findings indicate the diverse nature of the problems associated with diode laser failure mechanisms.
69
111111I
I
I I
II
11111111I I
I I 111 I
FCEFERENCES
1.
M. Yoshikawa, K. Shinoharaand
2.
H.
Preier, Appl.
3.
K.
J. Linden, K. 7 2 0( 1 9 7 7 ) .
4.
T. C. Harman, A. R. C a l a w a , I. M e l n g a i l i sa n d Lett. 14, 3 3 3 ( 1 9 6 9 ) .
70
Phys. W.
20,
R. Ueda, A p p l .
Phys. L e t t . 31,699(1977).
189(1979).
N i l 1 and J. F. B u t l e r , IEEE Jour.Quant.
E l e c . , QE-13,
J. 0 . D i m o c k , Appl. Phys.
TABLE 1.-STABILITY DATA ON LASERS CYCLED 500 TIMES BETWEEN 300K AND 77K.
Original Data
After 135 temp cycles Rf
8324-11 8324-12 8324-10 * (control)
-005
JICl~
(ohms)
1
th
(mA)
Rf
(ohms)
.005
1410
1
After 500 temp cycles
.005 I
> 2000 1270
.005
I
.005
13*0
*Laser 8324-10 was a control sample from the same lot. temperature cycled.
This laser was not
TABLE 11.-SUMMARY OF AES FINDINGS ON Pbl-xSnxSe
Wafer No. D700 p- s i d e
AES F i n d i n g s
Description
Surf ace : Trace of Au ( - .1%). S t r o n g I n More I ni n s i g n a l( - f e w %1. s t r i p e t h a n on i n s u l a t o r .
P - d i f f u s e d ,s t r i p e dw a f e ro f Pb.93Sn.07Se.
resisLasersexhibitedcontact tanceincreasesafter a few months.
~~
LASER WAFERS.
Below S u r f a c e : 50 o f material removed gave same r e s u l t s as above. 150 H of material removed,and t h eI n s i g n a l was gone.
a
~
Dl002 p-side
Surface: Au approximately 2 % I n a p p r o x i m a t e l y 2%
N-diffused,non-stripedwafer
Lasersdegradedin
a few days. Surface: Approximately5%In
n-side
D44 8 p-side
N - d i f f u s e d ,s t r i p e dw a f e ro f
Lasers were s t a b l e f o r o v e r one y e a r .
Surface: Approximately5% Infound.
Au, no trace of
TABLE 111.-COMPARATIVE STABILITY DATA FOR THREE Pb SnSe FOR OVER ONE YEAA RT ROOM TF,MPERA&b.X LASERSSTORED
Test Date
R€ (ohms )
11-20-78 12-07-78 12-18-78 1-08- 79 2-08-79 2-24-79 5-25-79 11-06-79 2-20-80
.007 .006 .006 .006 .005 .006 .006 .006 .006
>2000 1380 1350 1 2 80 1330 1350 1500 1330 1400
11-20-78 11-27-78 11-30-78 12-07-78 12-18-78 1-08-79 2-08-79 2-24-79 5-25-79 7-26-79 11-06-79 2- 20- 80
.005 .005 .006 -005 .006 .006 .005
11-20-78 11-28-78 11-30- 7 8 12-07-78 12-18-78 1-08-79 2-08-79 2-24-79 5-25-79 7-27-79 11-06-79 2-20-80 stored
.006 .005 .006 .006 .005 .006
.006
.005 .006 .006 .006
-006
.006 .006
.006 .006
AP "
P
max "
43 71 58 59 69 54 49 40
78 109 93 102 104 90 97 90
1490 1 410 1360 1320 1300 1260 1250 1220 1160 1250 1210 1180
50 49 52 53 50 59 52 52 50 48 46 52
84 88 90 95 90 10 3 95 96 98 98 96 100
1350 1270 1200 1200 1250 1220 1230 1260 1160 1360 1400
48 43 51 52 51 50 48 42 43 35 23
82 85 96 95 91 96 90 86 94 76 65
" - 1310 n a l r a t 'oom temp r a t u r e
70
Laser H i s t o r y " C o n t r o l sample No special cycling.
-
C y c l e d1 3 5 times Cycled365 times 77- 300K
C y c l e d 1 3 5 times Cycled365 times 77- 300K
betweer tests.
TABLE 1V.-STABILITYTEST
L a s e r No.
9093-21
9110-3
DATA FOR LASERS 9093-21 AND 9110-3
Test
Electrical Contact Resistance
Date
(ohms)
Threshold Current (W)
4-04-79
0.007
+330
4-09-79
0.008
+310
4-23-79
0.008
+320
7- 13- 79
0 -007
+315
1-11-80
0.004
+300
2-12-80
0.005
+305
4-20-79
0.007
-430
7-30-79
0 -007
- 395
8- 31- 79
0.006
-400
2-12-80
0.005
-4 30
I
Figure 1.- SEM photograph of a Pb-saltdiodelaserpackage.Thisdesign minimizes c r y s t a l s t r e s s by e l i m i n a t i n g p r e s s u r e .
'l
T
1
.
I
N-Type Bulk Samples Pb.9Z6 Sn.074Se
1
.01
DEVICE SHELF STORAGE TIME (MONTHS)
Figure 2.- C o n t a c t r e s i s t a n c e t o n - t y p e and p-type Pb Se m a t e r i a l 924Sn with etched surfaces and standard contacts showing.sever&'A$gradation.
75
I
I
2
0
4
6
10
8
14
12
16
18
E W E SHELF STORAGE TIME (MONTHS)
F i g u r e 3.- C o n t a c t r e s i s t a n c e t o n - t y p e and p-type P b 924Sn. 076Se w i t h improved c o n t a c t s .
.
1
I
,
,
-
,
I
,
,
,
I
,
,
,
I
,
8292-1 * 48292-2 """4 1792-10
I
2
4
8303-22
_"
==0
-
- +-
6
" " " 0
" "
I
8
I
I
10
I
I
12
I
14
16
AGE OF L A S B (MowTHS)
F i g u r e 4.- P l o t s of r e l a t i v e quantum e f f i c i e n c y , t h r e s h o l d c u r r e n t and l a s e r c o n t a c t r e s i s t a n c e as f u n c t i o n s of s h e l f s t o r a g e t i m e c o v e r i n g a p e r i o d o f overoneyear.Lasers 8292-1 and 2 were f a b r i c a t e d from Pb Sn and l a s e r s 8292-10 and 8303-22 were f a b r i c a t e d from Pb 966 S; c o r r e .92zs'. f 7 3 sponding t o t h e 1 1 . 5 and 17.2 pm s p e c t r a l r e g i o n s , r e s p e c l v e y.
0345e
76
-
I
T(K)
1
= 11
YmA)
= 2000 10120178
P(mW)=1.91
MmA) = 521
= 857
V&m-1)
T(K)
@A)
"".
,
= 11 = 2000
P(mW)
= 2.6
l,,,(mA)
= 711
v,,,(cm-')
I
~11113179
= 858 870 cm'' 880 cm"
F i g u r e 5.- Comparisonofemission s p e c t r a from l a s e r 8292-2 t a k e no n October 2 0 , 1978andagain onNovember 1 3 , 1 9 7 9 .D i f f e r e n c e s i n power r e a d i n g c o u l d ,i n p a r t , b e d u e t o d i f f e r e n t power m e t e r s . S h i f t of 6 cm-I t o w a r d s h i g h e r wavenumber is possiblydue t o slight thermal r e s i s t a n c ei n c r e a s e .
I = -2000mA
F i g u r e 6.-Emissionspectrum a t 10 K. The 338.5
of
l a s e r 9096-10 o p e r a t i n g CW a t a 2 Amp c u r r e n t h a sa p p r o x i m a t e l y 30 pW of power.
c m ' mode
71
"
r
B 0
n
n
I = 526mA
I
F i g u r e 7.- Emissionspectrum of l a s e r 9096-10 o p e r a t i n g s l i g h t l y a b o v e t h r e s h o l d a t 1 0 K s h o w i n g s i n g l e mode e m i s s i o n a t 330 c m - l (30.30 urn)
-
,
I
800
I
"""---""
400
0 20
I
I
I
I
I
I
f
t
4
"""_
~O~"""&""""~ "
" " " "
""".%=am""-*
I
I
I
1
I
I
2
4
6
8
10
12
A6E OF LASER (MONTHS)
F i g u r e 8.- S t a b i l i t y d a t a f o r l o n gw a v e l e n g t h lasers s t o r e d u n d e r a m b i e n t c o n d i t i o n s € o r 78
( 2 8 pm r e g i o n ) Pb0,99Sn0.10Se a 10% month p e r l o .
LONG WAFLENGTH
PblxSnxSe -
AND Pbl-xSnxTe
DIODE LASERS
*
AS LOCAL OSCILLATORS E. R. Washwell LockheedPaloAltoResearchLaboratory
SUMMARY A b r e a d b o a r dh e t e r o d y n e receiver i s d e s c r i b e d w h i c h h a s b e e n u s e d t o e s t a b l i s h t h ec h a r a c t e r i s t i c so fl e a d s a l t d i o d e lasers p e r t i n e n t t o t h e i r u s e as l o c a l o s c i l l a t o r s u s i n g laser d e v i c e s o p e r a t i n g i n t h e 15-25um s p e c t r a l r e g i o n . Heterodynedetectionefficiencyhasbeendirectlycorrelatedwiththetransv e r s e mode s t r u c t u r e a n d e m p h a s i z e s t h e n e c e s s i t y o f s t a b l e l o w e s t o r d e r mode o p e r a t i o nf o rl a s e r s when used as l o c a l o s c i l l a t o r s . The r e s u l t so b t a i n e d i n d i c a t et h a tt h ec o n t i n u e dd e v e l o p m e n to ft h e s e lasers w i l l p r o v i d e s u i t a b l e l o c a l o s c i l l a t o r s f o r a v a r i e t y of a p p l i c a t i o n s .
INTRODUCTION
The a b i l i t yt oo p e r a t e Pb SnxSe and Pb Sn T e d i o d e lasers a t a n yg i v e nf r e 1l-x. x quencyover a w i d ew a v e l e n g r hr a n g e( a p p r o x l m a t e l y7 - 3 0 ~ m )w i t h a n a r r o wl i n e widthhasresultedintheirapplicationtohighresolutioninfraredabsorption s p e c t r o s c o p y .T h e s e same f e a t u r e s p r o v i d e t h e b a s i s f o r t h e i r u s e as l o c a l o s c i l l a t o r si nh e t e r o d y n er e c e i v e r s . However, a d d i t i o n a lc h a r a c t e r i s t i c s are r e q u i r e d of lasers u s e d as l o c a l o s c i l l a t o r s w h i c h are r e l a t i v e l yu n i m p o r t a n t i nt h eu s u a li n c o h e r e n t( d i r e c t )d e t e c t i o n mode o fo p e r a t i o ne m p l o y e di na b s o r p t i o ns p e c t r o s c o p y .I np a r t i c u l a r ,t h et r a n s v e r s e mode s t r u c t u r ea n d mode stability play a keyroleintheabilityofthe l a s e r t op r o v i d eq u a n t u mn o i s e l i m i t e dp e r f o r m a n c e .D i o d e l a s e r s w h i c ho p e r a t ei nt h el o w e s to r d e rT E 0 , o mode provideexcellentheterodynemixingefficiencywhilethoseoperatinginnigher o r d e r modes a r e t y p i c a l l yi n e f f i c i e n ta n df r e q u e n t l yu n s t a b l e .T h e s ec h a r a c t e r i s t i c s are o f p r i m e i m p o r t a n c e f o r h e t e r o d y n e d e t e c t i o n b u t h a v e l i t t l e influenceondirectdetection.
l a s e r p o w e ra n dm i x e rc h a r a c t e r i s t i c sr e q u i r e d I nt h ef o l l o w i n gs e c t i o n s ,t h e f o r optimum h e t e r o d y n e receiver s e n s i t i v i t y a r e summarized.Thebreadboard h e t e r o d y n e receiver d e v e l o p e d f o r t h e o p e r a t i o n a n d c h a r a c t e r i z a t i o n o f l e a d s a l t d i o d e lasers as l o c a l o s c i l l a t o r s i s d e s c r i b e d i n d e t a i l a n d r e s u l t s t a i n e do nc o m m e r c i a l l ya v a i l a b l ed i o d e lasers p r e s e n t e d .
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T h i sw o r k w a s s u p p o r t e d b y t h e B a l l i s t i c Missile DefenseAdvancedTechnology C e n t e ru n d e rC o n t r a c t DASG 60-76-C-0061
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HETERODYNE SIGNAL-TO-NOISE
RATIO
The c u r r e n t s i g n a l - t o - n o i s e r a t i o o f a n o p t i c a l h e t e r o d y n e d i n gp o l a r i z a t i o nl o s s i s g i v e nb y( R e f . 1)
receiver i n c l u -
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The numerator i s t h e rms c u r r e n t r e s u l t i n g f r o m t h e m i x i n g o f t h e s i g n a l p o w e r , w i t ht h el o c a lo s c i l l a t o rp o w e r , PLOY f o l l o w e db ys e c o n dd e t e c t i o n . B 1 is t h eb a n d w i d t hf o l l o w i n gt h ef i r s td e t e c t o r ( I F ) and B2 t h a t f o l l o w i n g t h e second d e t e c t o r ; t h e i r r a t i o r e p r e s e n t s t h e s i g n a l - t o - n o i s e r a t i o i m p r o v e m e n t p o s s i b l e r.hrough t h eu s eo fs e c o n dd e t e c t i o nw i t hi n t e g r a t i o n . The denominator c o n t a i n st h es h o tn o i s eo ft h el o c a lo s c i l l a t o ra n d a t h e r m a l n o i s e term, c1 e q u a l s 2 f o r a p h o t o d i o d ea n d 4 f o r a p h o t o c o n d u c t o r , G i s u n i t y f o r a photodiodeand i s t h ep h o t o c o n d u c t i v eg a i nf o r a p h o t o c o n d u c t o r , n i s t h em i x e r quantum e f f i c i e n c y , TN and RA t h e I F a m p l i f i e r n o i s e t e m p e r a t u r e a n d i n p u t i m p e d a n c e ,r e s p e c t i v e l y , e t h ee l e c t r o n i cc h a r g e , k B o l t z m a n ' sc o n s t a n t , h P l a n c k ' sc o n s t a n t ,a n d v t h ef r e q u e n c yo ft h er a d i a t i o n .T h e term n b i s t h e number ofbackgroundphotonswhich a r e i n a s i n g l e s p a t i a l mode a n d w i t h i n t h e I F bandwidth B 1 ( a n d w h i c h t h e r e f o r e may i n t e r a c t c o h e r e n t l y w i t h t h e l o c a l o s c i l l a t o r ) , and i s e q u a lt o( e x p ( h v / k T ) - 1)-1 where T i s t h et e m p e r a t u r eo f t h eb a c k g r o u n db l a c k b o d yr a d i a t i o n .E q u a t i o n( 1 )c o n s i d e r sl o c a lo s c i l l a t o r s h o tn o i s e( o n l y )b u td o e sn o ti n c l u d ea n ye x c e s sn o i s e ,a n da s s u m e sp e r f e c t w a v e - f r o n tm a t c h i n go ft h es i g n a la n dl o c a lo s c i l l a t o r beams a t themixed.The signal-to-noisedegradationresultingfrom a w a v e - f r o n tm i s m a t c hh a sb e e n treate d e l s e w h e r e( R e f e r e n c e2 )a n d i s n o tc o n s i d e r e df u r t h e rh e r e . I t c a nb es e e n t h a ti no r d e rt oa c h i e v eq u a n t u mn o i s e - l i m i t e do p e r a t i o nw i t h a heterodyne r e c e i v e r , u s i n ga ni d e a l i z e dl o c a lo s c i l l a t o r ,t h es h o tn o i s eg e n e r a t e di nt h e m i x e rb yt h el o c a lo s c i l l a t o rm u s te x c e e dt h et h e r m a ln o i s ei nt h ef o l l o w i n g a m p l i f i e r . The l o c a l o s c i l l a t o r power P ' L O r e q u i r e d t o make t h e s en o i s e cont r i b u t i o n se q u a l i s
Ps,
Thepower r e q u i r e m e n tf o rt h e l a s e r i s a f u n c t i o no ft h em i x e re l e m e n t c h a r a c t e r i s t i c sa n dt h ea p p l i c a t i o n ,i np a r t i c u l a r ,t h e I F b a n d w i d t hr e q u i r e ment.Diode l a s e r s d e v e l o p e dt od a t eh a v eb e e nr e l a t i v e l y lowpower devices ( < < 1 mW p e r m o d e ) , n e c e s s i t a t i n g o p t i m i z a t i o n i n m i x e r a n d a m p l i f i e r s e l e c t i o n f o r a g i v e na p p l i c a t i o n .F o r a v a r i a b l el o c a lo s c i l l a t o r power PLO, t h e receiver w i l l a p p r o a c hq u a n t u mn o i s e l i m i t e d o p e r a t i o n as ( 1 + P ' L o / P ~ The a c t u a l s i g n a l - t o - n o i s er a t i oa c h i e v e dd e p e n d sl a r g e l y on s u c h i n h e r e n t Paser c h a r a c t e r i s t i c s as e x c e s sn o i s e (mode i n s t a b i l i t y ) a n d t r a n s v e r s e mode s t r u c t u r e( m i x i n g e f f i c i e n c y )a n do p e r a t i o n a lf a c t o r ss u c h as t e m p e r a t u r e s t a b i l i t y a n d v i b r a t i o n
).
80
( s h o c k )i s o l a t i o n ,b o t ho fw h i c ha f f e c tt h ef r e q u e n c ys t a b i l i t y .T h e low d i o d e laser p o w e r s a v a i l a b l e i n a s i n g l e l o n g i t u d i n a l mode d u r i n g t h e c o u r s e ofthisinvestigationdictatedtheuseofphotoconductive mixer e l e m e n t sa n d r e l a t i v e l y low I F b a n d w i d t h s .P h o t o c o n d u c t i v e HgCdTe mixers were usedbetween 1 0 pm a n d1 7 pm and Ge:Cu f o r t h e l o n g e r w a v e l e n g t h s .
D I O D E LASER HETERODYNE RECEIVER BREADBOARD
The l a s e r c h a r a c t e r i z a t i o n a n d h e t e r o d y n e m e a s u r e m e n t s were o b t a i n e d u s i n g t h ea p p a r a t u s shown i n F i g u r e 1. T h i ss y s t e m w a s u s e df o rt h ee v a l u a t i o no f d i o d e l a s e r s produced a t Laser A n a l y t i c s , I n c . ( B e d f o r d , MA) and New England ResearchCenter(Sudbury, MA) o p e r a t i n gi nt h e 10-25 Um s p e c t r a lr e g i o n . The a p p a r a t u s w a s alsousedforblackbodyandmolecular l i n e radiationheterodyne m e a s u r e m e n t s ,a n dh e t e r o d y n ed e t e c t i o no fs e m i c o n d u c t o r l a s e r e m i s s i o n a t 2 3 pm and 10.6 pm. The o p t i c s a r e a l l r e f l e c t i v e ( e x c e p t f o r KRS-5 l e n s e s i n t h e f r e q u e n c y c o n t r o ll o o pa n di nt h en e a r - f i e l dm e a s u r e m e n to p t i c a ll e g s )w h i c hp e r m i t st h e receivertobeusedover a w i d ew a v e l e n g t hr a n g ew i t h o u tr e f o c u s i n g ,i n c l u d i n g t h ev i s i b l ef o ra l i g n m e n t . The e n t i r e o p t i c a l p a t h c a n b e e n c l o s e d w i t h a p l e x i g l a s sc o v e r( n o ts h o w n )a n dt h es y s t e mp u r g e dw i t hd r yn i t r o g e nt o e l i m i n a t ea t m o s p h e r i ca b s o r p t i o n .T h e l a s e r s e x a m i n e dd i dn o te x h i b i th i g h beam d i v e r g e n c ei n a g i v e n s i n g l e mode. T h i s i s as e x p e c t e db e c a u s eo ft h e l o n gd i f f u s i o nl e n g t h sa n dh i g hr e f r a c t i v ei n d e xi nt h e Pb s a l t s . T h u s ,t h e f / 2c o l l e c t i o no p t i c s were m o r et h a na d e q u a t e .T h eo p t i c a lt r a n s f e rs y s t e m shown i n F i g u r e 1 m a t c h e st h ef / 8m o n o c h r o m a t o ro p t i c s . Themonochromator is u s e df o rs p e c t r a la n a l y s i sa n dl o n g i t u d i n a l mode i s o l a t i o n . The g r a t i n g i n t h e m o n o c h r o m a t o rc a nb ee a s i l yb y p a s s e db y a mirrorwhich i s usedmainlyfor i n i t i a l l a s e r a l i g n m e n t .T h er a d i a t i o nf r o mt h em o n o c h r o m a t o r is collimated b y a & i n .f o c a ll e n g t h ,o f f - a x i s ,p a r a b o l i cm i r r o rt or e d u c et h es i z er e q u i r e mentonthe beam s p l i t t e r . The beam s p l i t t e r f o r t h e 15-um s p e c t r a lr e g i o n c o n s i s t so f ZnSe 50% r e f l e c t i n g on o n e s u r f a c e ( a t 45O a n g l eo fi n c i d e n c e )a n d a n t i r e f l e c t i o nc o a t e do nt h eo t h e rw h i c h i s optimum f o r t h e weak s i g n a l case. F a r - f i e l dr a d i a t i o nm e a s u r e m e n t s a r e made i n t h i s c o l l i m a t e d beam u s i n g a small area HgCdTe o r p y r o e l e c t r i c d e t e c t o r m o u n t e d o n a n a u t o m a t e d x-y s t a g e w h i c hs a m p l e st h ec r o s ss e c t i o no ft h e beam i n t e n s i t yp r o f i l e .( F a r - f i e l d measurements were a l s o made a t a d i s t a n c e of s e v e r a l c e n t i m e t e r s f r o m t h e d i o d e l a s e r , u s i n gn oa d d i t i o n a lo p t i c s . ) The X 1 0 beam expander i s employed t o p r o v i d e a c o n v e n i e n tw o r k i n gd i s t a n c et ot h ef o c a lp o i n to ft h ef / 2f o c u s i n gm i r r o r . The laser r a d i a t i o nt r a n s m i t t e db yt h e beam s p l i t t e r i s p a s s e d t h r o u g h a 1 - i n .l o n gs o l i dg e r m a n i u me t a l o n (when u s e d )f o ra c c u r a t et u n i n g r a t e m e a s u r e m e n t sa n dt h er a d i a t i o n i s a l s op a s s e dt h r o u g h a g a sa b s o r p t i o n c e l l which i s u s e d f o r a c c u r a t e l a s e r f r e q u e n c ym e a s u r e m e n t s( r e f e r e n c e dt o a known a b s o r p t i o n l i n e ) a n d f o r h i g h - r e s o l u t i o n s p e c t r o s c o p i c m e a s u r e m e n t s ( i n c l u d i n gp r e s s u r eb r o a d e n i n gs t u d i e s ) .T h i ss i g n a lp a t ha l s o forms o n el e g o ft h ef r e q u e n c yc o n t r o ll o o pw h i c h w i l l b ed e s c r i b e db e l o w .T h es i g n a ll e g o f t h e receiver canemploy a b l a c k b o d y s o u r c e ( o p e r a b l e i n t h e D i c k e s w i t c h mode), a h e a t e dg a se m i s s i o n c e l l ( w h i c hc a nb eo p e r a t e di nt h eh e t e r o d y n ea b s o r p t i o n mode b y u s e of a b l a c k b o d y s o u r c e b e h i n d t h e c e l l ) , o r a second l a s e r a t t h e p o s i t i o n s shown i n F i g u r e 1. Theemission c e l l w a s d e s i g n e dt oh a v e a l o we m i s s i v i t y .N e a r - f i e l d( m i r r o rr a d i a t i o n )a n dp o l a r i z a t i o nm e a s u r e m e n t s 81
havealsobeen
made as shown.
T h em a j o re l e c t r o n i cc o m p o n e n t so ft h e receiver are t h e laser d r i v e r , mixer, I F a m p l i f i e r c h a i n , a n d t h e f r e q u e n c y s t a b i l i z a t i o n c o n t r o l l o o p . Thecomponentsand c i r c u i t sh a v eb e e nc u s t o md e s i g n e d when n e c e s s a r y . A l l o t h e rc o m p o n e n t si n v o l v ec o m m e r c i a li n s t r u m e n t a t i o n ( I F a m p l i f i e r ,s p e c t r u m a n a l y z e r ,l o c k - i nd e t e c t o r s ,e t c . ) . The laser d r i v e r w a s d e s i g n e df o r lown o i s e ,l o w - d r i f to u t p u t . A m e a s u r e dn o i s e l e v e l of20 UA r m s w a s a c h i e v e d . Two d r i v e r s were b u i l t : a m a n u a l l yo p e r a t e du n i tf o rg e n e r a lu t i l i t ya n d a r e m o t e l yc o n t r o l l e du n i tf o ru s ew i t ht h es t a b i l i z a t i o nc o n t r o ll o o p .T h e Amplica201 VSL I F a m p l i f i e r u s e d was t h o r o u g h l y c h a r a c t e r i z e d h a v i n g 5- t o 110-MHz b a n d w i d t h ,1 . 2 - d Bn o i s ef i g u r e ,a n d 50-dB g a i n .T h i sa m p l i f i e rw a s u s e dw i t ht h ew i d e rb a n d w i d t h Ge:Cu m i x e r . A P l e s s e y SL1205low n o i s e preampl i f i e r was u s e dw i t ht h e HgCdTe m i x e r .T h i sa m p l i f i e r w a s s p e c i f i c a l l yd e s i g n ed forusewith a HgCdTe d e t e c t o r h a v i n g a 50-dB g a i n( n o m i n a l ) , a 6.5"Hz bandwidthand a 0 . 8 n V / &e q u i v a l e n ti n p u tn o i s ev o l t a g e .I nt h ed i r e c td e t e c t i o n mode, t h e o u t p u t o f t h e d e t e c t o r ( m i x e r ) c i r c u i t i s c o n n e c t e dd i r e c t l y t o a l o c k - i na m p l i f i e ra n d i s u s e dt om o n i t o rt h e l a s e r mode p o w e r .I nt h e h e t e r o d y n e mode, t h e o u t p u t o f t h e I F a m p l i f i e r i s p a s s e dt h r o u g h a second d e t e c t o rw h i c h i s r e f e r e n c e dt ot h eD i c k es w i t c h .T h eo u t p u to ft h eI F amplifiercanbedirectlyconnectedto a spectrumanalyzerforsignalfrequency analysis. The m i x e r s u s e d i n t h e 1 5 Um s p e c t r a l r e g i o n were p h o t o c o n d u c t i v e HgCdTe 75 um and150 p m s q u a r ee l e m e n t sm o u n t e d on a common h e a ts i n k .T h ep e a k s p e c t r a lr e s p o n s i v i t yo c c u r s a t 1 4 . 3 um w i t hg o o dr e s p o n s et o1 6 . 5 um. The 40 n s e cr e s p o n s e time m e a s u r e db yt h em a n u f a c t u r e r (SBRC) u s i n g a n I d s emitter w a s v e r i f i e db yt h eg e n e r a t i o n - r e c o m b i n a t i o n( g - r )n o i s er o l l - o f ff r e q u e n c y d i s p l a y e do n a s p e c t r u ma n a l y z e ra n d shown i n F i g u r e 2. T h i sa l s od e m o n s t r a t e s thattheparticulardiode laser used w a s c a p a b l e o f g e n e r a t i n g a g - rn o i s e l e v e l t h a te x c e e d e dt h ea m p l i f i e rn o i s e .T h el i f e t i m eo ft h i sp a r t i c u l a r d e t e c t o r was t h e s h o r t e s t a v a i l a b l e i n h i g h q u a l i t y material. I t may b e p o s s i b l e t o r e d u c et h el i f e t i m eb yc o m p e n s a t i o n t o p r o v i d e a m i x e rw i t h a w i d e r bandwidthand flatresponsetohigherfrequencies,butthis i s advantageous o n l y when i n c r e a s e d l a s e r powersbecome a v a i l a b l e .T h eb r e a k d o w nf i e l d was measured t o b e 4OV/cm which i s s u f f i c i e n t l y h i g h t o e n s u r e a t t a i n m e n t of t h e s a t u r a t e dd r i f tv e l o c i t yc o n d i t i o n s a t o p e r a t i o n a l e l e c t r i c f i e l d s . T h ed e t e c t o r p r o p e r t i e s as a f u n c t i o n o f b i a s a n d b a c k g r o u n d f l u x as w e l l as r e s p o n s i v i t y were u s e dt oC a l c u l a t et h e quantum e f f i c i e n c y (q = 0 . 6 ) . The d e t e c t o r impedance i s a p p r o x i m a t e l y 30 R ( d e p e n d i n gu p o nb i a s )a n d is relatively w e l l m a t c h e dt ot h eI Fa m p l i f i e r . The m i x e re l e m e n tu s e df o rt h e 23-pm s p e c t r a l r e g i o n w a s Ge:Cu. The l i f e time a n d q u a n t u m e f f i c i e n c y were d e t e r m i n e df r o mm e a s u r e m e n t so ft h er e s p o n s i v i t y , D", a n d d e t e c t o r r e s i s t a n c e a t known b a c k g r o u n df l u xl e v e l sa n d as a f u n c t i o no fb i a sf i e l d .T h i sp a r t i c u l a re l e m e n th a da ni n t e r e l e c t r o d es p a c i n g of0.0152 c m , a q u a n t u me f f i c i e n c yo f0 . 1 5 , a l i f e t i m eo fa p p r o x i m a t e l y 5 nsec, and a s a t u r a t e d d r i f t v e l o c i t y o f 5 x lo6 cm/sec, r e s u l t i n g i n a photoconduct i v e g a i no f1 . 6 4 .T h ed e t e c t o ri m p e d a n c e a t t h e f l u x l e v e l s d u r i n gt h e exp e r i m e n t s w a s a p p r o x i m a t e l y 1 MR. T h i sh i g hi m p e d a n c e l e v e l r e s u l t e di np o o r c o u p l i n gt ot h e 50-R I F a m p l i f i e r u s e d ( A m p l i c a 201 VSL) a n dn o n l i n e a r resp o n s eo v e rt h ea m p l i f i e rb a n d w i d t h . The b e s t 22 Urn l a s e r d i d ,h o w e v e r ,i n d u c e a2
sufficientshotnoisetoexceedtheamplifiernoise, as observed on a spectrum a n a l y z e r , and p r o v i d e d a good b l a c k b o d y h e t e r o d y n e s i g n a l - t o - n o i s e r a t i o .
FREQUENCY STABILIZATION
AND CONTROL
A s i g n i f i c a n t a d v a n t a g e of a d i o d e laser LO h e t e r o d y n e r e c e i v e r o v e r f i x e d f r e q u e n c y laser systems is t h e c a p a b i l i t y t o b e o p e r a t e d a t a n yf r e q u e n c yi n t h e 3- t o 30-pm s p e c t r a l r e g i o n . Laser o p e r a t i o n a t a s p e c i f i cf r e q u e n c y i s p r i n c i p a l l y a f u n c t i o n of t h e laser c h e m i c a lc o m p o s i t i o n ,t e m p e r a t u r e ,a n d d r i v ec u r r e n t . A d i o d e laser c a nb ee a s i l yt u n e d( c o n t i n u o u s l yo v e r a 30-GHz r a n g e ,f o re x a m p l e ) b ys i m p l yc h a n g i n gt h el a s e rd r i v ec u r r e n t .T h i sv e r s a t i l i t y of d i o d e lasers h a s a p r i c e , e x p e c i a l l y when u s e d i n a h e t e r o d y n e a p p l i c a t i o nf o rt h ed e t e c t i o n of m o l e c u l a rl i n er a d i a t i o n .S i n c et h ed i o d e lasers a r e r e l a t i v e l y low-power d e v i c e s a n d e f f i c i e n t l o n g w a v e l e n g t h i n f r a r e d m i x e r s presentlyhavemodestbandwidths,the laser m u s t b e o p e r a t e d c l o s e i n f r e q u e n c y tothesource of i n t e r e s t ( w i t h i n a few hundred MHz, f o r example)andmustbe s t a b l e t o w i t h i n a few MHz. T h i sr e q u i r e m e n t makes f r e q u e n c y s t a b i l i z a t i o n and c o n t r o l a ni m p o r t a n ti s s u e . Once t h ec o m p o s i t i o na n dn o m i n a lo p e r a t i n g c o n d i t i o n sh a v eb e e ne s t a b l i s h e df o r a g i v e nd e v i c e ,t h ep r o b l e mr e d u c e st o o p e r a t i o n a t a s p e c i f i cf r e q u e n c yw i t h a g i v e nf r e q u e n c ys t a b i l i t y .S i n c et h e l a s e r t u n i n g r a t e s a r e t y p i c a l l y 1 0 0 MHz/mA and 100 MHz/mdeg, c l o s e c o n t r o l of t h e s e parameters i s e s s e n t i a l .I np r i n c i p a l ,t h ea b s o l u t e l a s e r frequencycan bedetermined by m i x i n g t h e d i o d e l a s e r f r e q u e n c yw i t ha n o t h e rf i x e d - f r e q u e n c y l a s e r orharmonics of microwavedevices i np o i n tc o n t a c td e v i c e s .T h i st e c h n i q u e i s c o n s i d e r e dt ob e complex, i n e f f i c i e n t , and u n n e c e s s a r y f o r many applications. The useof a s e c o n d a r yf r e q u e n c ys t a n d a r d i s c o n s i d e r e dt ob e a d e q u a t e .T h i ss t a n d a r dc a nb es i m p l y a v a p o rp h a s em o l e c u l a ra b s o r p t i o n which i s a t ( o rn e a r )t h el i n e of i n t e r e s t .T h i sa b s o r p t i o nn e e dn o te v e n occurinthe same m o l e c u l a r specie as t h e l i n e r a d i a t i o n of i n t e r e s t , b u t must b ea b l et ob ee s t a b l i s h e d( i s o l a t e d )t ot h er e q u i r e da c c u r a c y . Givensuchan a b s o r p t i o n ,t h ef r e q u e n c ys t a b i l i z a t i o nt e c h n i q u ei n v e s t i g a t e dd u r i n gt h i s p r o g r a mi n v o l v e do p e r a t i o no ft h ed e s i r e dl a s e rf r e q u e n c y on t h ee d g e of t h e a b s o r p t i o n l i n e and u s e ofan e l e c t r o n i cf e e d b a c kc o n t r o l of t h e l a s e r d r i v e c u r r e n tt om a i n t a i nt h e l a s e r o u t p u t a t t h ed e s i r e df r e q u e n c y . A laser f r e q u e n c y s t a b i l i z a t i o n and c o n t r o l l o o p w a s d e s i g n e d , f a b r i c a t e d , and t e s t e d . Two t e c h n i q u e s w e r e i n v e s t i g a t e d ;b o t h a r e b a s e do nt h es t a b i l i z a t i o n of t h e l a s e r f r e q u e n c y on t h e s l o p e of a r e f e r e n c e m o l e c u l a r a b s o r p t i o n l i n e . The f i r s t c o n t r o l l o o p u t i l i z e d a s i n g l ed e t e c t o r ;t h es e c o n du s e d two d e t e c t o r s . The s i n g l e mode s e l e c t e d by t h e monochromator i s passedthroughan a b s o r p t i o n c e l l whose l i n e w i d t h c a n b e p r e s s u r e b r o a d e n e d f r o m t h e D o p p l e r l i m i t t o a few GHz. With t h e l a s e r d i o d et u n e dt o a f r e q u e n c ya p p r o x i m a t e l y half-way down t h e a b s o r p t i o n l i n e e d g e , t h e o u t p u t v o l t a g e of a d e t e c t o r s e n s i n g t h i s power t r a n s m i t t e d t h r o u g h t h e a b s o r p t i o n c e l l w i l l increaseor decreaseproportionatelywithdiodefrequency,providedthatthere i s no change i n t h e l a s e r power. The change i n d e t e c t o r o u t p u t i s f e db a c kt h r o u g h a c o n t r o l l e r t o c h a n g et h ed i o d e laser c u r r e n t a n d , t h e r e f o r e , i t s frequency. This w i l l r e t u r n t h e d i o d e f r e q u e n c y t o t h e o r i g i n a l v a l u e s e l e c t e d on t h e abs o r p t i o nl i n e .F l u c t u a t i o n si nd i o d e power o u t p u t w i l l c a u s e e r r o r s i n t h e frequencychangemeasurementandanindependentmeasurement ofpowermust be made.
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T h ee l e c t r o n i cs y s t e mb a s e do nt h es i n g l ed e t e c t o rt e c h n i q u e w a s bench t e s t e dt oe n s u r ep r o p e ro p e r a t i o no ft h ec o n t r o l l e r . A t t h i ss t a g eo ft h e p r o g r a m ,t h ee x p e r i m e n t a le f f o r tc o n c e n t r a t e do nt h e 23-pm s p e c t r a l r e g i o n . T h ef r e q u e n c yc o n t r o ll o o p w a s , t h e r e f o r e , o p e r a t e d on t h e s l o p e o f a n Hz0 a b s o r p t i o n l i n e u s i n g a d i o d e laser w h i c h c o u l d b e t u n e d t o t h e a b s o r p t i o n line(althoughthe laser power w a s r e l a t i v e l y low a t t h i s p a r t i c u l a r f r e q u e n c y ) . The p e r f o r m a n c e o f t h e c o n t r o l l o o p i n terms o f f r e q u e n c y s t a b i l i t y c o u l d n o t b es a t i s f a c t o r i l ye v a l u a t e dd u et ol a r g ed e t e c t o ra m p l i t u d ef l u c t u a t i o n sc a u s e d b yt h er e f l e c t i v ec h o p p e rw h e e lu s e di nt h i s mode o fo p e r a t i o n .T h e metallic w h e e l was found t o h a v e s e v e r a l d i s t o r t e d b l a d e s w h i c h c a u s e d t h e f o c u s e d image a t t h e d e t e c t o r t o b e d i s p l a c e d o na n do f ft h ed e t e c t o re l e m e n t .T h e r e s u l t i n g n o i s e l e v e l o ft h es y s t e m was toolargeforeffectiveoperationof t h ec o n t r o ll o o p . A s e c o n ds y s t e m was c o n s t r u c t e dw h i c h made u s e of twodet e c t o r sr a t h e rt h a no n e .T h eb a s i cp r i n c i p l e i s the same as f o r t h e case of a s i n g l ed e t e c t o r ;t h ed i f f e r e n c e is i nt h ec o m p l e x i t yo ft h ed e t a i l e d electronics. The l o o p w a s t e s t e d e l e c t r o n i c a l l y b u t n o t o p e r a t e d w i t h a laser. It s h o u l db ep o i n t e do u tt h a tc o n s i d e r a b l ee f f o r th a sb e e na p p l i e d to t h e p a s s i v ef r e q u e n c ys t a b i l i z a t i o no ft h e laser o u t p u t .F o re x a m p l e ,t h e laser c u r r e n ts o u r c e( d r i v e )p r o v i d e s a l o w - n o i s e ,l o w - d r i f to u t p u ta n dt h el i q u i d h e l i u mD e w a r su s e df o r laser c o o l i n g h a v e b e e n s u p e r i n s u l a t e d t o m i n i m i z e v a r y i n ge x t e r n a lh e a tl o a d s . The m e c h a n i c a lr e f r i g e r a t o r( A i rP r o d u c t sM o d e l CS 202-modified)has a s p e c i a l l yd e s i g n e dt h e r m a ld a m p e n e rt or e d u c et e m p e r a t u r ef l u c t u a t i o n sc a u s e db yt h ep i s t o nc y c l e .I n d i r e c tf r e q u e n c ys t a b i l i t y m e a s u r e m e n t so ft h e laser o u t p u ti nt h ec l o s e dc y c l er e f r i g e r a t o r show t h a t frequencyvariationsgreaterthan a f e w MHz d i d n o t o c c u r i n g e n e r a l , a l t h o u g h s i g n i f i c a n t l a s e r a m p l i t u d e( p o w e r )f l u c t u a t i o n s a t t h e mixer c a no c c u rf o r a poorlymountedrefrigeratorinadditiontoexcess l a s e r n o i s ei n d u c e db yt h e m e c h a n i c a lr e f r i g e r a t o r .( F i g . 3 shows a D o p p l e rl i m i t e dl i n ew i d t h was a c h i e v e di nC 0 2 . )T h e s ea m p l i t u d ef l u c t u a t i o n s were t h e r e s u l t of a p h y s i c a l d i s p l a c e m e n t o f t h e l a s e r and i t s c o n j u g a t ei m a g e a t t h e small mixer e l e m e n t usedand were m i n i m i z e db yp r o p e rm o u n t i n ga r r a n g e m e n t s .I n a l l t h e measurements t o b e r e p o r t e d , t h e o n l y a c t i v e l a s e r f r e q u e n c yc o n t r o ll o o pi n v o l v e d t e m p e r a t u r es t a b i l i z a t i o no ft h em e c h a n i c a lr e f r i g e r a t o r .
A more d i r e c t m e a s u r e o f t h e f r e q u e n c y s t a b i l i t y a t t a i n a b l e u s i n g t h e s e t e c h n i q u e s i s shown i nF i g . 4 . T h eh e t e r o d y n eb e a tn o t eo b t a i n e du s i n g a C02 l a s e r as t h e l o c a l o s c i l l a t o r and a d i o d e l a s e r o p e r a t i n g i n t h e m o d i f i e d r e f r i g e r a t o r as t h e s i g n a l s o u r c e i s shown.The l i n ew i d t hr e m a i n e du n c h a n g e d when t h e r e f r i g e r a t o r was t u r n e d o f f a n d t h e l i n e p o s i t i o n s w e p t t h r u t h e IF bandpass as e x p e c t e d . The o b s e r v e dl i n ew i d t ho fa p p r o x i m a t e l y 5 MHz i n d i c a t e s e x c e l l e n tf r e q u e n c ys t a b i l i t y .
MECHANICALREFRIGERATORMODIFICATION The m a n u f a c t u r eo f a d i o d e laser o p e r a t i n g a t a s p e c i f i cf r e q u e n c y i s a t i m e - c o n s u m i n gp r o c e s s C . r y s t a lg r o w t h a, n n e a l i n g j, u n c t i o nf o r m a t i o n c, o n tacting,andyield l i m i t t h e numberof devicesthatcan meet a g i v e n s p e c i f i c a t i o n .I no r d e r t o i n c r e a s et h eu s e f u l n e s so f a g i v e n device’s c h e m i c a l c o m p o s i t i o na n d ,t h e r e f o r e ,r e d u c et h ef a b r i c a t i o nc o m p l e x i t y , l a s e r s are 84
operated in mechanical refrigerators which provide additional convenient ( t e m p e r a t u r e )t u n i n gc a p a b i l i t y . The u s e ofsuch a c o o l e r ,h o w e v e r ,c a ni n t r o d u c ea d d i t i o n a lu n w a n t e dm e c h a n i c a l l yi n d u c e d laser n o i s e . Q u a n t i t a t i v e measurements were made o f t h e s h o c k and v i b r a t i o n e f f e c t s i n t h e A i r P r o d u c t s ref r i g e r a t o r . The d i s p l a c m e n to ft h ec o l dh e a dd u et ot h ep i s t o nm o t i o n was measured t o b e 0.015 cm (0.006 i n )u s i n g a c a p a c i t a n c et e c h n i q u e . Shock e f f e c t s were i n v e s t i g a t e d u s i n g a n a c c e l e r o m e t e r a t t a c h e d t o thecoldhead. The d a t a were a c q u i r e d a n d a n a l y z e d by a Hewlett-Packard 54508 Fourier Analyzer. A t y p i c a ls p e c t r u ma n d i t s F o u r i e rt r a n s f o r m are shown i n F i g u r e s 5 and 6 r e s p e c t i v e l y . The a n a l y s i s shows a r e l a t i v e l y complexspectrumhaving a p e a kv a l u e of approximately8g with a majorcomponent a t 1500 Hz. The r e s u l t s o ft h e s ed a t a were used i n t h e s h o c k / v i b r a t i o n i s o l a t o r d e s i g n . T h i s d e s i g n i s similar t o t h a t u s e d by D r . Jennings a t NASA GoddardSpaceFlightCenter ( R e f . 31, b u t i s more v e r s a t i l e i n p r o v i d i n g f i v e d e g r e e s of freedom i n t h e l a s e r p o s i t i o n i n g , w h i c h w e havefound t o b e n e c e s s a r y t o p r o v i d e t h e p r o p e r a l i g n m e n to ft h e laser i n t h e o p t i c a l s y s t e m .
a comparison of The e f f e c t i v e n e s s of t h e s h o c k i s o l a t i o n c a n b e s e e n f r o m F i g u r e s 7 and 8. I d e n t i c a la c c e l e r o m e t e r s w e r e p l a c e d on t h ec o l dh e a d and t h e m o d i f i e dl a s e r mount. F i g u r e 7 shows t h ea c c e l e r o m e t e ro u t p u t a t t h er e f r i g e r a t o rc o l dh e a d and F i g u r e 8 t h e c o r r e s p o n d i n g d a t a a t t h e m o d i f i e d laser mount. A d r a m a t i cd e c r e a s ei nt h ep e a kg - v a l u e i s t ob en o t e d as w e l l as a r e d u c t i o ni nt h eh i g h - f r e q u e n c yc o n t e n t . A t o t a l of s i x l a s e r mounts were made t o accomodate a l l p r e s e n t l y e x i s t i n g l a s e r packagedesignsand comparativeaccelerometerdatawereobtained. The laser m e a s u r e m e n t s t a k e n w i t h t h e m o d i f i e d r e f r i g e r a t o r i n c l u d e d a comparison of t h e d i r e c t d e t e c t e d laser power l e v e l ( i n a g i v e n l o n g i t u d i n a l mode) , t h e s e c o n d d e t e c t i o n ( I F n o i s e ) l e v e l , andlow-temperatureblackbody h e t e r o d y n e SNR measurements when p o s s i b l e . The m a g n i t u d ea n dn o i s el e v e lo f t h eI Fs i g n a l( a n d ,t h e r e f o r e ,h e t e r o d y n es i g n a l - t o - n o i s er a t i o (SNR)) i s a s t r o n g f u n c t i o n of t h e l o n g i t u d i n a l mode s t a b i l i t y whichdepends upon t h e l a s e r o p e r a t i n ' gc o n d i t i o n s . The SNR a t t a i n a b l ef o rt h eh e t e r o d y n ed e t e c t i o n of t h e r m a lr a d i a t i o nc a nb ed i r e c t l yr e l a t e dt ot h e mode s t a b i l i t y . C e r t a i n modes of a g i v e n laser a r e q u i t e s t a b l e o v e r a widerange of c o n d i t i o n s and o t h e r s a r e u n s t a b l eu n d e ra n yc o n d i t i o n . A comparison of t h e I F n o i s e l e v e l produced by a l a s e r when o p e r a t e d i n t h e m o d i f i e d r e f r i g e r a t o r and a l i q u i d helium D e w a r showed t h a t t h e m o d i f i c a t i o n o f t h e r e f r i g e r a t o r g r e a t l y r e d u c e d t h e random ( u n d e s i r e d )n o i s e l e v e l b u t d i d n o t eliminate i t ; i . e . , l i q u i d cryogencoolingprovides a b e t t e r h e t e r o d y n e SNR t h a n m e c h a n i c a l r e f r i g e r a t o r c o o l i n g .I na n yc a s e ,t h em o d i f i e dr e f r i g e r a t o rd e c r e a s e dt h ee x c e s sI Fn o i s e level t o t h e e x t e n t t h a t t h e i n h e r e n t mode i n s t a b i l i t y a n d p o o r t r a n s v e r s e mode s t r u c t u r e of t h e d i o d e lasers become t h e m a j o r l i m i t a t i o n s i n a c h i e v i n g n e a rq u a n t u m - n o i s el i m i t e dh e t e r o d y n ep e r f o r m a n c e .
85
l1l l 11 I
I1
LASER MEASUREMENTS
T h e d i o d e lasers c h a r a c t e r i z e d a n d u s e d d u r i n g t h i s p r o g r a m i n c l u d e lasers o p e r a t i n gi nt h e 23- a n d 15-vm s p e c t r a lr e g i o n s .D e t a i l e dm e a s u r e m e n t s were made onmorethan 20 lasers. T h i s s e c t i o n w i l l r e p o r t r e s u l t s o b t a i n e d o n s e l e c t e dd e v i c e sw h i c ht y p i f yd i o d e l a s e r c h a r a c t e r i s t i c s . The I-V c h a r a c t e r i s t i c s o ft h e lasers had series r e s i s t a n c e ( R s ) v a l u e sf r o mt e n so fm i l l i o h m s t o as h i g h as 180 mil. Good I-V c h a r a c t e r i s t i c s i n g e n e r a l d i d n o t c o r r e l a t e w i t h laser p e r f o r m a n c e . A s anexample, lasers h a v i n gt h e same R s v a l u ew o u l d v a r yi no u t p u tp o w e rb y a f a c t o r o f 10 and lasers e x h i b i t i n g t h e well-known k i n k ,s u p p o s e d l yi n d i c a t i n gh i g hr a d i a t i v ee f f i c i e n c y ,f r e q u e n t l yw o u l dh a v e lowerpowerthan a l a s e r w i t h a more g r a d u a l l y r i s i n g I-V curve.Although a few lasers e x h i b i t e d a n e x t r e m e l y l a r g e n u m b e r o f l o n g i t u d i n a l m o d e s , a more t y p i c a l b e h a v i o r w a s t h r e e t o six l o n g i t u d i n a l modes o p e r a t i n g s i m u l t a n e o u s l y a t c u r r e n t levels s u f f i c i e n t l y a b o v e t h r e s h o l d t o p r o v i d e u s e f u l l a s e r power levels. The t y p i c a lc o n t i n u o u st u n i n gr a n g e of a g i v e n mode f o r a l l lasers examined w a s a p p r o x i m a t e l y 1 cm'l (30 GHz). T h ep o w e ro u t p u tc h a r a c t e r i s t i c f o r a c o n t i n u o u s l yt u n a b l e mode v a r i e dc o n s i d e r a b l y .T h eo u t p u tp o w e rp e a k e d a t a n yp o i n ti nt h er a n g ef r o ml a s i n go n s e tt ot h ed i s a p p e a r a n c eo ft h e mode. The l a s e r w h i c h g a v e t h e b e s t b l a c k b o d yh e t e r o d y n ed e t e c t i o nr e s u l t ss h o w e d e f f i c i e n t IF noisegenerationfor a l l modes as shown i n F i g . 9 . The beam d i v e r g e n c e w e h a v e o b s e r v e d h a s v a r i e d f r o m f / l t o f / 1 0 w i t h v a l u e sl a r g e rt h a n f / 2 b e i n g more common. Some o ft h eo b s e r v e df a r - f i e l d p a t t e r n sc a nb ee x p l a i n e db y a b l o c k a g e( r e f l e c t i o n )o f a p o r t i o no ft h ed i v e r g i n g beam b y t h e h e a t s i n k , w h i c h i s a f a b r i c a t i o np r o b l e ma n d i s n o tf u n d a m e n t a lt ot h ed e v i c e . A l l t h e lasers examined t o d a t e are p o l a r i z e d i n t h e p l a n eo ft h ej u n c t i o na n df r e q u e n t l yi nt h el o w e s to r d e r T E 0 , o mode. Thenearf i e l d( m i r r o rr a d i a t i o n )p a t t e r no f two lasers s e l e c t e d a t randomshowed only one a c t i v e r e g i o na n dn oe v i d e n c eo ff i l a m e n t a r ya c t i o n( w h i c h is understanda b l e i n l i g h t of t h e l o n g d i f f u s i o n l e n g t h s i n t h e Pb s a l t s ) . T h em a j o re m p h a s i st od a t ei nt h ed e v e l o p m e n to ft h el e a d s a l t lasers h a s b e e nc o n c e r n e dw i t hi n c r e a s i n gt h et o t a lo u t p u tp o w e r .C o n s i d e r a b l es u c c e s s h a sb e e n a c h i e v e di nt h i s area, p a r t i c u l a r l yi nt h ep a s t two y e a r s .O u t p u t powers i n t h e m i l l i w a t t r a n g e a r e common, comparedwithmicrowatt levels i n t h ee a r l ys t a g e so fd e v e l o p m e n t .A l t h o u g h a s u f f i c i e n t l y h i g h power l e v e l i s a necessaryconditionfor a localoscillatorsource, i t is n o t s u f f i c i e n t b y i t s e l f . Thepowermust b ea l s oa v a i l a b l e i n a s i n g l e l o n g i t u d i n a l mode. Unfortunately,thepresentlyavailablehigher power l e a d - s a l t lasers e x h i b i t a r e l a t i v e l y l a r g e n u m b e ro fl o n g i t u d i n a lm o d e s ,l i m i t i n gt h ep o w e ra v a i l a b l e i n a s i n g l e mode. F u r t h e r m o r e ,t h e laser e m i s s i o nr a d i a t i o np a t t e r nf r e q u e n t l y c o n s i s t so f several d i v e r g i n gl o b e s .T h ea n g u l a re x t e n t of a g i v e nl o b e i s r e l a t i v e l y small as m e n t i o n e d a b o v e , b u t t h e t o t a l a n g u l a r e x t e n t may b e l a r g e . T h i s s i t u a t i o n i s shown i n F i g . 10, which i s t h e f a r - f i e l d p a t t e r n o f a diode l a s e r o p e r a t i n g a t 15.4um(cw) o b t a i n e df r o m a c r o s s s e c t i o n a l s c a n o f t h e o u t p u t beam w i t h o u tu s i n ga d d i t i o n a lo p t i c s .T h r e el o b e s a r e e v i d e n ta n de a c h , inturn,couldbeisolatedbythemonochromatorbyusingtheangularadjustm e n t sa v a i l a b l ei nt h em o d i f i e dr e f r i g e r a t o r laser mount.Although f/loptics c a nc o l l e c t a l l o ft h i se m i t t e dr a d i a t i o n ,t h ea v a i l a b l e power i n a s i n g l e l o b e (mode) i s l i m i t e d .
The l o w e s t o r d e r TE mode i s r e q u i r e d f o r good h e t e r o d y n e m i x i n g e f f i c i e n c y a n dt h e r e f o r et h ei m p o r t a n c eo ft h et r a n s v e r s e mode s t r u c t u r e of t h e laser i n d e t e r m i n i n gt h em i x i n ge f f i c i e n c yc a n n o tb eo v e r s t a t e d . A t y p i c a lr e s u l t i s shown i n F i g u r e 11. It c a nb es e e nt h a ti n d i v i d u a ll o n g i t u d i n a l modes have v a r y i n g d e g r e e s of m i x i n g e f f i c i e n c y ; p o o r e f f i c i e n c y is correlatedwith a complex transverse mode s t r u c t u r e of a ni n d i v i d u a ll o n g i t u d i n a l mode. T h i s p a r t i c u l a r laser also exhibited significant excess noise resulting in a poor h e t e r o d y n es i g n a l - t o - n o i s er a t i o as compared t o t h e o r e t i c a l p r e d i c t i o n s . The far-field intensity distribution for a r e l a t i v e l y e f f i c i e n t mode i s shown i n F i g u r e1 2 ( a )a n di nF i g u r e1 2 ( b )f o ra ni n e f f i c i e n t mode. The p l o t sr e p r e s e n t a partial cross-sectional scan of t h e laser beam u s i n g a small d e t e c t o r i n t h e c o l l i m a t e d beam o ft h e laser. C l o s ei n s p e c t i o no fF i g u r e1 2 ( b ) reveals a more complex i n t e n s i t y d i s t r i b u t i o n t h a n t h a t i n F i g u r e 1 2 ( a ) . T h e s e r e s u l t s are g e n e r a l :h i g h e ro r d e rt r a n s v e r s e modes r e s u l t i n p o o r h e t e r o d y n e d e t e c t i o n efficiency. The t r a n s v e r s e mode s t r u c t u r e of a d i o d e l a s e r i s o n l y of minorimportance inhigh-resolutionspectroscopy and h a s c o n s e q u e n t l y r e c e i v e d l i t t l e a t t e n t i o n i nt h em a n u f a c t u r e of t h e s el a s e r s .I ng e n e r a l ,t oe n s u r el o w e s to r d e r mode o p e r a t i o n ,a d d i t i o n a lo p t i c a l and c a r r i e rc o n f i n e m e n t i s e x p e c t e d t o b e requiredoverthatoccurring i n the simple diffusedjunctiondevicespresently m a n u f a c t u r e d ,a l t h o u g hh o m o j u n c t i o nd e v i c e sw h i c ho p e r a t ei nt h el o w e s to r d e r mode h a v e b e e n p r o d u c e d w h i c h p r o v i d e e x c e l l e n t h e t e r o d y n e m i x i n g e f f i c i e n c y . The n a t u r e ofthe mode i n s t a b i l i t y ( e x c e s s n o i s e ) i s n o tw e l lu n d e r s t o o d at present. I t i s p o s s i b l et h a t improvements i nm a t e r i a lu n i f o r m i t y and c r y s t a l p e r f e c t i o n w i l l r e l i e v e t h i s p r o b l e m .S e l f - b e a t i n ge f f e c t sh a v ea l s ob e e n observedwithindividuallongitudinal modesand may bedue t o F a b r y - P e r o t c a v i t yi m p e r f e c t i o n sp r o d u c e dd u r i n gt h ec l e a v i n gp r o c e d u r et of o r mt h e c a v i t y .T h e s ee f f e c t sr e s u l ti ne x c e s sn o i s ei nt h ed e t e c t i o np r o c e s sb u t do notoccurforthemajority of t h e modes andcanfrequentlybeminimized by c h a n g e si nt h eo p e r a t i n gc o n d i t i o n s . These r e s u l t s emphasizetheimportance of t h ec a v i t yc o n f i g u r a t i o n of d i o d e l a s e r s to beused as l o c a lo s c i l l a t o r s . I t i s e s s e n t i a lt h a tt h ec a v i t y p r o v i d e a minimum i n l o n g i t u d i n a l mode c o m p e t i t i o n as w e l l as a s t r o n g r e j e c t i o no fh i g h e ro r d e rt r a n s v e r s e modes.
SUMMARY A b r e a d b o a r dh e t e r o d y n er e c e i v e r i s describedwhichhasbeenusedto establishthecharacteristics of l e a d s a l t d i o d e l a s e r s p e r t i n e n t t o t h e i r u s e as l o c a l o s c i l l a t o r s u s i n g l a s e r d e v i c e so p e r a t i n gi nt h e 15-25 pm s p e c t r a l r e g i o n .H e t e r o d y n ed e t e c t i o ne f f i c i e n c yh a sb e e nd i r e c t l yc o r r e l a t e dw i t ht h e t r a n s v e r s e mode s t r u c t u r e a n d e m p h a s i z e s t h e n e c e s s i t y o f s t a b l e l o w e s t o r d e r mode o p e r a t i o nf o r l a s e r s w h e nu s e d as l o c a l o s c i l l a t o r s . The r e s u l t so b t a i n e d i n d i c a t et h a tt h ec o n t i n u e dd e v e l o p m e n t of t h e s e lasers w i l l p r o v i d e s u i t a b l e localoscillatorsfor a v a r i e t y of a p p l i c a t i o n s .
87
IIIIIIIIII I
REFERENCES
1. Washwell, E . R.;
..
S.
K.:
SPIE Proc. V o l .
95,pp
41-46
(1976).
2.
Cohen, S. C.: H e t e r o d y n eD e t e c t i o n p: h a s ef r o n t a l i g n m e n t , a n dd e t e c t o ru n i f o r m i t y .A p p l .O p t .1 4 ,1 9 5 3 - 1 9 5 9( 1 9 7 5 )
3.
J e n n i n g s , D. E . ; andHillman, J. J . : A S h o c kI s o l a t o rf o rD i o d e Laser O p e r a t i o n on a C l o s e d - C y c l eR e f r i g e r a t o r .G o d d a r dS p a c eF l i g h tC e n t e r Document X-693-77-143, J u n e1 9 7 7 .
88
.
a n dI c h i k i ,
beam s p o ts i z e ,
LASER NO. 2 MONOCHROMATOR OFF AXIS PARABOLIC
MIRROR
SPECTRUM ANALYZER
OR LIQ. He DEWAR
n
'
Figure 1.- Diode laser heterodyne receiver breadboard.
89
Ill !I
Ill
(A) LASER OFF !%MHz BANDPASS FI LTER,
2 MHzlcrn HORl ZONTAL AX1 S
(B) LASER ON S-MHZ BANDPASS F I LTER 2-MHzlcrn HORIZONTAL AX1 S
F i g u r e 2.-
Generation-recombination n o i s e g e n e r a t e d i n HgCdTe.
12 TORR
A
=
12 TORR
6.8 TORR
1.4 TORR
0.60 TORR 0.12 TORR
3.4 TORR
by 15-!~n d i o d e laser
1.4 TORR
14.64yrn
co2 =
I
3000
-
684.078 crn
F i g u r e 3 . - Frequency s t a b i l i t y measurement i n f e r r e d fromDoppler
90
l i n e width.
10 MHz PER DIVISION;
2-ms SWEEP
F i g u r e 4.-
D i r e c t f r e q u e n c ys t a b i l i t ym e a s u r e m e n tf r o mh e t e r o d y n eb e a tn o t e .
0.1023 F i g u r e 5.- Refrigerator accelerometer s i g n a l .
91
I
1I
Figure 6.-
Figure 7.92
l! I l l
I1
Fourier transform
of accelerometer output.
Modified r e f r i g e r a t o r a c c e l e r o m e t e r s i g n a l
( a t cold head).
Figure 8.- Modified r e f r i g e r a t o r a c c e l e r o m e t e r s i g n a l ( a t l a s e r
23
23.38
J
t
+
mount).
23.43
4
23.32 b
LASER OUTPUT POWER
I F NO1 SE
23.50
i I
WAVELENGTH (pm) Figure 9.-
-
Correlation of l a s e r node powerand
I F noise level.
93
I
1 1 I1 Il 1 I I
= 15.4 p m
h
0 4.699.7814.6719.5624.4529.34 0 (deg) Figure 10.- F a r - f i e l d beam p r o f i l e
I BLACKBODY T
C "
Figure 11.- Heterodyne spectrun as
94
( 1 5 - v m l a s e r cw).
(CI HETERODYNE SIGNAL
- 6W"
~. .
?A.
34.23
'
J
a function of l a s e r power and I F n o i s e l e v e l .
...
__f
D I STANCE
k RESOLUTION
(a) E f f i c i e n t node.
D l STANCE
-
(b) I n e f f i c i e n t mode.
Figure 12.-
Laser f a r - f i e l d p r o f i l e .
95
1 l1 l1 1 1 1 I
DIAGNOSTICS AND CONTROL OF WAVENUMBER STABILITY AND PURITY OF TUNABLE DIODE LASERS RELEVANT TO
THEIRUSE
AS LOCAL OSCILLATORS IN HETERODYNE SYSTEMS*
S. P o u l t n e y , D. Chen, G. S t e i n b e r g , F. Wu, A. P i r e s , M. Miller, and M. McNally The Perkin-ElmerCorporation,Norwalk,
CT.
06856
SUMMARY Performance c h a r a c t e r i s t i c s i n h e r e n t t o b o t h a v a i l a b l e t u n a b l e d i o d e l a s e r s and t h e i re n v i r o n m e n t w i l l a f f e c t t h e i r u s e as l o c a l o s c i l l a t o r si nh e t e r o d y n es y s t e m s .T h i sp a p e rr e p o r t sd i a g n o s t i c so f wavenumber s t a b i l i t y and p u r i t y of t u n a b l e d i o d e l a s e r s i n c l o s e d c y c l e c o o l e r environments, and a new c o n c e p t f o r c o n t r o l o f t h e wavenumber i n h e t e r o dyne s y s t e ma p p l i c a t i o n s . The i n h e r e n t TDL c h a r a c t e r i s t i c si n c l u d e d t h ec a p a b i l i t yt ob et u n e dt ot h e wavenumber d e s i r e d ,t h ec h a n g ei n o p e r a t i n gp o i n t as a f u n c t i o no ft i m e and t h e r m a lc y c l i n g ,t h en o n l i n e a r dependence of wavenumberon drivecurrent,theaxial mode s t r u c t u r e , and t h e o b s e r v e d f i n e s t r u c t u r e o f " s i n g l e " a x i a l modes. I n i t i a lo p e r a t i o no ft h e TDL showed t h a t i t was n o t p o s s i b l e t oa d j u s tt h e wavenumber t o one s e l e c t e d a p r i o r i i n t h e TDL t u n i n g range. I t s o p e r a t i n gp o i n t was r e c o r d e da s a f u n c t i o n of time and t h e r m a lc y c l i n g by h e t e r o d y n e and a b s o r p t i o nt e c h n i q u e s .D u r i n go p e r a tion,theoperatingpoint wouldchange by 0.1 cm- o v e r t h e l o n g e r t e r mw i t he v e nl a r g e rc h a n g e so c c u r r i n gd u r i n g some t h e r m a lc y c l e s . Most c h a n g e sd u r i n gt h e r m a lc y c l i n gr e q u i r e du s i n gl o w e rt e m p e r a t u r e s and h i g h e rc u r r e n t st or e a c ht h ef o r m e r wavenumber (when i t c o u l db e r e a c h e d ) .I n many c a s e s ,a no p e r a t i n gp o i n tc o u l db es e l e c t e d by changi n g TDL c u r r e n t and t e m p e r a t u r e t o g i v e b o t h t h e d e s i r e d wavenumber andmostofthe power i n a s i n g l e mode. The s e l e c t i o np r o c e d u r e had t o beused a f t e re a c ht h e r m a lc y c l i n g . Wavenumber n o n l i n e a r i t i e so f a b o u t 10% o v e r a 0.5-cm t u n i n gr a n g e were o b s e r v e d .D i a g n o s t i c so f t h e s i n g l e mode s e l e c t e d by a g r a t i n g monochromator showed wavenumber f i n es t r u c t u r eu n d e rc e r t a i no p e r a t i n gc o n d i t i o n s . The c h a r a c t e r i s t i c s due t o t h e TDL e n v i r o n m e n ti n c l u d e ds h o r t - t e r m wavenumber s t a b i l i t y , t h ei n s t r u m e n tl i n e s h a p ef u n c t i o n , and i n t e r m e d i a t e - t e r m wavenumber
'
*This
work was p a r t i a l l y s u p p o r t e d by t h e High A l t i t u d e P o l l u t i o n Program, C o n t r a c t No. FAA-AEE-79. However, t h ep r i m a r ys o u r c e so fs u p p o r t were Perkin-Elmer'sIndependentResearch andDevelopmentProgramsand c a p i t a le x p e n d i t u r e s . S. P o u l t n e yw i s h e st ot h a n k D r . David A. H u c h i t a l , DirectorofResearch,forhostingthemeasurementprogramsandfor h a v i n gt h ef o r e s i g h tt op r o v i d et h ei n i t i a lc a p i t a ls u p p o r t . 97
stability. Steinberg (ref. 1) has reported wavenumber set-point stabilities of several MHz on the short term (up to several minutes). His heterodyne measurementof this stability indicated that the instrumental line-shape function hada half-width of 10 MHz and a shape with broad wings and that the wings were due to residual cooler vibrations which could not be removed. Preliminary work with digital control of TDL current indicates an intermediate-term wavenumber stability (up to -1 an hour) of 1 6 cm or better which is related to cycling of the temperature control system. These wavenumber stability and purity characteristics will require supplementary systems to ensure that the TDL willbe an appropriate local oscillator, especially if thermal cycling may occur. We have developed the following concept for wavenumber control such that the TDL wavenumber can be adjusted to and held at the desired wavenumber location in the absence of a calibration cell containing the gas to be measured. The wavenumber control algorithm is designed to initialize the TDL wavenumberto that of an absorption line of a control gas after thermal cycling, step the TDL wavenumber in the presenceof scan nonlinearities to the desired wavenumber which is assumed to be within a mode tuning range, lock to that desired wavenumber in spite of intermediate and long-term wavenumber drifts, and monitor continually the stability of the stable etalon vernier. The stepping and locking is accomplished using fm/harmonic detection of etalon fringes in quadrature in an auxiliary absorption spectrometer branch. This technique allows use of moderate finesse fringe patterns through its unique filtering capabilities and is insensitive to variations inTDL power. Details of the software and hardware design will be given including the design of the stable etalon. We conclude that the characteristicsof available laser diodes and aspects of their performance in closed-cycle cooler environments which may have made space flight systems impractical can be overcome through realizable control techniques. INTRODUCTION
In 1976, S . K . Poultney initiated a two-pronged research program to enable Perkin-Elmer to return to the flight spectroscopic sensor business. This program emphasized the FTS interferometer as a quantitative-survey instrument and the tunable laser spectrometer a asspeciesspecific monitor. The tunable laser spectrometer was based on the Pb-salt tunable diode lasers. It encompassed the applications accessible to the Laser Heterodyne Spectrometer (LHS) and to the Tunable Diode Laser Absorption Spectrometer System(TDLSS). Following the assembly and testing of a COz-laser based LHS, S.K. Poultney led the assembly and development of the state-of-the-artTDLSS for trace gas detection.
98
The following section describes the Perkin-Elmer TDLSS. The trace gas detection measurements with this system have been supported by the Federal Aviation Administration; first as a feasibility study (ref. 2) and more recently asa flight prototype activity led by M.D. Miller. These measurements with the TDLSS as well as many performance diagnostics supported by independent research programs have allowed us to obtain the measurements that we report in this paper. The goals of our investigations of the tunable laser spectrometer have been to determine those characteristicsof the spectrometer which will limit its use for eitherof its two applications and to develop a potential solution for one of the most critical performance aspects. That aspect of performance is wavenumber purity and stability. Table I summarizes the wavenumber purity and stability required by the most demanding application; a laser heterodyne spectrometer measurement from space of reactive trace gases which cannot be contained a inreference cell and whose absorption lines have Doppler widths. The next two sections report the results of our wavenumber purity and stability diagnostics for performance characteristics inherentto tunable diode lasers and their e n v i r o n m e n t , r e s p e c t i v e l y . The final section describes a new concept for automatic wavenumber control which has been developed on an independent research program, and briefly summarizes our design for the necessary stable Fabry-Perot interferometer. Methods for ensuring a single mode at the detector are not discussed. DESCRIPTION OF THE TUNABLE DIODE SPECTROMETRIC SYSTEM (TDLSS)
LASER
The tunable diode laser spectrometric system consists aoflaser source, its control environment, a sample cell, a detector, connecting optics, and a data acquisition system as shown in figure 1. The laser must normally bekept at temperatures near25 K with a precision of tenths of millidegrees. It is normally tuned in wavenumberby applying an incremental drive current to the necessary bias current froma control module. This particular system possesses a digital control box which steps the drive current in small steps and synchronously clocks the A/D converter in the data acquisition system. This TDLSScan be operated in either the conventionalam detection mode using the chopper or the fm/harmonic mode using one or more fm modulations superposed on the drive current. The signal out of the detector is appropriately demodulated and filtered before being digitized. A number of auxiliary optical devices are needed to control the laser output and to calibrate the laser wavenumber. These devices include a mode selector, a coarse wavenumber locator, calibration gas cells, calibration etalons, feedback isolators, etc. For use as a laboratory spectroscopy system, the TDLSS requires all these components as well as an executive software program.
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I II I1 1I I1 I
The t u n a b l e laser s o u r c e is a s e m i c o n d u c t o rd i o d eo ft h eP b - s a l t familywhichcan e m i t s u b s t a n t i a l amounts o f power i n a n a r r o w s p e c t r a l r a n g ei nt h em i d i n f r a r e du n d e rt h ep r o p e rc o n d i t i o n s . A l l of t h e lasers t h a t we haveusedhavebeenpurchasedfromonecommercialsource, Laser A n a l y t i c s ,I n c . These d i o d e sh a v ey i e l d e da b o u t1 0 0m i c r o w a t t s i ne a c ho f several modes i n t h e wavenumber r e g i o ns p e c i f i e d .T h e i r l i f e t i m e sh a v eb e e ne x c e l l e n t( e . g . many months). Our s p e c i f i e d wavenumbershavebeenreachedwithintheavailablerangesofclosed-cycle c o o l e r t e m p e r a t u r e s a n dc o n t r o l - m o d u l eb i a sa n dd r i v ec u r r e n t s . The TDL's mustbeoperatedunderthepropertemperature,mechanicalmount,and c u r r e n tc o n d i t i o n st oi n s u r eb e s tp e r f o r m a n c e .F o rt h e l a b o r a t o r y TDLSS we s e l e c t e d a n AIRCO c l o s e d - c y c l e c o o l e r t o p r o v i d e t h en e c e s s a r yc r y o g e n i ce n v i r o n m e n t .I nt h i se n v i r o n m e n t ,t h e TDL c a no p e r a t eb e t w e e na b o u t 20 K and 50 K dependingon i t s composition and t h e wavenumber d e s i r e d .T e m p e r a t u r ec o n t r o la n dv i b r a t i o nm o u n t i n g o ft h e TDL was e n g i n e e r e d byPerkin-Elmerand i s d e s c r i b e d by S t e i n b e r g ( r e f .1 ) . The TDL r e q u i r e s a c u r r e n t b i a s o f up t o 2 A i n o r d e r t o emit l i g h t . It i s n o r m a l l yt u n e di n wavenumber by a p p l y i n g a v a r i a b l e d r i v e c u r r e n t . T y p i c a lc u r r e n tt u n i n gc o e f f i c i e n t s are 0.011 c m - I / m A . W e selected t h ea n a l o gc u r r e n t - c o n t r o l module made by Laser A n a l y t i c s I n c . f o r t h eL a b o r a t o r y TDLSS. W e havefound t h i st ob e a w e l l - e n g i n e e r e du n i t . W e have a l s o found i t c o n v e n i e n tt os u p p l e m e n tt h ea n a l o gc u r r e n t - c o n t r o l module w i t h a d i g i t a l c o n t r o l box.This d i g i t a l c o n t r o l box s t e p s t h ed r i v ec u r r e n ti n small s t e p s and s y n c h r o n o u s l yc l o c k sb o t h a chart r e c o r d e r and t h e A/D c o n v e r t e ri nt h ed a t aa c q u i s i t i o ns y s t e m . The d a t aa c q u i s i t i o ns y s t e mr e c o r d st h ed i g i t i z e d TDLSS s c a n s f o r l a t e r a n a l y s i s on thePerkin-Elmertime-sharingcomputer. The d i g i t a lc o n t r o l boxcanscan a t d i f f e r e n t r a t e s and d i f f e r e n t t u n i n g r a n g e s , c a n s c a n down, c a n d i g i t a l l y s e t t h e b i a s c u r r e n t , and c a n a c c e p t e i t h e r wavenumber c o n t r o l s i g n a l s o r s e v e r a l TDL m o d u l a t i o n s i g n a l s a t a n e x t e r n a l summing j u n c t i o n . This TDLSS c a n b e o p e r a t e d i n e i t h e r o f two d e t e c t i o n modes: a m choppingorfm/harmonicdetection. The c o n v e n t i o n a l am mode u s e s t h ec h o p p e rt om o d u l a t et h el i g h t beam and t h es y n c h r o n o u sa m p l i f i e r t o demodulate and f i l t e rt h ed e t e c t o rs i g n a l . W e u s et h i s mode f o r s p e c t r o s c o p i cm e a s u r e m e n t so fl i n ep o s i t i o n s ,s t r e n g t h s , and w i d t h s ( r e f . 3 ) . The fm/harmonic mode u s e s one o r more f m modulationssuperposed on t h e TDL d r i v e c u r r e n t and t h es y n c h r o n o u sa m p l i f i e rt o demodula t e and f i l t e r t h e d e t e c t o r s i g n a l . We u s e t h i s mode f o rt r a c eg a s d e t e c t i o nb e c a u s e o f i t s a d v a n t a g e sf o rt h i sa p p l i c a t i o na sd i s c u s s e d by Poultney e t a l . ( r e f . 2 ) .
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The d e t e c t o r is a b r o a dr e s p o n s e HgCdTe d e t e c t o r from Hughes/SBRC. It was purchased i n c o n j u n c t i o n w i t h a matched, low n o i s e p r e a m p l i f i e r . The d e t e c t o r h a s p e r f o r m e d w e l l , b u t it was n o t s p e c i f i c a l l y matched f o r any s i n g l ea p p l i c a t i o n .N e i t h e rt h ef i e l do fv i e w ,t h es i z e ,n o r a c o l df i l t e rh a sb e e ns e l e c t e dt op r o v i d et h el o w e s tn o i s ea c h i e v a b l e inthepresent TDLSS. The TDLSS o p t i c s c o n s i s t e n t i r e l y of o f f a x i s p a r a b o l a s and p l a n o mirrorsinordertominimizeparasiticetalonnoise and t o make a l i g n m e n t straightforward. The o p t i c s a r e modular i n form t oa l l o wc o n v e n i e n t changeoffunctions. A number o f a u x i l i a r y o p t i c a l d e v i c e s are needed t o c o n t r o l t h e l a s e ro u t p u t and t o c a l i b r a t e t h e l a s e r wavenumber. A g r a t i n g monochromator ( 1 / 2 m J a r r e l l - A s c h )h a sb e e nu s e dt ol o c a t et h e l a s e r wavenumber ( t o a b o u t 0.5 cm-’), t o s t u d y t h e a x i a l mode s t r u c t u r e o f t h e TDL, and t os e l e c t a s i n g l e mode f o rt r a n s m i s s i o nt h r o u g ht h e TDLSS. The monochromatorhasworked well i n a l a b o r a t o r ye n v i r o n m e n t and i s e s s e n t i a l f o rt h e TDL d i a g n o s t i c s and c o a r s e wavenumber a d j u s t m e n t . The a b s o l u t e wavenumber i s b e s td e t e r m i n e d by t h eu s eo f a g a sw i t h known a b s o r p t i o nl i n e si n a calibrationcell.
For u s e as a l a b o r a t o r ys p e c t r o s c o p ys y s t e m ,t h e TDLSS r e q u i r e s a ne x e c u t i v es p e c t r o s c o p yp r o g r a mw h i c hc a nr e a dt h ed i g i t a lr e c o r d s , n o r m a l i z e and c a l i b r a t ee a c hr e c o r d ,c o n n e c te a c hs h o r tr e c o r d( e . g . 1 ern-') i n t ot h e more n o r m a ls p e c t r o s c o p i cd i s p l a y ,p r i n to u tt h es p e c t r o s c o p i cd i s p l a y ,u t i l i z ea n a l y s i ss u b p r o g r a m sf o rt h ea n a l y s i s of t h ed a t a , and a r c h i v et h ed a t a and r e s u l t so ft h ea n a l y s i s . The flow c h a r to ft h a te x e c u t i v es p e c t r o s c o p yp r o g r a m i s shown i n f i g u r e 2. Some o ft h ea n a l y s i sp r o g r a m sa r el i b r a r yp r o g r a m sw h i l eo t h e r sh a v e b e e ns p e c i f i c a l l yw r i t t e nt oa c c o m p l i s hn o n l i n e a r , l e a s t s q u a r ef i t t i n g o fs p e c t r a l i n e s h a p e s( r e f 4. ) .
DIAGNOSTICS OF INHERENT TDL CHARACTERISTICS
C a p a b i l i t yt ob e
Tuned t o D e s i r e d
Wavenumber
Our i n i t i a l work w i t h t h e NO TDL showed t h a t we c o u l d n o t o b t a i n anoperatingpointwhichgaveanoutput a tt h eR ( 4 . 5 ) NO l i n e s , b u t c o u l do b t a i n a good o u t p u ta tt h en e a r b y R(5.5) NO l i n e s .T h i s wavenumber s e a r c h is e x e c u t e d by making a series o f c h a n g e s i n b o t h TDL h e a ts i n kt e m p e r a t u r e and d r i v ec u r r e n t . The d r i v ec u r r e n tt u n e st h e TDL t h r o u g h j o u l e h e a t i n g i n a f i n e rd e g r e et h a nt h et e m p e r a t u r ec o n t r o l l e r . The r e a s o nt h a t some wavenumbers c a n n o tb er e a c h e d is related t ot h en a t u r eo ft h e TDL o u t p u t .T h i so u t p u tc o n s i s t so f a number o f modes w i t h a f a i r l y r e g u l a r s p a c i n g and w i t h v a r y i n g a m p l i t u d e s .
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The number o f modes i s determined by t h e e x t e n t o f t h e TDL g a i n c u r v e . As t h e TDL t e m p e r a t u r e i s t u n e d ,t h e s e modes t u n e as much as a mode s p a c i n g , a t which p o i n t t h e TDL c a v i t y e n e r g y is s e n t i n t o a n o t h e r mode. C e r t a i no p e r a t i n gp o i n t sc a nb e foundwhereno modes are o p e r a t i n g o r where s e v e r a l modes are competing f o r t h e e n e r g y i n an u n s t a b l e fashion. Once f o u n d ,t h eo p e r a t i n gp o i n t s a r e a good g u i d e as t o how t o o b t a i n l i g h t a t t h ed e s i r e d wavenumbers d u r i n gt h e same t h e r m a lc y c l e . These o p e r a t i n g p o i n t s do changewith time and t h e r m a lc y c l i n g so t h a t t h e y may have t o beoptimizedeach time t h e TDL i s u s e d . The NO R(4.5) l i n e s were a c c e s s i b l ea f t e rt h es e c o n dt h e r m a lc y c l i n g ,b u tt h es e t t i n g o ft h eo p e r a t i n gp o i n t was o f t e nt e d i o u s . A s c a no ft h e s el i n e s is shown i n f i g u r e 3.
O p e r a t i n gP o i n tH i s t o r y The o p e r a t i n g p o i n t h i s t o r y o f a v a i l a b l e TDL's i s i m p o r t a n tb e c a u s e i t w i l l a l l o w us t op l a nt h o s ef i e l dd e p l o y m e n tt e c h n i q u e sw h i c h will make p o s s i b l e t h e e a s y t r a n s f e r o f a c h a r a c t e r i z e d TDL t o a f i e l d TDLSS o r t h eq u i c kr e c o v e r yo f a s p e c i f i e d wavenumber a f t e r a t h e r m a lc y c l i n g i nt h ef i e l d . We d i s c u s si n some d e t a i lt h eh i s t o r yo ft h e NO TDL. This TDL h a s b e e n i n u s e s i n c e March 1979and h a s b e e n k e p t c o l d c o n t i n u o u s l ye x c e p tf o r 5 t h e r m a lc y c l e s . Most o ft h e s ec y c l e s were caused by power o u t a g e s . W e f i n di ng e n e r a lt h a tt h eo p e r a t i n gp o i n tc h a n g e s s u c ht h a tt h eh e a ts i n kt e m p e r a t u r e mustbedecreased and t h e d r i v e c u r r e n t i n c r e a s e d as a f u n c t i o no f time t o a c h i e v e t h e same wavenumber a s b e f o r e . The l o c a t i o no ft h i s wavenumber is t a k e nt ob et h ep e a k o ft h e room w a t e r - v a p o rl i n ea t 1897.52cm-l. F i g u r e 4 p r e s e n t st h e h i s t o r y of t h i so p e r a t i n gp o i n t . W e found i t c o n v e n i e n tt op r e s e n t t h i s d a t a as, a graphof how t h e TDL wavenumber wouldhavechanged if t h eo p e r a t i n gp o i n t had n o tb e e nc h a n g e d .I no r d e rt oc a l c u l a t et h i s h y p o t h e t i c a lp e r f o r m a n c e , i t was n e c e s s a r yt od e t e r m i n et h et h e r m a l and c u r r e n t t u n i n g r a t e s o f t h i s TDL. The TDL c u r r e n t t u n i n g r a t e was measured by s c a n n i n gt h r o u g ht h e we11 known R(5.5) NO l i n e s a t 1897cm-l. The c u r r e n tt u n i n gr a t e was 0.011 cm-l/mA. The t h e r m a lt u n i n gr a t e was c a l c u l a t e d from t h eo p e r a t i n g p o i n td a t ao ff i g u r e 4 f o rt h ed a t e sb e t w e e n8 / 2 3a n d9 / 1 9 .S i n c e t h e same a b s o r p t i o nl i n e was usedeach time, t h et e m p e r a t u r ec h a n g e s mustcorrespondtothecurrentchanges. Use o ft h ec u r r e n tt u n i n g r a t el e a d st o a t h e r m a lt u n i n g r a t e of1.2 cm"/K. The h y p o t h e t i c a l performanceofthe TDL wavenumber was t h e n c a l c u l a t e d r e l a t i v e t o t h e o p e r a t i n g p o i n t x on 3 / 2 9 ,u s i n gt h e s er a t e s and t h e a c t u a l o p e r a t i n g p o i n tc h a n g e sf o re a c hd a t e .T h i sp e r f o r m a n c e i s shown i nf i g u r e 4 . Most ofthechanges were small, which i n d i c a t e dt h a tt h e same mode was u s e df o r a l l measurements.Thisobservation means t h a t onecould
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".
. ..
r e c o v e r o p e r a t i o n on a g i v e n l i n e by a s i m p l e t u n i n g o f t h e d r i v e c u r r e n t . Changes d u r i n g a c y c l e are t y p i c a l l y less t h a n0 . 1 cm The l a r g ec h a n g ei n wavenumber a f t e r c y c l e 2 i s p r o b a b l y a n i n d i c a t i o n t h a t t h e TDL c h a r a c t e r i s t i c s change so as t o modify d r a s t i c a l l y t h e mode s t r u c t u r e . The magnitudeofthechange i s p r o b a b l ya b o u te q u a l t ot h e mode s p a c i n gi nt h i sp a r t i c u l a r TDL. I nf a c t ,a f t e rt h i st h e r m a l c y c l e , t h e TDL c o u l db et u n e d so as t oa l l o wt h em e a s u r e m e n t so fb o t h t h e R(5.5) and R(4.5) NO l i n e t r a n s i t i o n s .
.
It may a l s o b e h e l p f u l f o r f i e l d deployment c o n s i d e r a t i o n s t o summarize t h e o p e r a t i n g p o i n t o p t i m i z a t i o n p r o c e s s a f t e r a thermal change.Figure 5 shows a c o m p o s i t ev i e wo ft h i sp r o c e s sa f t e rt h e s e c o n dt h e r m a lc y c l e .F i g u r e5 ( a )n o t e st h e TDL o u t p u tb e f o r ec y c l i n g as measured by t h eg r a t i n gs p e c t r o m e t e rs u p e r p o s e d on a TDLSS t r a c e oftheambient water v a p o rl i n e s .F i g u r e5 ( b ) shows t h eg r a t i n gs p e c t r o m e t e rt r a c eo ft h e TDL o u t p u t a f t e r t h e c y c l e a t t h e same h e a t s i n k t e m p e r a t u r e ,b u ta t a d r i v e c u r r e n t w h i c hp l a c e st h eo u t p u ta tt h e p r e v i o u s wavenumber. F i g u r e5 ( c ) shows t h eg r a t i n gs p e c t r o m e t e rt r a c e o ft h e TDL o u t p u t a f t e r b o t h t e m p e r a t u r e and c u r r e n th a v eb e e no p t i m i z e d t og i v en e a rs i n g l e mode performance st t h ep r e v i o u s wavenumber.Optimiza t i o no ft h eo p e r a t i n gp o i n ta f t e r a t h e r m a lc y c l e may t h u s r e q u i r e o p t i m i z a t i o no ft h e mode s t r u c t u r e as w e l l .
Wavenumber N o n l i n e a r i t y o f t h e
TDL
The f i n e wavenumber t u n i n go ft h e TDL i s accomplished by t h e j o u l e h e a t i n go ft h ed r i v ec u r r e n t . For s m a l ld r i v ec u r r e n t s compared t o t h e TDL b i a s c u r r e n t , t h e wavenumber t u n i n g i s n e a r l y l i n e a r w i t h d r i v e current. Scans l i k ef i g u r e 3 i ng e n e r a lp o s s e s s a n o n l i n e a rc u r r e n t d r i v ea x i s whichmustbe c a l i b r a t e df o ra c c u r a t e work. One cannot s i m p l yi n t e r p o l a t e between NO l i n e st ol o c a t et h e N,O l i n e ,f o re x a m p l e , t ot h ep r e c i s i o nr e q u i r e d( e. g .1 0 cm- '1. The c u r r e n td r i v ea x i s i s commonly c a l i b r a t e d t h r o u g h u s e o f a scanof a s t a b l e e t a l o n as shown i nf i g u r e s 6 and 7 . The s t a b l ee t a l o nu s e d was a t e m p e r a t u r e s t a b i l i z e d Ge e t a l o nc o m m e r c i a l l yp u r c h a s e dw i t h a f i n e s s e o f 3 and a f r e es p e c t r a lr a n g eo f0 . 0 4 8 cm-I i nt h e 1897 cm-I r e g i o n .E t a l o n f r i n g ep e a k s are t h u s known t o be e q u a l l ys p a c e di nt h e wavenumber doma i n . One c a ne x p e c t a 5% n o n l i n e a r i t y o v e r f o r 100 mA d r i v e and 1 A b i a s .
a 1 cm-l t u n i n g i n t e r v a l
We h a v e s t u d i e d t h e s c a n n o n l i n e a r i t i e s f o r a v a r i e t yo fp u r p o s e s u s i n go u rd i g i t a lc o n t r o l and d a t aa c q u i s i t i o ns y s t e m .F i g u r e 6 shows a ne a r l ys c a no fa ne t a l o nf r i n g ep a t t e r n w h e r et h ed r i v ec u r r e n t at
103
e a c h ‘ f r i n g ep e a kh a sb e e nr e c o r d e d . The p e a ks p a c i n gi nc u r r e n ts t e p s are l i s t e d i n t a b l e 11. This l i s t i n g and t h et u n i n g rate neareachpeak as t h e d r i v e c u r r e n t i n c r e a s e s w h i l e shows t h a t t h e ‘ s p a c i n g i n c r e a s e s t h et u n i n g rate d e c r e a s e s .T h e s er e s u l t s are c o n s i s t e n tw i t ht h ej o u l e 7 shows a d i g i t a lr e c o r do f a f r i n g es c a n h e a t i n gp i c t u r e .F i g u r e o v e r l a y i n gt h e room w a t e r - v a p o rl i n e s . W e h a v ea n a l y z e dt h i sd i g i t a l r e c o r dw i t ho u re t a l o nc a l i b r a t i o ns o f t w a r ei n d i c a t e d i n f i g u r e 2 and as o u t l i n e di nt a b l e 111. This more g e n e r a la n a l y s i si n d i c a t e st h a t i s more c o m p l i c a t e d t h a n t h e q u a d r a t i c e x p e c t a t h et u n i n gn o n l i n e a r i t y tion. W e are c o n t i n u i n go u ra n a l y s i so ft h en o n l i n e a r i t i e s by t r y i n g t of i te v e r yp o i n to ft h ee t a l o nc u r v er a t h e rt h a nj u s tt h ep e a kv a l u e s . Axial Mode S t r u c t u r e A x i a l mode s t r u c t u r e i s importanttotheperformanceof a TDLSS modes a t f o r a t l e a s t t h r e er e a s o n s .F i r s t ,t h ep r e s e n c eo fs e v e r a l t h e same o p e r a t i n g p o i n t r e q u i r e s i n g e n e r a l t h e u s e o f a g r a t i n g monochromator (or o t h e r mode s e l e c t o r )t os e l e c tt h ed e s i r e d mode. The u s eo f a g r a t i n gs p e c t r o m e t e rs e r v e st oi n c r e a s et h ec o m p l e x i t yo f t h e TDLSS, i n t r o d u c et r a n s m i s s i o n loss, and i n t r o d u c eo p t i c a ln o i s e , a l t h o u g h i t i s c o n v e n i e n tf o r making c o a r s e wavenumber i d e n t i f i c a t i o n s . Second,the mode s t r u c t u r e may b es u c ht h a tt h e mode s e l e c t o r may n o t b ea b l et or e j e c tv e r yn e a ro rv e r yf a r modes. T h i r d ,t h es e l e c t e d modemay h a v e f i n e w a v e l e n g t h s t r u c t u r e w h i c h c a n o n l y b e d i a g n o s e d by o t h e rt e c h n i q u e s . The l a t t e r two s i t u a t i o n s would s e r v et od i m i n i s h t h es p e c i f i c i t yo ft h e TDLSS.
We p r e s e n t h e r e some r e p r e s e n t a t i v e r e s u l t s o f s p e c t r a l s c a n s o ft h e TDL outputwhich a r e r e l e v a n tt of i e l dd e p l o y m e n t .F i g u r e 8 shows a TDL mode scanwhichindicatestheverywiderange ( % 38 cm-3 overwhich a mode s e l e c t o r may have t o o p e r a t e . Most TDL emission rangesspanonlyabout 10 t o 15cm-l. Figure 9 shows a mode scanwhich i n d i c a t e st h ec l o s e s ts p a c i n go f modes t h a t may occur ( i . e . about 2.2 cm-’). P o t e n t i a l modes a r ea c t u a l l ys p a c e d by 1.1 cm-’ i n t h i s TDL, b u to p e r a t i o n a t l o w e r b i a s c u r r e n t s c a u s e s e v e r y o t h e r mode t o d r o p o u t . A t c e r t a i no p e r a t i n gp o i n t s ,t h es e l e c t e ds i n g l e mode h a sf i n e s t r u c t u r e .T h i sf i n es t r u c t u r eh a sb e e ns t u d i e di ns e v e r a l ways,includingheterodynemeasurementswith a localoscillator,calibrationetalon we show examplesofsuch s c a n s , and s c a n so fg a sl i n e s .I nf i g u r e1 0 , NO d o u b l e t . The scanherehappenstobethefm/second s c a n so ft h e NO d o u b l e to f R 1 , ( 5 . 5 ) .F i g u r e harmonictransformofthefamiliar 1 0 ( a )i l l u s t r a t e st h e mode s t r u c t u r e( l o w e rt r a c e ) and t h ea b s o r p t i o n l i n es c a n( u p p e rt r a c e ) when a s i n g l e a x i a l mode h a sb e e ns e l e c t e d by t h e monochromatorand when f i n es t r u c t u r e is n o tp r e s e n t .F i g u r e 1 0 ( b )i l l u s t r a t e sa b s o r p t i o nl i n es c a n s when t h es e l e c t e d mode has f i n es t r u c t u r e . Each t r a c e i s t a k e nw i t h a d i f f e r e n to p e r a t i n gt e m p e r a A t y p i c a ls p a c i n g i s 0.075 t u r e andshows d i f f e r e n tf i n es t r u c t u r e . cm- or 2.25 GHz. Heterodynemeasurementswith a 10micron TDL i n d i c a t e d
104
fine structure with separations of 250 MHz. (See figure 11). Scans of a stable etalon also provide information on the fine structureof TDL axial mode from the discontinuity of the fringe pattern. Figure 12 shows a scan of a solid Ge etalon with a finesse of 3 and a free spectral range of 0.047 cm-' using940 a cm" TDL. At the left, the trace has the expected amplitude and shape. However, in the center a discontinuity appears and the amplitude of modulation drops sharply. This suggests theTDL sends out several closely spaced modes at this point. Figure 5a and figure 5c show typical monochromator scans of TDL output after operating point optimization. For a common slit setting (i.e. 0.27 mm or 1.6 cm-'), we typically see about5% of the TDL output in other distant modes. True single mode behavior to the 0.1% level was very seldom observed, as was the mode fine structure described above. Powers up to 201-1W were delivered to the detector in a single mode selected by the monochromator of 25% transmission. DIAGNOSTICS OF TDLSS CHARACTERISTICS DUE TO TDL ENVIRONMENT The characteristics dueto TDL environment included short-term wavenumber stability, the instrument lineshape function, and intermediateterm wavenumber stability. The short-term wavenumber stability was measured by using the heterodyne technique which was described G. by Steinberg (ref. 1). This study w a s carried out usinga TDL chosen to overlap the available CO, -laser lines near 10.6 microns. Figure 13 shows the heterodyne spectrometer and figure 14 shows the results achieved. The two lower traces indicate that the lineshape of the TDLSS is about 10 MHz in half-width (i.e.Q 3 ~ 1 0 -ern-') ~ for averaging times up to several minutes. Single traces at the top left show that the central wavenumber remains constant to about 2 MHz. Single traces at the top right indicate that the wings of the lineshape are due to large wavenumber excursions caused by the residual vibrations from the closed-cycle cooler. These residual vibrations could not be eliminated. The goal of resolving Doppler lines was, however, substantially met. The lineshape study of Doppler and Voigt lines reported by Poultney et al. (ref. 3 ) confirmed the heterodyne measurementsof instrumental lineshape. Figure 15 is the measured shape of a NH, line under Doppler conditions. The curve which connects the data points is the best fit Gaussian profile, with an instrument linewidth ~ of x I O cm-'. -~ The fitting procedure is described by No11 and Pires (ref. 4 ) . Intermediate-term wavenumber stability was estimated by using the TDLSS near5.3 microns which was tuned to the half-width position
105
of NO Rll(5.5), where the slope is the greatest. A sample cellof the spectrometer was filled with 0.9 Torr of NO, a pressure low enough for the absorption line to be Doppler limited, awith half-width at half maximum of about 0.002 cm-'. The vertical deflectionof the recorder pen then corresponds approximately 0.004 to cm-l, orFWHM of the line. The diode current was then adjusted so that the diode wavelength was halfway down the absorption line; the point of maximum sensitivity to wavelength changes. Two traces are shown in figure16. The upper trace shows the intermediate-tern stability was about 0.001 cm-l, withTDL temperature controller gain 50, while the short-term noise was much smaller. The intermediate fluctuation had a period of about 3 minutes. The short-term noise was larger(- 0.0008 cm-' 1, but the intermediate-term noise was smaller,with TDL the temperature controller gain 100.
WAVENUMBER
CONTROL
TECHNIQUE
The use of tunable diode lasers for laser heterodyne spectrometry requires that the issue of wavenumber control be addressed. There are various techniques to counteract the effects of thermal drift and thermal cycling of the laser. Oneof the most promisingmethods, based on counting and interpolating fringes of a stable Fabry-Perot interferometer, is described here. The requirements forTDL wavenumber control imposed by spectroscopic and mission considerations have been summarized in table I. When the laser is turned onremotely, it must be tuned to the desired wavenumber even though its initial wavenumber may be uncertain due to thermal cycling. The tuning accuracy must be f cm-1 to ensure operation at the center ofa Doppler-broadened line. Ideally, a cell containing the gas to be detected would be available at all times to provide a reference absorption line at precisely the correct wavenumber. However, in many casesit is not possible to carry a sample of a particular species with a flight spectrometer. Such tuning accuracy is useful only if the technique can also reduce the intermediate- and long-term drift of thelasertolessthancm-l. Our concept for wavenumber control is depicted with theof aid figure 17. First the TDL wavenumber is swept across its tuning range until an absorption line in a reference gas cell is found. We assume that the TDL wavenumber remains within a mode tuning rangeof the reference line during thermal cycling and operation. The reference line need only be as close as1 or 2 cm-I to the desired line position. Then the TDL mode structureis optimized for operation in this spectral region. The TDL is locked to the gas reference line and the position (in wavenumber space)of its output with respect to the etalon transmission curve is recorded. Next the TDL is tuned across a predetermined number of Fabry-Perot interferometer transmission peaks to the wavenumber
106
of the spectral line of interest. Finally the TDL is stabilized at that wavenumber by monitoring transmission through the etalon, and long-term drift of the etalon is compensated by a separate control system. Detection of reference gas absorption and interferometer fringes is accomplished by modulation of theTDL at a frequency f and modulation index A,. As shown in figure 18, the first and second fmlharmonic signals are available to identify the position in wavenumber space of the laser output. The precise locationof the TDL wavenumberto within one hundredth of a free spectral range of the interferometer requires accurate interpolation between transmission peaks. One simple, elegant method of interpolation uses information found in first and second fmlharmonic detection of the etalon transmission function (also known as the Airy function). For very low valuesof finesse the Airy function is nearly sinusoidal on top a of DC component, resulting in nearly sinusoidal fm/harmonics (see figure19a). Since the first two fm/harmonics have a 90" relative phase difference, their ratio is approximately proportional to the tangent aofquantity which varies linearly with wavenumbero, as shown in figure19(b). Therefore, by computing 8 , the arctangent of the ratio of the two harmonics of the Airy function, one obtains the wavenumber difference between the TDL output and any point on the etalon transmission function, except for an ambiguity due to the periodicity of the fringes. This ambiguity is resolved by counting the number of periods traversed as theTDL is tuned away from the reference line. Variations in TDL power do not present a problem because the ratio of fmlharmonic signals is used. Use of moderate finesse etalons may degrade linearity below acceptable levels. The nonlinearity in the relationship between 8 and u is due to the deviation of the Airy function from a sinusoid. The nonlinearity can be reduced by judicious choice of the modulation index A,. As shown in figure 20, fmlharmonic detection is equivalent to filtering by a Bessel function in Fourier space. Here we use results of our complete analysis of the operation of a wavelength modulated spectrometer (ref. 5 ) . The Fourier transform of the Airy function is a series of impulses at0 , 2L/c, 4L/c, etc., of decreasing magnitude. The impulse at 2L/c represents the sinusoidal component of interest here; the others cause the nonlinearity. Detectionof the first fmlharmonic, H,, results in filtering byJ,(A,); detection of the second fmlharmonic, Hz, results in filtering by J,(A Variation of A , greatly affects linearity, as is apparent from figure 21a. WhenA, is about 0.4 times the etalon free spectral range, a minimum in nonlinearity is achieved. This minimum is not sufficient to allow utilization of a moderate finesse etalon of practical length (e.g. 0.01 cm-,). Further improvement can be obtained by adding a second modulation at frequency f, with modulation indexA,. As shown in figure 20, the second modulation has the effect of filteringJo(by A,), provided
,
107
l l11111l I I 1 I
_I
the detection bandwidth is smaller than I f,-f, I. The value ofA, may be chosen to cancel the contribution of the delta function at 4L/c, the major sourceof nonlinearity. The improvement in interpolation linearity is seen in figure 21b, foran A, of 0.19 times the free spectral range. Less than 1% nonlinearity may be achieved with reasonable etalon plate reflectivities of about 30%. Drift of the etalon transmission curve,e.g. due to thermal expansion, can seriously degrade the accuracy of this technique. With a second laser (TDL21, the drift of the etalon can be monitored and taken into account. If TDL 2 is continuously locked to a reference gas absorption line, its transmission through the etalon can accurately measure etalon drift. Use of fm/harmonic quadrature detectionof TDL-2 transmission makes the monitor signal independent of TDL-2 power. The two lasers can be isolated by having their beams counterpropagating and orthogonally polarized. The detected etalon drift can provide continuous updating of the tuning of TDL 1, or can merely signal the need to repeat theTDL 1 wavenumber acquisition process. In conclusion, a wavenumber control technique capable of meeting the most stringent requirements of a tunable laser heterodyne spectrometer system has been described and analyzed. Straightforward signal analysis principles have been used to adapt the method to practically realizable hardware. The requisite accuracy and long-term stability can be achieved with this system as long as the vernier is etalon stable between updates. Interpolation to 1% of 0.01 a cm-' period etalon achieves the cm-l goal. The control technique and algorithms are discussed in greater detail by Wu and Poultney (ref.6 ) . STABLE
WAVENUMBER
CONTROL
ETALON
Our experience with solid Ge calibration etalons and adjustable air-space Fabry-Perot interferometers has led us to design a special purpose, air-space interferometer for the wavenumber control etalon. Table IV lists the features of this control etalon. An air space etalon eliminates the refractive index inhomogeneity associated with thick germanium or silicon etalons. In addition, the temperature dependence of the index of air can be eliminated by sealing the space, and a spacer can be chosen with a lower coefficient of thermal expansion than thatof a solid etalon.
Our air space interferometer uses a 25-cm reflector spacing to achieve a 0.02 cm-l free spectral range. Figure 22 shows a schematic of the interferometer. Fabricating a 50-cm spacer would yield 0.01 cm- l, which is our goal. The spacer is a fused silica tube, which c1 = 5 provides inherently low thermal expansion. For fused silica, x OC. Only the so-called zero-expansion glasses have lower values,
108
and t h e y are h i g hc o s t s i l ip c ar o v i d setsa b i l iot yf
items n o tr e a d i l ya v a i l a b l ei nt u b e cm-l/"C.
form.Fused
The 2" o u t e r d i a m e t e r was c h o s e n t o b e c o m p a t i b l e w i t h s t o c k f l a t reflectors. The r e f l e c t i n gs u r f a c e s are o p t i c a l l yc o n t a c t e dt ot h e t u b ee n d s ,a s s u r i n g a m o d e r a t e l yr u g g e d ,p e r m a n e n t l ya l i g n e di n t e r f e r ometer. The u s eo fu n c o a t e dr e f l e c t o r sa l l o w sb r o a d b a n dp e r f o r m a n c e . Z i n cs e l e n i d ep r o v i d e s1 7 %r e f l e c t i v i t y and v i s i b l et r a n s m i s s i o nf o r ease ofsystemalignment. Germanium would g i v e t w i c e t h e r e f l e c t i v i t y a t t h ee x p e n s eo ft h i s l a s t f e a t u r e , b u t we h a v ed e t e r m i n e dt h eh i g h e r finesseunnecessary. The o u t e r s u r f a c e s o f t h e r e f l e c t o r s are wedged w i t h r e s p e c t t o t h ei n n e rs u r f a c e s , and d e l i b e r a t e skewingofthe wedges i n b o t h p l a t e s p r e v e n t st h ef o r m a t i o no f a s e c o n de t a l o nc a v i t y . AR c o a t i n g sc o u l d b e u t i l i z e d on t h e o u t e r s u r f a c e s i f e x p e r i m e n t p r o v e s them n e c e s s a r y , b u t a t t h es a c r i f i c eo fb r o a d b a n dp e r f o r m a n c e . With f i v e - m i l l i m e t e rt u b e walls, t h e c l e a r a p e r t u r e is forty m i l l i m e t e r s . Tube f l e xn e g l i g i b l yd i s t o r t st h e low f i n e s s e ,e v e ni ft h e mounting i s s i m p l e . The d e s i g no ft h es t a b l eF a b r y - P e r o ti n t e r f e r o m e t e r i n more d e t a i l by McNallyand P o u l t n e y( r e f . 7).
is described
CONCLUSIONS We c o n c l u d e t h a t t h e c h a r a c t e r i s t i c s o f a v a i l a b l e l a s e r d i o d e s and a s p e c t s of t h e i rp e r f o r m a n c ei nc l o s e d - c y c l ec o o l e re n v i r o n m e n t s which may have made s p a c e f l i g h t s y s t e m s i m p r a c t i c a l c a n b e overcome t h r o u g hr e a l i z a b l ec o n t r o lt e c h n i q u e s .I np a r t i c u l a r , wavenumber p u r i t y and s t a b i l i t yi s s u e sh a v eb e e nt h o r o u g h l yd i a g n o s e d and a c o n c e p t u a l d e s i g nh a sb e e nd e v e l o p e df o r a wavenumber controlsystemwhichcan r e s o l v ep e r f o r m a n c eq u e s t i o n sr e l a t e dt ot h e s e key i s s u e s .
109
REFERENCES 1.
Steinberg, G.N.: Wavenumber Stability of a Laser Diode Mounted in a Closed Cycle Helium Refigerator. Rev. Sci. Instrum., vol. 50, no. 12, Dec. 1979, pp. 1622-1625.
2.
Poultney, S.; Steinberg, G.; Pires, A.; Miller, M.; Chen, D.; Macoy, N.; Noll, R.; and Grossman, W.: Tunable Diode Laser Absorption Spectrometer as the Stratospheric Measurement System for the High Altitude Pollution Program - A Feasibility Study. Perkin-Elmer Report No. 14752,The Perkin-Elmer Corporation, Danbury, CT., 1980.
3.
Poultney, S.; Steinberg, G . ; Noll, R.; Pires, A.; Chen, D.; and Miller, M.: The Position of Water Vapor Lines near 5 Microns, NJ3 Line Shape Measurements, and Comments on Wavenumber Calibration using Tunable Diode Laser Spectrometer. Published in 34th Annual Symposium on Molecular Spectroscopy, Ohio State University, Columbus, Ohio, June11-15, 1979.
4.
Noll, R.; and Pires, A.: A New Nonlinear Least Square Algorithm for Voigt Spectral Lines. Applied Spectroscopy,v o l . 3 4 , no. 3 , May 1980, pp. 351-360.
5.
Pires, A.; Poultney, S.; Chen, D.; and Miller, M.: Theory and Practice of the Wavelength Modulated Spectrometer. Preliminary report published in Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, New Jersey, March 1014, 1980.
6.
Wu, F.; and Poultney, S.: Wavenumber Control Techniques for Tunable Diode Laser Spectrometer. Perkin-Elmer ReportNo. 14762, The Perkin-Elmer Corporation, Danbury, CT., 1980.
7.
Design of a Stable Fabry-Perot McNally, M.; and Poultney, S.: Etalon for TDL Spectrometer Calibration and Control. PerkinElmer Report No. 14763, The Perkin-Elmer Corporation, Danbury, CT., 1980.
110
. . ".
" " "
--'"*-."'---~-
"."...
11
I
-. . 1 "" . 1
,.",
~
..._.....__
TABLE I.- REQUIREMENTS
FORTDL WAVENUMBER
CONTROL
o
Following Turn-On, TDL must Automatically Tune to Desired Wavenumber
o
Tuning Accuracy must be better than + cm-’ (10% of Doppler Width), even if a reference cell with absorption line at desired wavenumber is not available
o
Short-term Drift must be less thanlo-‘ cm-’
o
Intermediate-TermDriftmustbelessthan
o
Long-term Drift must be less than of hours
o
TDL Output at Detector must consist of Single Mode of Adequate Power
cm” cm-
’ for periods
111
TABLE 11.- PEAK SPACING I N CURRENT STEPS AND TUNING RATE NEAR EACH PEAK
TDL Current (Amp)
Fringe
_........
........_ ._.. .
a0 -
a
(cm-l 1
)
1
1.7222
4.0+0.1 -
0.0120 = 0.0003
2
1.7262
3.9
0.0123
3
1.7301
3.9
0.0123
4
1.7340
4.1
0.0117
5
1.7381
3.9
0.0123
6
1.7420
4.1
0.0117
7
1.7461
4.1
0.0117
8
1.7502
4.2
0.0114
9
1.7544
4.2
0.0114
10
1.7586
4.2
0.0114
11
1.7628
4.2
0.0114
12
1.7670
4.3
0.0112
13
1.7713
4.3
0.0112
14
1.7756
112
. .".
AI (ma 1
....._... . .
,
I.. ., I
/ma
TABLE 111.- PROCEDURE FOR WAVENUMBER SCALE CALIBRATION
1. Find Etalon Peak Locations o
locate peak vicinity
o
quadratic fit fringe to locate fringe center
2. Expand peak positions in nth order polynomial o
assign fringe number to peak
o
least-squares fit nth order polynomial y=a
0
+ax+ax2+... 1 2
where x
=
fringe position
y = fractional fringe number 3 . Calibrate o
wavenumber scale using known FSR and reference line
a(x> = FSR
*
2
[ao ref+ a 1 x + a2 x
...I
+
0
113
l1l1111l1l I
TABLE 1V.- ETALON
FEATURES
o
Inherent Temperatue Stability - lom3 cm-’/OC with fused silica spacer
o
Short Free Spectral Range - 0.02 cm-I with 25 cm cavity
o Permanent o Simple
o Large
Alignment- Solid Assembly of Reflectors to Spacer
System Insertion- Transparent in Visible for Alignment
o Broadband
114
DESIGN
Reflectivity- Available through Fresnel Reflection
Aperture- 40 mm C.A.
o
Freedom from Secondary Ref lectors
o
Freedom from Index Air-Space Design
Etalon
Effects - Uses
Wedged
Inhomogeneity - Assured by Closed
Laser Diode
Compressor
I
-
Grating Monochromator
k
I
Dw a r
Calibration Cell
-
('
Chopper kHz) Etalon
w d supplySync
Digital Control Box
5ca
II
Control
"
Bias Circuit
Temperature Controller
Diode Power
Sample Cell
di'll Detector Detector
-
--C
Ampl.
-
I
J
Clock
Lock- In and Harmonic Detector
-
Kennedy Formatter I
1
Deck
-
Digitizer
f Clock
x
Tape Recorder
1T-
I
Kennedy
1
Figure 1.- Tunablediode l a s e ra b s o r p t i o ns p e c t r o m e t e r ;u s e s all r e f l e c t i v e o p t i c s and can p r o v i d e g r a p h i c a l o r IBM-compatible d i g i t a l r e c o r d s .
~ Etalon & Calib. 0% Line
~
;
~
Quick Look Plotting (Every nth P o i n t )
Read Tape
~
Master Data
Calib. Sam l e
Read StorelRead Spectral
Constsnts
Spectral Simulation
Spectral Constants
l8 , H'
Spectrdl
,
I"
I
Calibrated
"I
s ;s i;;
I
I
m L,r.
I
,
Calibration Ralon Frequency
Figure 2 . -
Data Transmission
"^._.. m - M Line
I
1 MHz) c h a r a c t e r i s t i c sa r ea l s oi m p o r t a n t .T h e r e f o r e , we have s t u d i e d e highfrequencynoisecharacteristicsofthe TDL a s a p a r t of a f u l l TDL a r a c t e r i z a t i o n programwhich has beenimplemented a t Langley Research Center r t h e improvement of t h e TDL a s a l o c a l o s c i l l a t o r i n t h e LHS system. I n t h i s udy which i n v o l v e d t h e c h a r a c t e r i z a t i o n o f T D L ‘ s from two commercialsources a s e r A n a l y t i c s and New EnglandResearchCenter) and one p r i v a t e s o u r c e e n e r a l MotorsResearch L a b o r a t o r y ) , i t hasbeenobserved that all the devices owed s i m i l a r h i g h f r e q u e n c y n o i s e c h a r a c t e r i s t i c s eventhoughtheywere all n s t r u c t e du s i n gd i f f e r e n tt e c h n i q u e s . W e w i l l now r e p o r tt h e s e common high equency n o i s e C h a r a c t e r i s t i c s . ”
EXPERIMENTAL DETAILS
The TDL n o i s e c h a r a c t e r i s t i c s f a c i l i t y i s shown i n f i g u r e 1. The TDL i s unted i n a commercial c l o s e d c y c l e c o o l e r which has been m o d i f i e d t o minimize equency f l u c t u a t i o n s which a r e inducedduringthecoolingcycle.Radiation is upledthrough a wedged AR coated ZnSewindow t o a 37 nun f / l Ge l e n s which l l i m a t e s t h e TDL emission. T h i s c o l l i m a t e d beam p a s s e s e i t h e r t o a highspeed CdTe d e t e c t o r ( I F bandwidth 2 GHz) o rt o a monochromator. The monochromator s used t o i n v e s t i g a t e t h e s p e c t r a l c h a r a c t e r i s t i c s ofthe TDL o u t p u t . The gh s p e e d d e t e c t o r was used t o measure t h e h i g h f r e q u e n c y n o i s e c h a r a c t e r i s t i c s t h e TDL o u t p u t . The detectorhighfrequencyoutput was a m p l i f i e d by two scaded I F a m p l i f i e r s whichwere followed by a square law d e t e c t o r u s e d t o
I
1 29
r e c t i f y and i n t e g r a t e t h e I F o u t p u t . The low frequencyportion of t h e d e t e c t o r output and t h e i n t e g r a t e d I F were monitoredon a dualchanneloscilloscope. At times,thefrequencydistributionofthe I F w a s displayed on a spectrum analyzer. The high frequency noise present i n t h e TDL o u t p u t f o r t h e numerous devices, studiedcan be c a t e g o r i z e da c c o r d i n gt of r e q u e n c yc h a r a c t e r i s t i c s .F i g u r e2 ( a ) shows a t y p i c a l example of what we have c a t e g o r i z e d as l o w frequencynoise ( 30 which i s m a i n t a i n a b l e f o r several lays.These t e s t s a l s od e m o n s t r a t et h a tt h ee t a l o ns y s t e m i s c a p a b l eo fr e s o n AV -7 2 x 1 0d u r i n g similar time p e r i o d sW . i t hc u r i n c ef r e q u e n c ys t a b i l i t y V
l e v e l ofperformance - e n t l ya v a i l a b l em i r r o rc o a t i n g s ,t h i s :n o p t i c a l b a n d w i d t h Ah = 3 um c e n t e r e d a t X = 10 pm.
i s a c h i e v a b l eo v e r
INTRODUCTION
W i t ht h ei n c r e a s e dc o n c e r na b o u tp o s s i b l e damage t o t h e a t m o s p h e r e b y c h e m i c a lp o l l u t i o n , much i n t e r e s t h a s a r i s e n i n t h e c h e m i s t r y o f t h e s t r a t o s p h e r e .S e v e r a li m p o r t a n tc o n s t i t u e n t sw h i c h a r e p r e s e n ti no n l y t r a c e amounts, s u c ha so z o n e 0 ( r e f e r e n c el ) ,c h l o r i n em o n o x i d e C 1 0 ( r e f e r e n c e s 2 and 3 ) , and 3 t h ev a r i o u sc h l o r o f l u o r o c a r b o n s( r e f e r e n c e 4 ) h a v ep r o m i n e n ta b s o r p t i o n - e m i s s i o n s p e c t r a in t h e 8 t o 1 2 urn w a v e l e n g t hr e g i o n . Remote s e n s i n go ft h e s ea n d many o t h e r g a s e s c a n b e a c c o m p l i s h e d w i t h i n f r a r e d h e t e r o d y n e s p e c t r o m e t e r s u s i n gt u n a b l ed i o d el a s e r s (TDL) as l o c a l o s c i l l a t o r s (LO) ( r e f e r e n c e s 5 and 6 ) . " D L ' Se x h i b i tn a r r o ws p e c t r a le m i s s i o nw i d t h sw h i c h are piecewise-continuo u s l yt u n a b l eo v e rm o r et h a n1 0 0w a v e n u m b e r s( r e f e r e n c e s 7 , 8 , 9 , and 1 0 ) . W h i l es u c ht u n a b i l i t yg r e a t l yi n c r e a s e st h es p e c t r a lc o v e r a g eo b t a i n a b l ew i t h a s i n g l e - T D Ls p e c t r o m e t e r , i t introduces a d e g r e eo fu n c e r t a i n t yi nt h e LO f r e quencywhich i s n o tp r e s e n t when a l o w - p r e s s u r eg a s l a s e r i s u s e d .I ns t u d i e s o f s p e c t r a l l yn a r r o wf e a t u r e ss u c h as s t r a t o s p h e r i c g a s a b s o r p t i o n l i n e s , t h i s LO f r e q u e n c y u n c e r t a i n t y c a n h a v e s e r i o u s e f f e c t s on m e a s u r e m e n t a c c u r a c y ( r e f e r e n c e1 1 ) . It i s o f t e nn e c e s s a r yt od e t e r m i n et h e LO f r e q u e n c yw i t h a r e s o l u AV -7 5 MHz a t X = 1 0 . 6 um). t i o n - = 1 . 8 x 1 0( a b o u t V
"Thiswork
w a s s u p p o r t e d by NASA, L a n g l e yR e s e a r c hC e n t e r ,
Hampton, V i r g i n i a 23665.
1 43
Ill I l l
WAVELENGTH I D TECHNIQUES
Measurementsof "DL o u t p u t p a r a m e t e r s s u c h as w a v e l e n g t hv e r s u sd r i v ec u r rent have generally shown l a r g e t e m p o r a l v a r i a b i l i t y r e n d e r i n g w a v e l e n g t h ID v i a d r i v ec u r r e n tc a l i b r a t i o nn e a r l yu s e l e s s( r e f e r e n c e s 7 a n d1 0 ) . A successf u l meansofaccuratewavelengthdetermination w i l l r e l y as l i t t l e a s p o s s i b l e onthestabilityofthe TDL.
Anotherdevicecapableofhighwavelengthresolution is t h eF a b r y - P e r o t e t a l o n( F P E ) ,w h i c h i s a no p t i c a l l yr e s o n a n tc a v i t yf o r m e db y two p a r a l l e l p l a n ep a r t i a lr e f l e c t o r s .M i r r o rc o a t i n g sp r e s e n t l ya v a i l a b l ec a na l l o wa n FPE t o o p e r a t e w i t h a f i n e s s e F > 30 t h r o u g h o u t a s p e c t r a l i n t e r v a l AX = 3 um n e a r X = 10 um. U n t i lr e c e n t l yt h e s ed e v i c e s were n o t s t a b l e enough t o p e r m i t their potentially high resolution to be used for TDL w a v e l e n g t h c a l i b r a t i o n . An e t a l o n h a s b e e n d e s i g n e d a n d c o n s t r u c t e d w h i c h o v e r c o m e s t h e s e s t a b i l i t y p r o b l e m s ,m a i n t a i n i n g a f i n e s s e F = 33 +3 f o r a p e r i o d o f 2 d a y s ,a n ds h o w i n g r e s o n a n c ef r e q u e n c ys t a b i l i t yo f +10 o v e r 4 h o u r s . The d e s i g n i s c a p a b l e o fe v e nl o n g e r term s t a b i l i t y .
MHz
144
H I G H RESOLUTION TUNABLE FPE
The e t a l o n e m p l o y s a c a p a c i t a n c e - b r i d g e s e r u o t e c h n i q u e t o m a i n t a i n m f r z o r Farallelism and t op r o v i d e . c o n t i n u o u sp a s s b a n dt u n i n g( r e f e r e n c e s 1 3 , 14, and 15). The FPE u n i q u e l y u s e s a C e r - V i t m i r r o r s p a c e r a s t h e p r i m a r y l e n g t h s t a n d a r d f o rt h eo p t i c a lc a v i t y . ( C e r - V i t i s manufacturedbyOwens-CorningFiberglas C o r p o r a t i o n . ) The t h e r m a ls t a b i l i t yo ft h es p a c e rr e d u c e st h en e e df o rp r e c i s e t e m p e r a t u r ec o n t r o lo ft h ee t a l o nt o a manageable l e v e l . The m i r r o r sw h i c h formtheopticalcavity are a t t a c h e dt op i e z o e l e c t r i ce l e m e n t s (PZT) whichcan a x c u r a t e l yv a r yt h ec a v i t yl e n g t ha n d ,t h e r e f o r e ,s p e c t r a l l yt u n et h ee t a l o n . The m i r r o r s , w h i c h f o r m t h e e t a l o n shown s c h e m a t i c a l l y i n F i g u r e 1, c o n s i s t c fm u l t i l a y e rd i e l e c t r i cc o a t i n g sd e p o s i t e do n5 - c m - d i a m e t e r ZnSe s u b s t r a t e s . They have a r e f l e c t i v i t y R > 0 . 9 0 o v e r t h e s p e c t r a l i n t e r v a l 8 . 9 pm 5 X 1 1 . 2 pm. The s u b s t r a t e s a n t i r e f l e c t i o nc o a t e do no n es i d ea n dh a v e small wedge a n g l e s . The p l a t e s a r e matched i n f l a t n e s s t o b e t t e r t h a n A/175 a t X = 1 0 . 6 pm. The s p a c e r i s a 4-cm-long c y l i n d e rw h o s es u r f a c e s were ground f l a t a n dp a r a l l e lt on e a r l y X/lOOO a t X = 1 0 . 6 pm.
are
G o l dh a sb e e ns p u t t e r e do n t ot h em i r r o ra n ds p a c e rs u r f a c e si n a pattern 2 . When t h em i r ' r o r sa r ep o s i t i o n e dc l o s et +t h es p a c e r ,t h e o p p o s i n gg o l dp a d sf o r mp a r a l l e lp l a t ec a p a c i t o r s . The p a d so nt h es p a c e r are s l i g h t l yo v e r s i z e dt of a c i l i t a t em i r r o r - s p a c e ra l i g n m e n t . The g a p sb e t w e e nt h e m i r r o r sa n dt h es p a c e r a r e m a n u a l l yp r e s e tt o a nominal50 pm. 1
shown i nF i g u r e
E a c hm i r r o ro ft h e FPE i s i n d e p e n d e n t l y c o n t r o l l e d b y a n e l e c t r o n i c S e r v o mechanism. The two p a i r so fo r t h o g o n a lc a p a c i t o r s ,a r b i t r a r i l yd e s i g n a t e d X1 - X and Y - Y2, a r e u s e dt om a i n t a i nm i r r o r - s p a c e rp a r a l l e l i s m ,t h e r e b ya l s o
2
1
i s a c c o m p l i s h e db yb a l a n c i n gt h ep a i r s a s s u r i n gm i r r o r - m i r r o rp a r a l l e l i s m .T h i s i n s e p a r a t ec a p a c i t a n c eb r i d g e s . A s t h em i r r o r sd r i f to u to fp a r a l l e l ,t h e resultingbridgevoltages a r e s e n s e da n du s e dt od r i v et h eP Z T ' ss u p p o r t i n gt h e ' r r o r s .C a v i t yl e n g t hc o n t r o la n dt h e r e f o r er e s o n a n c ef r e q u e n c yc o n t r o l i s perby b a l a n c i n g t h e t w o u n p a i r e d Z - c a p a c i t o r s a g a i n s t f i x e d - v a l u e r e f e r e n c e a c i t o r s i n two o t h e rb r i d g e s . The f r e q u e n c ys t a b i l i t yo ft h ee t a l o n i s ded e n to nt h es t a b i l i t yo ft h ef i x e dr e f e r e n c ec a p a c i t o r sa n dt h e Cer-Vit
S i n c e t h e FPE m i r r o r s a r e i n d i v i d u a l l y c o n t r o l l e d , s i x h i g h - r e s o l u t i o n a n c eb r i d g e s a r e r e q u i r e dt of o r mt h es e r v os y s t e m . A b l o c kd i a g r a mo f c t r o n i c su s e dt oc o n t r o lo n eo ft h em i r r o r s i s shown i n F i g u r e 3 ( r e f 13). E a c ho ft h et h r e eb r i d g e sh a s a b a l a n c e - c o n t r o lt oc o m p e n s a t ef o r ' raycapacitancesarisingfromeithertheetalonstructureortheservoitself. ! e c t r a l t u n i n go ft h e FPE i s a c c o m p l i s h e d by c o m b i n i n g t h e o u t p u t o f t h e bridgewithanadjustableoffsetvoltage;thecombinationthen 1 is usedto C r i v et h eP Z T ' s .T h i st u n i n gm e t h o d is r e a d i l ya d a p t a b l et od i g i t a lc o n t r o l i Id a l l o w s c o n t i n u o u s s p e c t r a l c o v e r a g e w i t h i n limits s e t b y t h e maximum PZT T Ition.
1 45
I-
-
i n gt ot h ee x p r e s s i o n :
dR - dC "n &
C
( E q u a t i o n (1) a s s u m e s t h a t t h e r e i s a n e g l i g i b l e area d i f f e r e n c e b e t w e e n t h e p a i r so fc a p a c i t o r s . ) A l s o , c h a n g e si nt h ec a v i t yl e n g t h L a r e r e l a t e dt o c h a n g e si nt h ep a dg a p sb y :
a r i s e s when b o t h p l a t e s The f a c t o r o f 1 1 2 dL. F o ro n ep l a t ef i x e d ,t h er e l a t i o nr e d u c e st o
a r e t r a n s l a t e d by t h e same d i s t a n c e dR = dL. To a c h i e v et h e -7 1 . 8 x 1 0 , i t c a nb e shownfrom t h ee t a l o n
d e s i r e dw a v e l e n g t hr e s o l u t i o no f r e s o n a n c ec o n d i t i o nt h a t :
"
-8 L
C
F o rt h ec a s ei nw h i c h L = 4 cm and R = 50 um, t h e n e c e s s a r y c a p a c i t a n c e b r i d g e r e s o l u t i o nc o r r e s p o n d i n gt oa n FPE r e s o n a n c ef r e q u e n c yr e s o l u t i o no f 5 MHz dC -5 X = 10.6 pm i s - = 7 . 2 x 1 0
at
C
.
Stabilizationofan FPE w i t h s u c h h i g h p r e c i s i o n r e q u i r e s s p e c i a l c o n s i d e r a t i o no ft e m p e r a t u r ea n dp r e s s u r ee f f e c t s .S i n c et h es p a c e r i s t h el e n g t hs t a n dard of t h ec a v i t y , i t m u s tb ed i m e n s i o n a l l ys t a b l et ot h e same d e g r e e as t h e d e s i r e df r e q u e n c yr e s o l u t i o n .S i n c e Cer-Vit h a sa ne x p a n s i o nc o e f f i c i e n to f
-6
-
0
0.12 x lo t e m p e r a t u r es t a b i l i t yo fa b o u t 1 . 5 C i s n e c e s s a r yt oa c h i e v et h e cm"C d e s i r e dr e s u l t .T h i se t a l o n i s mounted i n a vacuumchamber t oe l i m i n a t et h e effectsofatmosphericpressurevariationsonthedielectricconstant of t h e c a p a c i t o rg a p sa n dt h ei n d e xo fr e f r a c t i o no ft h e a i r b e t w e e nt h em i r r o r s .
MeasuredPerformance
The measurements of t h e FPE f i n e s s e a n d f r e q u e n c y s t a b i l i t y a t A = 10.6 pm were p e r f o r m e du s i n gt h es e t u p shown i n F i g u r e 4 . The c o l l i m a t e do u t p u to ft h e HeNe l a s e r w a s d i r e c t e d t h r o u g h t h e e t a l o n t o a p y r o e l e c t r i cd e t e c t o r . As the s p a c i n gb e t w e e nt h em i r r o r s i s v a r i e d by t h e P Z T ' s , p e a k s i n t h e o u t p u t o f t h e d e t e c t o ro c c u rw h i c hc o r r e s p o n dt oc a v i t yl e n g t hc h a n g e so f A12 o r 0.3164 urn. The v o l t a g ee x p a n s i o nc o e f f i c i e n to ft h e PZT's is t h e ne a s i l yc a l c u l a t e d . 146
I
ecausethemirrors are notoptimizedforoperation at visiblewavelengths,the e a k s t h a t are observed are n o tv e r ys h a r p ,i n t r o d u c i n ga ne r r o ro fa b o u t1 0p e r e n ti n t ot h em e a s u r e m e n to ft h ee x p a n s i o nc o e f f i c i e n t . The o u t p u to f a waveu i d e CO2 laser o p e r a t i n g a t X = 10.588 pm w a s t h e n d i r e c t e d t h r o u g h t h e e t a l o n i t h a n FPE p a s s b a n d t u n e d a c r o s s t h e l a s e r o u t p u t b y PZT v o l t a g e a d j u s t m e n t . hemeasuredtransmissionversuslengthchangecan be u s e d t o estimate t h e i n e s s eb e c a u s et h ec a v i t yL e n g t h i s known. F i g u r e 5 showstwo c u r v e so f calu l a t e dp a s s b a n ds h a p ef o r two v a l u e s of f i n e s s e a n d PZT e x p a n s i o n c o e f f i c i e n t . l s o shown a r e d a t a p o i n t s m e a s u r e d o n two o c c a s i o n s two d a y s a p a r t . The m e a s u r e m e n t o f t h e f r e q u e n c y s t a b i l i t y o f t h e FPE was p e r f o r m e d u s i n g e same o p t i c a l s e t u p b y t u n i n g t h e FPE s o t h a t a h a l f - t r a n s m i s s i o n p o i n t o f a s s b a n do v e r l a p p e dt h e CO l a s e r o u t p u t . The d e t e c t e d l a s e r p o w e rt r a n s m i t t e d t h e e t a l o n w a s m o n i t o r e 2b y a s t r i p - c h a r t r e c o r d e r . A d j u s t i n g t h e FPE t o a s s b a n ds k i r ti n c r e a s e s i t s f r e q u e n c yd i s c r i m i n a t i o n . Thelong-termfrequency abilityofthe laser i s b e l i e v e d t o b e b e t t e r t h a n 3 MHz and i t s o u t p u t power s c o n t i n u o u s l ym o n i t o r e d . The FPE wasmounted i n a vacuumchamber,but its m p e r a t u r e was n o t a c t i v e l y c o n t r o l l e d . F i g u r e 6 shows t h e m e a s u r e d r e s o n a n c e f r e q u e n c y s t a b i l i t y o f t h e e t a l o n . e d a s h e dc u r v es h o w st h a tt h ef r e q u e n c yd e v i a t i o nw a s l e s s t h a n 1 0 MHz o v e r p e r i o do f 4 h o u r si nt h ee a r l ya f t e r n o o n . The s o l i dc u r v e shows a c o n t i n u o u s i f t t h r o u g ht h ee v e n i n g .T h i sd r i f t i s caused by a d e c r e a s ei nc e r a m i cr e f e n c ec a p a c i t o rt e m p e r a t u r e a s t h ea m b i e n tl a b a i r cooled.Measurementsof acer t e m p e r a t u r e d u r i n g t h i s p e r i o d i n d i c a t e d a stabilityofbetterthan l0C. e f i g u r e c l e a r l y shows t h a t t h e FPE i s s p e c t r a l l y s t a b l e t o a n u n p r e c e d e n t e d g r e ep r o v i d e ds u f f i c i e n tr e f e r e n c ec a p a c i t o rt h e r m a ls t a b i l i t y is achieved. improved a i r g a p r e f e r e n c e i s b e i n gd e v e l o p e d .
WAVELENGTH I D WITH AN FPE
a p o s s i b l e TDL w a v e l e n g t h c a l i b r a t i o n s y s t e m u s i n g The s c h e m a t i cd i a g r a mo f - r e s o l u t i o n FPE i s shown i n F i g u r e 7 . The o u t p u to f a TDL i s f i r s t p r e ed t o i s o l a t e a s i n g l ef r e q u e n c y mode. P a r t of t h i s power i s d i r e c t e d h t h e FPE t o a d e t e c t o r w h o s eo u t p u t i s t h e n f e d b a c k t o c o n t r o l t h e TDL a t u r e o r d r i v e c u r r e n t .D e p e n d i n go nt h ea v a i l a b l e TDL power, FPE ters, a n dc a l i b r a t i o nd e t e c t o rs e n s i t i v i t y , i t may b en e c e s s a r yt oi m p r e s s i g h tf r e q u e n c ym o d u l a t i o no nt h e TDL o u t p u t t o e n h a n c e c a l i b r a t i o n a c c u r a c y . modulationwouldhave no s i g n i f i c a n t e f f e c t o n t h e laser's performance as
is a b s o l u t e l y n e c e s s a r y t o p r e f i l t e r t h e TDL o u t p u t ; t h i s c a n b e d o n e small monochromator.Theremovalofextraneous "DL f r e q u e n c y modes i s edby t h es p e c t r o m e t e rt oa v o i dm o d a ls e l f - b e a t i n ge f f e c t sa n dp o s s i b l e i g u i t y( r e f e r e n c e 10). The bandpassofthismonochromatorcanbe as -1 as a TDL mode w i d t h (%lcm ), i fn e c e s s a r y ,b u t some a d d i t i o n a l f i l t e r i n g required in the calibration system to position the TDL o u t p u t w i t h i n a u l a r h i g h - r e s o l u t i o n FPE p a s s b a n d . 1 47
I Il1Il I I
H i g h - f i n e s s e ,l o n g - c a v i t ye t a l o n sn e e ds p e c i a la t t e n t i o nf o rt h e i r beam q u a l i t yr e q u i r e m e n t s .B o t h beam d i v e r g e n c ea n dv a r i a t i o n so fa n g l eo fi n c i d e n c c m u s t b em i n i m i z e dt oe x t r a c tt h eb e s t FPE s y s t e mp e r f o r m a n c e .F o rt h ee t a l o n p r e v i o u s l y d e s c r i b e d , t h e beam d i v e r g e n c e h a l f - a n g l e s h o u l d b e less t h a n a f e w t e n t h s of a d e g r e e t o a v o i d e x c e s s i v e r e s o n a n c e f r e q u e n c y s h i f t s a n d f i n e s s e d e g r a d a t i o n .S i n c et h ee t a l o n i s used a t n o r m a li n c i d e n c e , its s e n s i t i v i t yt o c h a n g e si ni n c i d e n c ea n g l e i s somewhat r e d u c e d ,b u tt h e s ec h a n g e ss h o u l d still b ek e p tb e l o w several t e n t h s o f a d e g r e e t o a v o i d t o o much f r e q u e n c y s h i f t .
CONCLUSIONS
F o rt u n a b l ed i o d e lasers t o r e a l i z e t h e i r p o t e n t i a l i n h i g h - r e s o l u t i o n h e t e r o d y n es p e c t r o s c o p y , some m e a n s o f a c c u r a t e l y d e t e r m i n i n g t h e i r e m i s s i o n f r e q u e n c y i s r e q u i r e d .I ng e n e r a l ,c h a r a c t e r i s t i c so f a TDL a r e n o ts u f f i c i e n t s t a b l eo v e rt h el o n g term t oa l l o wt h e i ro n e - t i m ec a l i b r a t i o n . Several p o t e n t i c a l i b r a t i o nt e c h n i q u e sh a v eb e e nd i s c u s s e dw h i c hh a v ev a r y i n gd e g r e e so ff l e x i b i l i t ya n da c c u r a c y . Any of t h e s em e t h o d sc o u l db ei n c o r p o r a t e di n t o a spectrc e t e r a s a subsystem. A F a b r y - P e r o te t a l o nh a sb e e nd e v e l o p e dw h i c h is capable o f c a l i b r a t i n g a TDL o u t p u t w i t h a r e s o l u t i o n o f a b o u t 5 MHz a t w a v e l e n g t h s ne2 1 0 pm a n dm e a s u r e dd a t ah a sb e e np r e s e n t e d .T h i sd e v i c ec a nf o r mt h eb a s i so f a h i g h l y f l e x i b l e and a c c u r a t e w a v e l e n g t h I D s y s t e m .
148
REFERENCES
1.
R. A. M c C l a t c h e y , W. S . B e n e d i c t , S . A. C l o u g h , D. E. B u r c h , R . F. Calfee, K. Fox, L.S.Rothman,and J. S .G a r i n g , AFCRL A t m o s p h e r i cA b s o r p t i o n L i n eP a r a m e t e r sC o m p i l a t i o n , AFCRL-TR-73-0096 (1973).
2.
R . T . M e n z i e s , J. S .M a r g o l i s , 1 6 5, 2 3( 1 9 7 7 ) .
3.
R . S. Rogowski, C. H. B a i r , W. R . Wade, J. M. AppO l p1 t 7 1, 3 0 1( 1 9 7 8 ) .
4.
J. M. H o e l l , C . H. Bair, C. Harward,and Dec 1 9 7 9 .
5.
M. A. F r e r k i n ga n d
6.
R.
7.
G . A. A n t c l i f f ea n d
8.
K . J. L i n d e n , K . W . N i l l , a n d J . F. B u t l e r , I E E E J-QE
T . Ku a n d D. L.
E. D . H i n k l e y ,a n d
R. A. T o t h ,A p p lO p t
H o e l l ,a n d
G . E. C o p e l a n d ,
B. Williams, Geophys R e s L e t t 1 2 ,
D. 3 . M u e h l n e r ,A p p lO p t1 6 ,5 2 6( 1 9 7 7 ) . S p e a r s , I E E E J-QE QE-13,
72D ( 1 9 7 7 ) .
S . G . P a r k e r , J . A p p lP h y s4 4 ,4 1 4 5( 1 9 7 3 ) .
QE-13,
7 2 0( 1 9 7 7 ) .
J r . , a n d D . E. S w e t s , J . Appl P h y s 4 7 2, 6 7( 1 9 7 6 ) .
, 9.
W.
L o , G. P.Montgomery,
10.
M.
G.
11.
R . K . S e a l sa n d B. J . P e y t o n ,P r o c e e d i n g so f t h eI n t e r n a t i o n a lC o n f e r e n c e Vol 1, p 1 0 - 4 ,S e p t1 9 7 5 . o nE n v i r o n m e n t a lS e n s i n ga n dA s s e s s m e n t ,
12.
A. S a n c h e z , C . F. Davis, J r . , CC. . 5 2 7 0( 1 9 7 8 ) .
13.
T. R. H i c k s , (1974).
Reay,and
R.
J . S c a d d e n , J . P h y s E:
14.
T. R. H i c k s , N . K. Reay,and (1976)
C.
L . S t e p h e n s ,A s t r o na n dA s t r o p h y s
15.
R. V.
B. J. P e y t o n , NASA C o n t r a c t o rR e p o r t1 5 9 1 2 1 .
S a v a g e , R . C . A u g e r i ,a n d
N.
K.
.
J o n e sa n d
J. C.
S.
L i u ,a n d
R i c h a r d s , 3. P h y s E:
A.
J a v a n , J . A p p lP h y s4 9 ,
SCI INSTR 7 ,2 7 ,
51, 367
SCI INSTR 6 ,5 8 9( 1 9 7 3 ) .
149
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A REVIEW OF THE NASA/OAST CRYOGENIC COOLERS TECHNOLOGY PROGRAM J . G. Lundholm, J r . NASA H e a d q u a r t e r s A l l a nS h e r m a n G o d d a r dS p a c eF l i g h tC e n t e r
SW R Y
An ongoing NASA r e s e a r c h a n d t e c h n o l o g y p r o g r a m p r o v i d e s f o r t h e d e v e l o p ment of l o w a n d u l t r a - l o w t e m p e r a t u r e c r y o g e n i c c o o l e r s y s t e m s f o r f u t u r e s p a c e m i s s i o n s .T h e s ei n c l u d em e c h a n i c a l ,s o l i dc r y o g e n ,g a sa d s o r p t i o n ,s u p e r f l u i d h e l i u m ,h e l i u m - 3 ,a n dm a g n e t i c( a d i a b a t i cd e m a g n e t i z a t i o n )c o o l e r s . a fewweeks f o r a S h u t t l e / S p a c e l a b O p e r a t i n gl i f e t i m e sr e q u i r e dv a r yf r o m Temperam i s s i o nt o as l o n g as n i n ey e a r sf o rm i s s i o n st ot h eo u t e rp l a n e t s . A t t h eh i g h e rt e m p e r a t u r er e q u i r e m e n t sv a r yf r o mt e n s, t ot e n t h so fk e l v i n . t u r e ,c o o l i n gl o a d sf o rd e t e c t o r s ,i n s t r u m e n t sa n da s s o c i a t e ds h i e l d s may b e as h i g h a s 1 5w a t t s . A t t h eu l t r a - l o wt e m p e r a t u r e s ,c o o l i n gl o a d sm u s tb el i m i t e d A l l s y s t e m sm u s ts u r v i v el a u n c hl o a d sa n dt h e nf u n c t i o n t o t e n s o fm i c r o w a t t s . i nt h ez e r og r a v i t ye n v i r o n m e n to fs p a c e . a v a r i e t y of T h er e q u i r e m e n t so fl o n g - l i f e t i m e ,z e r o - go p e r a t i o na n d t e m p e r a t u r e sa n dc o o l i n gl o a d sp r e s e n tc o m p l e xt e c h n o l o g i c a lp r o b l e m s . Much p r o g r e s s i s b e i n g made i n s o l v i n g t h e s e p r o b l e m s . A l a r g e numberof p r o j e c t e d NASA m i s s i o n s w i l l u t i l i z e i n s t r u m e n t s t h a t r e q u i r et h ec r y o g e n i cc o o l e r s .T h e s ei n c l u d ee a r t hr e s o u r c e s ,h i g he n e r g y a s t r o n o m y ,i n f r a r e da s t r o n o m y ,a n dg r a v i t a t i o n a lp h y s i c s .
INTRODUCTION
The u t i l i z a t i o n a n d a p p l i c a t i o n o f l o w a n d u l t r a - l o w t e m p e r a t u r e d e v i c e s a t a r a p i dp a c e .T h i s i s t r u eb o t hf o r NASA a n ds y s t e m sh a v eb e e ni n c r e a s i n g and DOD s p a c e f l i g h t p r o g r a m s a s w e l l a s many r e s e a r c h l a b o r a t o r i e s o u t s i d e of NASA. P l a n n i n gf o rf u t u r e NASA m i s s i o n si nt h en e x td e c a d ei n d i c a t e s i n c r e a s i n g u s e of l o w a n d u l t r a - l o w t e m p e r a t u r e s e n s o r s a n d i n s t r u m e n t s . A l t h o u g hs o m et y p e so fc o o l e r sh a v eb e e nf l o w no n a fewmissionstodate, at the noneofthecoolerscanoperateforthedesiredlifetimeinzero-gand NASA i s e x t r e m e l yl o wt e m p e r a t u r e sr e q u i r e di n some cases. F o rt h i sr e a s o n , c o n d u c t i n g a b r o a d - r a n g ec o o l e rR e s e a r c ha n dT e c h n o l o g y (RhT) p r o g r a m .T h i s w i l l a s s u r et h a tt h er e q u i r e dt e c h n o l o g y w i l l b e available i n t i m e t os u p p o r t future missions. 153
T h eo b j e c t i v e so ft h i sp a p e r are to: (1) a c q u a i n tt h er e a d e rw i t ht h e presentcoolerprogrammanagedbytheOffice of AeronauticsandSpaceTechnol o g yI n f o r m a t i o nS y s t e m sO f f i c e ,a n d ( 2 ) d e s c r i b ec h a n g e si np r o g r a mt h r u s t f r o mt h a td i s c u s s e d i n p r e v i o u sp a p e r s( R e f . 1 and 2 ) . T h en e x ts e c t i o n will discussthegeneralrequirementsforlowandultra-lowtemperatureandtechniq u e st h a tc a nb ee m p l o y e dt oa c h i e v et h e s et e m p e r a t u r e s a t e x p e c t e dh e a tl o a d s . Thispartofthepaper w i l l be f o l l o w e d w i t h a d i s c u s s i o n o f t h e NASA C e n t e r s involvedinthe R&T e f f o r t s a n d t h e t y p e of c o o l e rs y s t e m so nw h i c ht h e y are f o c u s i n gt h e i re f f o r t . A b r i e fd e s c r i p t i o na n dt h r u s to f R&T e f f o r t w i l l t h e n b e g i v e n of e a c hc o o l e rt y p e .
COOLING REQUIREMENTS
T h ee x p e c t e dh e a tl o a d st h a tt h ec o o l e r sm u s tb ec a p a b l eo fh a n d l i n g are l i s t e di nT a b l e I. It s h o u l db en o t e dt h a tt h eh e a tl o a d sm u s tb ek e p t e x t r e m e l y small ( m i c r o w a t t s )f o ro p e r a t i o n a t t h eu l t r a - l o wt e m p e r a t u r e s . A c o m p a r i s o no ft h ed a t ai nT a b l e s I and I1 a l l o w s o n e t o d e t e r m i n e t h e t y p e o f c o o l e rm o s tl i k e l yt ob eu s e df o r a p a r t i c u l a rm i s s i o nc a t e g o r y .E a r t ho b s e r v a t i o nm i s s i o n sw i t hi n s t r u m e n t su s i n ga r r a y so f I R s e n s o r sl o o k i n g a t t h e e a r t h w i l l requiremechanicalcoolerswhichcansupply a few watts c o o l i n g c a p a c i t ya n dt e m p e r a t u r e so f a f e wt e n so fk e l v i n .I n f r a r e da s t r o n o m ym i s s i o n s s u c h as IRAS ( I n f r a r e d A s t r o n o m y S a t e l l i t e ) w i l l u s es u p e r f l u i dh e l i u mw h i c h canprovidetemperatures down t o 1 . 5 K w i t h t e n s t o a fewhundred m i l l i w a t t s c o o l i n gl o a d s .I n f r a r e db o l o m e t e r s may b er e q u i r e d t o b ec o o l e dt o 0.3K u s i n g a h e l i u m - 3c o o l e rt oo b t a i nd a t a a t l o n g e rw a v e l e n g t h s .F o ri n s t a n c e ,t h e sensitivityof a b o l o m e t e rc o n t i n u e st oi n c r e a s e a s i t s o p e r a t i n gt e m p e r a t u r e i s l o w e r e d .I nt h el o n g e r term, r e l a t i v i t y o r g r a v i t a t i o n a l p h y s i c s m i s s i o n s , which w i l l a t t e m p t t o d e t e r m i n e t h e e x i s t e n c e of g r a v i t a t i o n a l w a v e s , may r e q u i r et e m p e r a t u r e so f a f e wm i l l i k e l v i n .T h e s el o wt e m p e r a t u r e sc a no n l yb e a c h i e v e dw i t ha na d i a b a t i cd e m a g n e t i z a t i o nr e f r i g e r a t o r .I na d d i t i o nt o m i s s i o nr e q u i r e m e n t s ,s p e c i a lc o o l e r s w i l l berequiredto c o o l experimental f a c i l i t i e st o . p e r f o r mv a r i o u sb a s i cs c i e n c ee x p e r i m e n t si ns p a c e . One o ft h e p r e s e n t l ya p p r o v e dS p a c e l a be x p e r i m e n t s w i l l m e a s u r ef u n d a m e n t a lp r o p e r t i e so f s u p e r f l u i dh e l i u m (He 1 1 )u n d e rz e r o - gc o n d i t i o n s .T h e s e a r e measurementson a " q u a n t u mf l u i d "w h i c hc a no n l yb ep e r f o r m e du n d e rz e r o - gc o n d i t i o n s . T a b l e s I and I1 c l e a r l y show t h a t d i f f e r e n t t y p e s o f c o o l e r s m u s t b e a v a i l a b l e i n o r d e r t o meet t h ev a r y i n gr e q u i r e m e n t s( t e m p e r a t u r e ,h e a tl o a d / o p e r a t i n g l i f e t i m e )p l a c e do nt h ec o o l e rb yd i f f e r e n tm i s s i o n s .
PROGRAM MANAGEMENT STRUCTURE
Because o ft h ed i v e r s i t yo ft h ec r y o g e n i cc o o l e rp r o g r a m ,t h r e e C e n t e r s a r e i n v o l v e di np e r f o r m i n gt h eR e s e a r c ha n dT e c h n o l o g ye f f o r t s .T h e major areas of e f f o r t b yC e n t e r s a r e as f o l l o w s : G o d d a r dS p a c eF l i g h tC e n t e r o o
154
(GSFC)
Long-LifetimeMechanicalCoolers S o l i dC r y o g e nC o o l e r s
NASA
Ames R e s e a r c hC e n t e r o o o
(ARC)
Helium-3 Coolers A d i a b a t i cD e m a g n e t i z a t i o nC o o l e r s L o n g - L i f eS u p e r f l u i dH e l i u mC r y o s t a t s
J e t P r o p u l s i o nL a b o r a t o r y( J P L ) o
o o o
R a d i a nCt o o l e r s A d s o r p t i o nC o o l e r s M a g n e t iC c oolers S p a c e l a bE x p e r i m e n t Helium i n Zero-g
-
Measure Basic P r o p e r t i e so fS u p e r f l u i d
i s managedby t h e NASA H e a d q u a r t e r s O f f i c e o f A e r o T h ec o o l e rp r o g r a m n a u t i c sa n dS p a c eT e c h n o l o g y (OAST). C o o r d i n a t i o n i s m a i n t a i n e dw i t ht h e NASA H e a d q u a r t e r sp r o g r a mo f f i c e s ,w h i c h w i l l betheeventualusers of t h e new t e c h n o l o g y .T h i si n c l u d e st h eO f f i c eo fS p a c eS c i e n c e (OSS) a n dt h eO f f i c eo f Spaceand T e r r e s t r i a l A p p l i c a t i o n s (OSTA). The c o o l e rp r o g r a ma c t i v i t y is a l s o DOD. reviewedincoordinationmeetingswith
CRYOGENIC COOLER TYPES
R a d i a n tC o o l e r s R a d i a n tc o o l e r sa c h i e v ec r y o g e n i ct e m p e r a t u r e sb yr a d i a t i n gt h ei n s t r u m e n t l o a d st oc o l ds p a c e .T y p i c a lo p e r a t i n gt e m p e r a t u r e sf o rp r e s e n tr a d i a n t c o o l e r s a r e a b o u t lOOK w i t h a c a p a c i t y o n t h e o r d e r o f 30 mW a n d t h e s e n s o r h e a td i s s i p a t i o nb u t a f r a c t i o n of t h et o t a l . A d e t a i l e d d i s c u s s i o n of t h e i s g i v e ni nr e f e r e n c e 3 . A r e c e n ta d d i t i o nt ot h e d e s i g no fr a d i a n tc o o l e r s c o o l e r R&T program i s a n e f f o r t by J P L t o i m p r o v e t h e p e r f o r m a n c e o f r a d i a n t coolers.
a r a d i a t o rp l a t es u r r o u n d e db yf i x e dr a d i a t i o n T h eJ P Lc o n c e p tc o n s i s t so f a r e r e d u c e db yt h eu s eo f s h i e l d s .C o n d u c t i o nh e a tl o a d st ot h er a d i a t o rp l a t e a small c r o s s - s e c t i o n a l area t h a t a r e c a p a b l eo f f i b e r g l a s st a p es u p p o r t sw i t h a l ge n v i r o n m e n t ,b u tn o td u r i n gl a u n c h . A c a g i n ga n d s u p p o r t i n gt h ep l a t ei n r e l e a s e mechanism i s t h e ne m p l o y e df o rt h ep l a t e .T h ec o n c e p tc o u l dr e s u l ti n much l a r g e r r a d i a n t c o o l e r s i n f u t u r e m i s s i o n s w i t h g r e a t e r h e a t r e j e c t i o n capacity. a t JPL i s p a r t o f a n o v e r a l l JPL p l a n The r a d i a n t o r p a s s i v e c o o l e r e f f o r t t o employ a c o m b i n a t i o n of m a g n e t i c a n d g a s a d s o r p t i o n c o o l e r s t a g e s t o pump h e a tf r o mt h ed e s i r e dl o wo ru l t r a - l o wt e m p e r a t u r et o a sufficientlyhigh a r a d i a n tc o o l e r .W i t hn o t e m p e r a t u r ew h e r e i t c o u l d b e r a d i a t e d t o s p a c e w i t h e x p e n d a b l e s ,t h es y s t e mo p e r a t i n gl i f e i s d e t e r m i n e do n l yb yt h er e l i a b i l i t yo f t h em a g n e t i ca n dg a sa d s o r p t i o nc o o l e rs t a g e s .W i t ht h eJ P Li n t e r e s ti n 6 to 9 m i s i o n st ot h eo u t e rp l a n e t s ,t h i sc o n c e p th a s many d e s i r a b l e f e a t u r e s .
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SolidCryogenCoolers S o l i dc r y o g e nc o o l e r su t i l i z et h eh e a to fs u b l i m a t i o no fc r y o g e n st op r o v i d et h ec o o l i n g . To d a t e ,t e m p e r a t u r e s down t o 15K, u s i n gs o l i dn e o n , c o n s i d e r e dp r a c t i c a l . A d e t a i l e dd i s c u s s i o no ft h eo p e r a t i o no fs o l i dc r y o g e n coolers is in reference 4.
are
The s p e c i f i c o b j e c t i v e s o f t h e s o l i d c r y o g e n c o o l e r R e s e a r c h a n d T e c h n o l o g y (R&T) p r o g r a m sb e i n gc o n d u c t e db yG o d d a r dS p a c eF l i g h tC e n t e r (GSFC) a r e c a p a c i t ye n h a n c e m e n tt o 3 w a t t - y e a r sa n de x t e n s i o no ft h eu s a b l et e m p e r a t u r e r a n g e down t o 8 K u s i n gs o l i dh y d r o g e n . To d a t e ,t h ee m p h a s i sh a sb e e np l a c e d upon t h e l a t t e r o b j e c t i v e . For many m i s s i o n s , s o l i d c r y o g e n c o o l e r s h a v e t h e a d v a n t a g e s o f r e l a t i v e l y s i m p l ed e s i g na n de l i m i n a t i o no fz e r o - gf l u i dm a n a g e m e n tc o n s i d e r a t i o n s . Extendingthecoolingcapacityto 3 w a t t - y e a r so rm o r e w i l l a l l o w f o r a wide r a n g eo fu s e s ,i n c l u d i n go p e r a t i o n a lm i s s i o n s .E x t e n d i n gt h et e m p e r a t u r er a n g e t o 8 K may a l l o w f o r t h e c o o l i n g o f i n f r a r e d d e t e c t o r s f o r e a r t h r e s o u r c e s s a t e l l i t e m i s s i o n sb y a s i m p l e ra n dm o r ec o m p a c tc o o l e r a s compared t o a h e l i u m c o o l e r .T h i sc o u l dr e s u l ti nl o w e rs y s t e mc o s t s . Temperaturesof 8 K t o 13 K s h o u l d b e a b l e t o b e a c h i e v e d w i t h s o l i d h y d r o gen.Thesubliminghydrogen i s exposed t o t h e vacuumof s p a c ei no r d e rt o a c h i e v et h i s l o wt e m p e r a t u r e .S o l i dc r y o g e n su s e du pt o now i n c l u d es o l i dn e o n , w h i c hc a np r o v i d et e m p e r a t u r e so f 13-24 K, s o l i d n i t r o g e n (43-63 K), s o l i d a g r o n ( 4 8 - 8 3 K) , a n d s o l i d m e t h a n e (60-90 K).
A c o n t r a c t u a l e f f o r t i s u n d e r w a yw i t hL o c k h e e dR e s e a r c hL a b o r a t o r i e st o performfill,boil-offandvent tests on a f l i g h t - t y p e s o l i d h y d r o g e n c o o l e r . F i g u r e 1 shows a s c h e m a t i c of t h e t e s t s e t u p f o r t h i s e f f o r t . T e s t i n g of t h e h y d r o g e nc o o l e r w a s s u c c e s s f u l l y c o m p l e t e d a n d t h e r e s u l t s w i l l soon b e documented. A smaller c o n t r a c t u a le f f o r tw i t hB e e c h c r a f tC o r p o r a t i o n ,i n which a l a b o r a t o r y t y p e c o o l e r was f i l l e d w i t h l i q u i d h y d r o g e n a n d t h e n t h e hydrogen was f r o z e n , i s a l s o now complete. I no r d e rt oa c h i e v et h e 3 w a t t - y e a rc o o l i n gc a p a c i t y ,h e a tl e a k sa l o n gt h e t a n ks u p p o r t sm u s tb ek e p t lowbyemployi.nglowconduction materials and a l o n g c o n d u c t i o np a t h . To r e d u c et h er a d i a n ti n p u t , many l a y e r so fs u p e ri n s u l a t i o n are u t i l i z e d . The c o o lg a sf r o ms u b l i m i n gs o l i dc r y o g e nc o o l sm u l t i p l e metal b a r r i e r s t h a t a r e p l a c e db e t w e e nt h e many l a y e r s o f s u p e r i n s u l a t i o n . It may b e n e c e s s a r y t o p l a c e a l o w t e m p e r a t u r ec r y o g e ns t a g ei n s i d e a higher-temperature c r y o g e ns y s t e mo fs o l i d ammonia o rc a r b o nd i o x i d e ,w h i c h i s u s e dt oc o o lt h e o u t e rt h e r m a ls h i e l do ft h el o wt e m p e r a t u r es y s t e m .T h i s i s known as a two s t a g eo rg u a r d e ds y s t e m .
A s anexampleof a f l i g h ts o l i dc r y o g e ns y s t e m ,F i g u r e 2 i s a photograph of a t w o - s t a g e c o o l e r f o r t h e Limb R a d i a n c e I n v e r s i o n R a d i o m e t e r t h a t was o r b i t e da b o a r d Nimbus-F. F i g u r e 3 i s a cut-away view o ft h et w o - s t a g ec o o l e r . The c o o l e r , w h i c h w a s d e v e l o p e db yL o c k h e e dR e s e a r c hL a b o r a t o r i e s ,u s e sm e t h a n e as t h ep r i m a r yc r y o g e n a t 6 3 K and ammonia a t 152 K a s t h es e c o n d a r y .W i t h
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6 . 4 k i l o g r a m so fs o l i dm e t h a n ea n d 5.4 k i l o g r a m s of s o l i d ammonia, a n o p e r a t i n g l i f e t i m eo f 7 months w a s a c h i e v e d( . r e f . 4 ) . T h i sc o o l e ra l s of l e wa b o a r dt h e Nimbus-G s p a c e c r a f t w i t h similar performance. M e c h a n i c a lC o o l e r s Many o f t h e f u t u r e NASA m i s s i o n s r e q u i r e l o n g - t e r m c o o l i n g i n t h e 5-10 w a t t s load. A 3-5 y e a rl i f e t i m em e c h a n i c a lc o o l e ro f f e r s a compact,low-weight s y s t e mf o ra c c o m p l i s h i n g t h i s g o a l . I t i s v i r t u a l l yt h eo n l yp r a c t i c a ls y s t e m f o rc o v e r i n gt h i sr e q u i r e m e n t .P r i m em i s s i o nt y p e si n c l u d el o n g - d u r a t i o n gamma-ray a s t r o n o m y , a t m o s p h e r i c r e s e a r c h , e a r t h r e s o u r c e s a n d w e a t h e r satell i t e s . Many t y p e s o f S h u t t l e S o r t i e m i s s i o n s ( 1 t o 4 w e e k sd u r a t i o n )w o u l d also benefit from a reliable mechanical cooler. G o d d a r dS p a c eF l i g h tC e n t e r is theleadCenterintheeffortstodevelop r e l i a b l em e c h a n i c a lc o o l e r s .T h es p e c i f i co b j e c t i v e i s t od e v e l o pt h et e c h n o l o g y f o r a 3-5 y e a r l i f e t i m e c o o l e r t h a t w i l l p r o v i d e 5 w a t t s of r e f r i g e r a t i o n a t 65 K. A f t e ra c h i e v i n gt h i so b j e c t i v e ,e f f o r t s w i l l b ec o n c e n t r a t e do n e x t e n d i n gt h i st e c h n o l o g yt ot h e 1 2 K region. The b a s i c p r o b l e m i s t o d e v e l o p a c l o s e d c y c l e m a c h i n e w i t h m o v i n g p a r t s t h a t w i l l r u nr e l i a b l yf o rb i l l i o n s of c y c l e sw h i l eu n a t t e n d e d .P a s ta t t e m p t s t os o l v et h i sp r o b l e mh a v eu s e dl i q u i do rs e m i - l i q u i dl u b r i c a n t s ,d r yl u b r i c a n t s ,a n d" h a r d - o n - h a r d "b e a r i n g sw i t h l i t t l e o r no l u b r i c a n t .V a r i o u s seal d e s i g n s were a l s o t r i e d . To d a t e ,t h e s ea p p r o a c h e sh a v en o ty i e l d e d a longl i f e t i m es p a c e b o r n ec o o l e rs y s t e m . The new a p p r o a c h i n i t i a t e d by NASA G o d d a r d S p a c e F l i g h t C e n t e r h o p e f u l l y e l i m i n a t e st h ep r o b l e m - c a u s i n gf e a t u r e so ft h ep a s t .T h ek e ye l e m e n t so ft h e approach are: ( 1 )t h er e c i p r o c a t i n gc o m p o n e n t sa r ed r i v e nd i r e c t l yw i t hl i n e a r m o t o r s ,a n d ( 2 ) w h i l eo p e r a t i n g ,t h e r e i s n oc o n t a c tb e t w e e nt h em o v i n g comp o n e n t sa n dt h em a c h i n eh o u s i n go rm o t o r .T h en o n c o n t a c to p e r a t i o nc a nb e a c h i e v e db ye i t h e rm a g n e t i co rg a sb e a r i n g sa n dc l e a r a n c e seals. Consequently, two m a j o r c o n t r a c t u a l e f f o r t s h a v e b e e n f u n d e d - o n et od e v e l o pt h em a g n e t i c b e a r i n gc o o l e ra n dt h eo t h e rt od e v e l o pt h eg a sb e a r i n gm a c h i n e . To d a t e , t h e P h a s e 1 designandcomponent test a c t i v i t i e s f o r b o t h t h e g a s a n dm a g n e t i cb e a r i n gc o o l e r s a r e c o m p l e t e .F a b r i c a t i o no ft h em a g n e t i cb e a r i n g d e s i g nh a sb e g u n ,w i t h a s c h e d u l e dd e l i v e r yo fa ne n g i n e e r i n gm o d e lc o o l e ri n J a n u a r y 1981. From t h i s p o i n t o n , m o s t o f t h e e f f o r t w i l l b ec o n c e n t r a t e do n t h em a g n e t i cb e a r i n gc o o l e r ,a l t h o u g h some component l e v e ld e v e l o p m e n to fg a s bearings w i l l continue. A s c h e m a t i co ft h eP h i l i p sL a b o r a t o r ym a g n e t i cb e a r i n gc o o l e r i s shown i n F i g u r e 4 . T h ep i s t o na n dt h ed i s p l a c e r are s i n u s o i d a l l y o s c i l l a t e d by t h e l i n e a rm o t o r ss h o w n .T h e s er e c i p r o c a t i n ge l e m e n t s a r e a c c u r a t e l yp o s i t i o n e di n t h e i rr e s p e c t i v ec y l i n d e r su s i n ga c t i v em a g n e t i cb e a r i n g s . These b e a r i n g s e m p l o ye d d yc u r r e n ts e n s o r st om o n i t o rt h eg a p a t two p o s i t i o n s , 90° a p a r t , a r o u n dt h ec y l i n d e rc i r c u m f e r e n c e .T h es i g n a l s a r e u s e dt oa d j u s tt h ec u r r e n t s i nt h ef o u rc i r c u m f e r e n t i a le l e c t r o - m a g n e t s .T h ec e n t e r i n ga c c u r a c yo ft h e s e
157
b e a r i n g s i s b e t t e r t h a n 0.025 mm ( 0 . 1 m i l ) a n d t h e y p e r m i t t h e e f f i c i e n t u s e ofpistonanddisplacerclearance seals which are a n o r d e r o f m a g n i t u d e l a r g e r . M a g n e t i cb e a r i n g ss u p p o r tt h ep i s t . o no ne a c hs i d e of t h ep i s t o nm o t o r . In this manner,theyequallyshareanyradialinstabilityforcedevelopedbythemotor. A m o v i n gm a g n e tm o t o rd e s i g n , as opposed t o a moving c o i l , p r o v i d e s h i g h r e l i a b i l i t yw i t h o u tf l e x i n gl e a d s .T h em o t o rc o i l s are h e r m e t i c a l l y s e a l e d i n metal c a n s t o e l i m i n a t e o u t g a s s i n g p r o d u c t s w h i c h c a n a f f e c t l o n g - t e r m c o o l e r stability. The room t e m p e r a t u r e p o r t i o n of t h e d i s p l a c e r i s guidedby two m a g n e t i c b e a r i n g s .T h e s eb e a r i n g s are l o c a t e d a t t h e 300 K a m b i e n tt e m p e r a t u r eh e a t exchanger as shown. I nt h ee n g i n e e r i n gm o d e lc o o l e r ,t h ed i s p l a c e r is d r i v e n i nb o t hd i r e c t i o n sa n d a r e s t o r i n g s p r i n g i s n o te m p l o y e d . A penaltyindrive power w i l l b e p a i d f o r t h e s i m p l i c i t y o f t h i s d e s i g n . A r e s t o r i n g' ' m a g n e t i c s p r i n g " may b e i n c l u d e d i n f u t u r e m o d e l s . The p i s t o n a n d d i s p l a c e r a m p l i t u d e s a n d r e l a t i v e p h a s e a r e u n d e rc l o s e d l o o pc o n t r o l .T h ep e a k - t o - p e a ka m p l i t u d eo ft h ep i s t o n i s m o n i t o r e du s i n g two LVTD'S. P h a s ec o n t r o l i s r e q u i r e dt om a x i m i z ec o o l i n gc a p a c i t ya n dt h e r m a l s t a b i l i t y . T h ep o w e rr e q u i r e m e n tf o rt h e 5 w a t t , 65 K P h i l i p sc o o l e r i s a b o u t 170 W a n d t h e m a c h i n e i s a b o u t 7 0 cm l o n g . Thequestfor a reliablespacebornemechanicalcoolerhasbeenan e x t r e m e l yd i f f i c u l to n e . I t i s h o p e dt h a tt h ei n n o v a t i v ea p p r o a c ho ft h e G o d d a r dS p a c eF l i g h tC e n t e r w i l l r e s u l t i n a m a j o rb r e a k t h r o u g h .
AdsorptionCooler
A conceptforanadsorptionrefrigeratorthat i s b e i n gd e v e l o p e db yt h e J e t P r o p u l s i o nL a b o r a t o r y( J P L ) i s shown i n F i g u r e 5. Heat i s used t o compress t h ew o r k i n gg a s by d r i v i n g i t from a z e o l i t ea d s o r b e r .T h eg a s i s t h e nd r i v e n t h r o u g ha ne x p a n s i o ne n g i n eo r a J o u l e Thomson d e v i c e t o p r o v i d e t h e c o o l i n g . T h ed o w n s t r e a ms e c t i o no ft h ec o o l e r i s m a i n t a i n e d a t n e a r vacuum c o n d i t i o n s b y m a i n t a i n i n gt h ea d s o r b e r a t a l o w t e m p e r a t u r et h r o u g ht h ep a s s i v er a d i a t o r . When a l l of t h e w o r k i n g f l u i d e n d s u p i n t h e d o w n s t r e a m a d s o r b e r , t h e c y c l e m u s tb er e v e r s e d .T h i s i s a c c o m p l i s h e db ya p p r o p r i a t e l yr e v e r s i n gt h ep o s i t i o n s( i . e . ,o p e n to c l o s e d a n d v i c e v e r s a ) of thedownstreamandupstream h e a ts w i t c h e sa n d valves. The a d s o r p t i o n r e f r i g e r a t o r h a s t h e a d v a n t a g e o f r e l a t i v e l y f e w moving p a r t s .Q u e s t i o n a b l e areas a r e i t s e f f i c i e n c ya n dn e e df o r a r a d i a n tc o o l e r . Work i s p r o g r e s s i n go nt h ed e v e l o p m e n ta n de n g i n e e r i n gm o d e lc o o l e r a t JPL.
M a g n e t i cC o o l e r s
a c o n t i n u o u s l yo p e r a t i n gm a g n e t i cc o o l e r Theconceptof pursuedbyJPL. W. A. S t e y a r to fL o sA l a m o sS c i e n t i f i cL a b o r a t o r y p u b l i s h e dr e s u l t so fh i sw o r k( r e f . 5) o nt h em a g n e t i cc o o l e r . presentlycooperatingwithJPLtofurtherdefinetheconcept. 158
is alsobeing (LASL) h a s LASL i s
T h em a g n e t i cc o o l e rc a no p e r a t ew i t he f f i c i e n c i e so f 80% o f C a r n o t , o r g r e a t e r ,i nt h et e m p e r a t u r er a n g eo f 2 t o 20 K. No h i g hp r e s s u r e seals a r e r e q u i r e d . Low s p e e d o p e r a t i o n w i t h a minimum numberofmoving p a r t sa p p e a r s p o s s i b l e .H o w e v e r , a l a r g em a g n e t i cf i e l do f 5 t o 7 Tesla i s r e q u i r e d f o r efficientoperation. F i g u r e 6 s h o w so n ep r o p o s e ds y s t e mc o n c e p t .T h em a g n e t i cc o o l e r pumps h e a tf r o mt h es e n s o rl o a d a t 2.2 K t o a t e m p e r a t u r eo f 1 7 K. A d d i t i o n a l m a g n e t i co ra d s o r p t i o nc o o l e rs t a g e sw o u l dt h e n pump t h e h e a t t o a t e m p e r a t u r e of 180 K where i t w o u l d b e r a d i a t e d t o s p a c e by a l a r g e , b u t l i g h t w e i g h t , radiator. T h eo p e r a t i o no ft h er o t a t i n gm a g n e t i cc o o l e r i s as f o l l o w s :T h e r i m of t h ew h e e l i s made o fp o r o u s Gd2 (SO4) 3, 8H20, Gd2 (SO41 3, o r Gd PO4 The applicationofthemagneticfieldtothewheelinthe small r e g i o n shown w i l l p r o d u c ea d i a b a t i ch e a t i n g .T h eh e a t i s removedby t h eh i g ht e m p e r a t u r el o o p . A s t h e Gd moves o u t o f t h e m a g n e t i c f i e l d i t d r o p si nt e m p e r a t u r e (as much a s 1 4 K) d u e t o a d i a b a t i c c o o l i n g . The r o t a t i n gm a g n e t i cc o o l e rc o n s i d e r e db yS t e y a r t ( r e f . 5) a l l o w e dc y c l e times of much l e s s t h a n 1 s e c o n d , t h a t i s , a w h e e l r o t a t i o n r a t e of 1 t o 5 r e v o l u t i o n sp e rs e c o n d .C i r c u l a t i n gf l u i d sf o r c e dt h r o u g ht h ep o r o u sw h e e l e x c h a n g et h eh e a t .
S t o r e dL i q u i dH e l i u mC o o l e r s a t a t m o s p h e r i c p r e s s u r e i s a t a t e m p e r a t u r eo f Liquidhelium(He4) 4.2K. By pumpingon t h el i q u i dh e l i u mt h et e m p e r a t u r ec a nb el o w e r e dt o 1.2 t o 1.5R. A t t h e s et e m p e r a t u r e s , a l l o t h e rn o r m a ls u b s t a n c e s ,e x c e p t helium-3, are s o l i d s .B e c a u s e of t h e l o w l a t e n th e a to fl i q u i dh e l i u m , h i g hq u a l i t yd e w a r so fc r y o s t a t sm u s tb eu s e dt op r e v e n tr a p i d l o s s o ft h e liquid.
The common i s o t o p e ofhelium(He4)above 2.17K ( t h el a m b d ap o i n t ) is a n o r m a ll i q u i di n a l l r e s p e c t s .S p e c i a le f f o r tm u s tb et a k e nt om a i n t a i nt h e r m a l c o n t a c tb e t w e e nt h ec r y o g e na n dt h eh e a tl o a d .A p p r o a c h e si n c l u d es u r f a c e t e n s i o n ,p o s i t i o nr e t e n t i o n ,t h e r m a l l yc o n d u c t i v en e t s ,a n dc o n t i n u o u s accelerat i o n by r o t a t i o n of t h ed e w a r . A t t e m p e r a t u r e sb e l o w 2.17K,normalhelium (He I ) becomes a s u p e r f l u i d (He 11) w i t h a t h e r m a lc o n d u c t i v i t ya l m o s t 1000 times t h a t of copper a t room t e m p e r a t u r ea n da l m o s tz e r ov i s c o s i t y . I t forms a t h i nf i l mo ne x p o s e ds u r f a c e s .F u r t h e r m o r e , i t moves t o t h e w a r m e r r e g i o n which i s whatonewouldwant a l i q u i dt od oi fo n ed e s i r e da ne x t r e m e l yu n i f o r m t e m p e r a t u r et h r o u g h o u t a l a r g ev o l u m e . A t p r e s e n t , i t a p p e a r st h a ts u p e r f l u i dh e l i u mh a s many d e s i r a b l e p r o p e r t i e s which make i t t h e p r i m e c h o i c e f o r c r y o g e n i c c o o l i n g i n t h e 1 . 5 t o 10K temperaturerangeinsystemswithheatloadsintherangeoftento a fewhundred m i l l i w a t t s . R e a s o n a b l ys i z e dc r y o s t a t sc a np r o v i d ec o n t i n u o u sc o o l i n gf o r infraredastronomymissionsforperiodsofoneyearinspacewithexpectedheat l o a d sf r o md e t e c t o r sa n dp r e a m p l i f i e r s( r e f . 6 ) . W i t hi m p r o v e dt e c h n i q u e sf o r 159
Il I I I I
i n s t a l l i n g m u l t i l a y e r s of i n s u l a t i o n a n d new d e s i g n s f o r t h e i n n e r t a n k s u p p o r t s ,o p e r a t i n gl i f e t i m e sw i t he x p e c t e dh e a tl o a d so fu pt ot h r e ey e a r s a p p e a rp o s s i b l e .C o n s i d e r a t i o n i s b e i n gg i v e nt o new d e s i g n s f o r f i b e r g l a s s t a n ks u p p o r t s . Retractable s u p p o r t s a r e a l s ob e i n gc o n s i d e r e d . However, p r a c t i c a lm e t h o d s are n e e d e d t o " f i l l " t h e h o l e i n t h e s u p e r i n s u l a t i o n b l a n k e t when t h et a n ks u p p o r t s a r e r e t r a c t e d .G u a r d e ds y s t e m su s i n gl o w - t e m p e r a t u r e s o l i d - c r y o g e n( e . g . ,h y d r o g e n )o r a m e c h a n i c a lc o o l e rt oc o o lt h eo u t e r metal s h i e l d a r e a n o t h e ra p p r o a c ht oa c h i v i n g a 3 to 5 year life cooler.
Helium-3Coolers The a v a i l a b i l i t y of q u a n t i t i e so ft h ep u r ei s o t o p eh e l i u m - 3 (He 3 ) i n t h e l a t e 1950 t i m e p e r i o d l e d t o i t s w i d e s p r e a d u s e a s a m e a n so fa c h i e v i n gl o w e r t e m p e r a t u r e s .T h i s i s b e c a u s et h ev a p o rp r e s s u r eo ft h i si s o t o p e i s much h i g h e r a t a g i v e nt e m p e r a t u r et h a nt h a to fh e l i u m - 4 (He 4 ) . Forexample, a t lK, l i q u i d H e 3 ( n o r m a lb o i l i n gp o i n t 3 . 2 K ) h a s a s a t u r a t e dv a p o rp r e s s u r e 80 times l a r g e r t h a n H e4 By a p p r o p r i a t e l y pumpingon H e 3 , t e m p e r a t u r e s of a b o u t 0.3K can be attained.
.
Ames R e s e a r c hC e n t e r i s p u r s u i n g R&T e f f o r t s o n H e 3 c o o l e r s t h a t c a n o p e r a t ei nz e r o - g .B e c a u s eo ft h eh i g hc o s to f H e 3 , i t m u s tb eu s e di n a c l o s e d - c y c l e mode. S p e c i a lt e c h n i q u e sm u s tb ee m p l o y e dt oc o n t r o lt h el o c a t i o n of t h e l i q u i d a n d t o m a i n t a i n t h e r m a l c o n t a c t w i t h t h e s e n s o r o r p a r t s t o b e cooled. F i g u r e 7 i s a p h o t o g r a p h of a n ARC p r o t o t y p e H e 3 c o o l e r . The H e 3 i s a b s o r b e di n a z e o l i t e bed as i t e v a p o r a t e sf r o mt h ec o p p e rf o a m , To r e c y c l e , t h e z e o l i t e is h e a t e d t o p r e s s u r i z e t h e He3 v a p o ra n dt h ec o p p e rf o a m i s cooled t ol i q u i dh e l i u m (He4) t e m p e r a t u r e s( p r o b a b l ys u p e r f l u i dh e l i u m a t less t h a n 2K t e m p e r a t u r e ) . The He3 i s t h e nc o n d e n s e do n t o / i n t ot h ec o p p e rf o a m I. f c o n t i n u o u so rs e m i - c o n t i n u o u sc o o l i n g is d e s i r e d ,m u l t i p l eu n i t sm i g h tb e e m p l o y e dw i t ho n eu n i tb e i n gr e c y c l e dw h i l et h eo t h e ru n i t is providing temp e r a t u r e s a s low a s 0.3K. I n i t i a l t e s t s h a v e shown t h a t t h e u n i t w i l l operateineitheranupright o ri n v e r t e d( n e g a t i v eg )p o s i t i o n .O t h e r t e s t s w i l l b ep e r f o r m e do nt h e NASA L e a r J e t and KC-135 a i r p l a n e s w h i c h c a n p r o v i d e a b o u t 3 0 s e c o n d s o f n e a r l y zero-genvironment. T h el o w e rt e m p e r a t u r ep r o v i d e db yt h e H e 3 c o o l e r ( 0 . 3 K ) as compared t o a s u p e r f l u i d H e 4 c o o l e r ( 1 . 5 K )c a np r o v i d eg r e a t l yi n c r e a s e dp e r f o r m a n c ef o r b o l o m e t e ri n f r a r e dd e t e c t o r s .I nf a c t ,t h es e n s i t i v i t yo f a bolometer I R d e t e c t o r w i l l i n c r e a s e b y a f a c t o r of 50 i f c o o l e d t o 0.3K i n s t e a do f1 . 5 K .
Joule-ThomsonCooler A n o t h e rt y p eo fc o o l e ri nw h i c h Ames R e s e a r c hC e n t e r (ARC) a n dt h e J e t P r o p u l s i o nL a b o r a t o r y( J P L )h a v ep e r f o r m e dp r e l i m i n a r y tests i s t h eJ o u l e Thomson e x p a n d e r( J T X ) .F o rt h i sc o o l e r ,t h eh e l i u m may b es t o r e du n d e rs u p e r -
160
c r i t i c a l c o n d i t i o n s .T h i se l i m i n a t e sc o n s i d e r a t i o no fl i q u i d l g a sl o c a t i o na n d m a i n t a i n i n gt h e r m a lc o n t a c ti nz e r o - g .T e m p e r a t u r e s down t o 1.4K h a v eb e e n achieved. T h ec o n c e p th a st h ea d v a n t . a g eo f a simplifiedheliumstoragesystem p r e s s u r e sb e t w e e n0 . 3 5a n d2 . 0 7 MPa ( 5 0a n d3 0 0p s i ) . Tests r u no nt h e shown s u f f i c i e n t l y low n o i s e levels g e n e r a t e d b y t h e e x p a n s i o n p r o c e s s t h a t w i l l not a f f e c t t h e p e r f o r m a n c e o f i n f r a r e d d e t e c t o r s .
at JTX have it
Tests t o d a t e h a v e shown t h a t s u p e r c r i t i c a l h e l i u m s t o r a g e w i t h a JTX i s a viablealternativetothesuperfluidheliumcoolerforobtainingtemperatures a t l e a s t down t o 2K w i t h h e a t leaks i n t h e t e n s t o f e w h u n d r e d milliwatts range. I t i s b e i n gc o n s i d e r e d as o n eo ft h ec a n d i d a t ec o o l e r sf o rt h eS h u t t l e I n f r a r e dT e l e s c o p eF a c i l i t y( S I R T F ) .T h ec o n c e p ta l s oh a st h ea d v a n t a g et h a t there is n ol i q u i ds l o s h i n gt oa f f e c tt e l e s c o p ep o i n t i n gs t a b i l i t y .
A d i a b a t i cD e m a g n e t i z a t i o nC o o l e r T h el o w e s tp r a c t i c a lt e m p e r a t u r et h a tc a nb ep r o d u c e db yp u m p i n go n He3 l i q u i d i s around0.3K,and,on H e 4 l i q u i d i s i nt h er a n g e of1.2K t o 1.5K. A t e c h n i q u e by w h i c h t e m p e r a t u r e s a p p r o a c h i n g a fewmicrokelvinhasbeenachieved i nt h el a b o r a t o r yd e p e n d so nt h em a g n e t o - t h e r m o d y n a m i cp r o p e r t i e so fc o m p l e x s a l t s . T h e p r i n c i p l e of o p e r a t i o n i s a sf o l l o w s :t h ep a r a m a g n e t i c salt is s u s p e n d e di nl i q u i dh e l i u m . A m a g n e t i cf i e l d i s a p p l i e d .T h e salt w i l l d e r i v ee n e r g yf r o mt h em a g n e t i cf i e l dw h i c ha p p e a r s as h e a ta n d raises t h e t e m p e r a t u r e of t h e s a l t . However, t h e s a l t i s s o o nc o o l e d down t o t h e l i q u i d h e l i u mt e m p e r a t u r e .T h es a l t i s t h e nr e m o v e df r o mt h el i q u i dh e l i u ma n dt h e n t h em a g n e t i cf i e l d i s removed.The s a l t now c o o l sp r a c t i c a l l yi n s t a n t a n e o u s l y t o a v e r y low t e m p e r a t u r e . The s a l t w i l l w a r m up a s i t p r o v i d e st h ed e s i r e dc o o l i n gf u n c t i o n .T h e r a t e a t which i t w a r m s up dependsontheamountof s a l t , t h ec o o l i n gl o a d ,a n d t h ed e g r e eo fi s o l a t i o np r o v i d e db yt h ec r y o s t a td e s i g n .A f t e r a p e r i o do f t i m e , t h em a g n e t i z a t i o n ,c o o l i n g ,a n dd e m a g n e t i z a t i o nc y c l em u s tb er e p e a t e d . M u l t i p l eu n i t sm i g h tb ee m p l o y e dw h e r e b yo n e is beingrecycledwhileother unitsareacting as c o o l e r s . NASA Ames R e s e a r c hC e n t e r i s p e r f o r m i n gp r e l i m i n a r ys t u d i e so nt h i s c o o l i n gc o n c e p tf o rs p a c em i s s i o n s .G r o u n d - b a s e ds y s t e m sh a v eb e e nu s e df o r numberof y e a r s .T h eo b j e c t i v eo ft h e NASA program i s t op r o v i d et h et e c h n o l o g yt oa c h i e v e a reliablespacequalifiedcooler.
a
M i l l i k e l v i n t e m p e r a t u r e s may b e n e c e s s a r y f o r o p e r a t i n g Weber b a r - t y p e g r a v i t a t i o n a l wave d e t e c t o r s i n s p a c e . T h i s lowtemperature may p r o v i d e t h e r e q u i r e ds e n s i t i v i t y( h i g h Q f o rt h es i n g l ec r y s t a ls i l i c o no rs a p p h i r er o d ) and stability
.
I t i s e x p e c t e dt h a tt h e R&T p r o g r a m c a n p r o v i d e t h e t e c h n o l o g y r e q u i r e d t o design a flighttypeadiabaticdemagnetizationcoolertoachievetemperatures of a f e wm i l l i k e l v i n .T h ep a r t i c u l a rc o o l e rs y s t e mc h o s e nf o r a g i v e nm i s s i o n
161
lllllllIll
.
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w i l l dependonthesimplicityandreliabilitythatfuture R&T e f f o r t s c a n p r o v i d ef o re a c h of t h e s es y s t e m s .I n a l l cases, c o o l i n gl o a d s( i n c l u d i n gh e a t l e a k s )m u s tb el i m i t e dt ot e n so fm i c r o w a t t s .
PRESENT/FUTUREAPPLICATIONS
T a b l e I11 p r o v i d e s a s e l e c t e d l i s t i n g o f p o t e n t i a l f u t u r e NASA m i s s i o n s which w i l l u s ec r y o g e n i cc o o l e r s .T h es t a t u s of t h ep r o j e c t ,t h et e m p e r a t u r e r e q u i r e d ,a n dt h ep o t e n t i a ll a u n c hd a t e a r e i d e n t i f i e d f o r e a c hm i s s i o n identified. I t s h o u l db e c l e a r t ot h er e a d e rt h a tc r y o g e n i cc o o l e r sf o r low andultra-lowtemperatures w i l l eventuallybeinuseon many NASA s p a c e c r a f t . R&T e f f o r t s t o e i t h e r o b t a i n o r i m p r o v e t h e r e q u i r e d t e c h n o l o g y f o r t h e s y s t e m d e s i g no b v i o u s l yh a v eh i g hp r i o r i t y .G r e a t l yi m p r o v e dd a t aw i t hi n c r e a s e d a c c u r a c ya n dr e s o l u t i o nc o u l db eo b t a i n e dw i t h new s e n s o r s .H o w e v e r ,t h e s e s e n s o r sm u s tb ec o o l e dt ot h et e m p e r a t u r e s as i n d i c a t e do nT a b l e 111.
CONCLUSION
An o v e r v i e w of t h e NASAIOAST Low andUltra-LowTemperatureCoolerResearch a n dT e c h n o l o g yP r o g r a mh a sb e e np r e s e n t e d . I t s p u r p o s eh a sb e e n t o acquaint t h er e a d e rw i t ht h ef u t u r er e q u i r e m e n t sf o rc o o l e r s ,t h ep r e s e n ts t a t u so f t e c h n o l o g y ,a n dt h e activities inprogresstoprovide new t e c h n o l o g y t o meet therequirementsofcoolersforfuturemissions. The areas ofemphasis i n a R e s e a r c ha n dT e c h n o l o g ye f f o r t w i l l naturally change a s r e q u i r e m e n t se v o l v ea n d as r e s u l t s f r o m t h e i n v e s t i g a t i o n s become a v a i l a b l e .T h u s ,t h i sp a p e rs h o u l db ec o n s i d e r e d a s t a t u s r e p o r t of p r e s e n t activities.
162
REFERENCES
1.
Lundholm, J. G . : NASA Low S y s t e m sf o rS p a c eM i s s i o n s .
2.
Sherman, A . : C r y o g e n i cC o o l i n gf o S r p a c e c r a fS t e n s o r s I, n s t r u m e n t s a, n d E x p e r i m e n t s .A e r o n a u t i c sa n dA s t r o n a u t i c s ,V o l .1 6 , No. 11, November 1978,pp.39-47.
3.
Donohoe, M . ; and Sherman, A.: RadianC t o o l e r s - T h e o r yF , lighH t istories, A I A A P a p e r No. 75-184, D e s i g nC o m p a r i s o n sa n dF u t u r eA p p l i c a t i o n s . CA, J a n u a r y1 9 7 5 . P r e s e n t e d a t 1 3 t hA e r o s p a c eS c i e n c e sM e e t i n g ,P a s a d e n a ,
4.
Nast, T. C . ; B a r n e s , C. B . ; andWedel, R. K . : D e v e l o p m e n at n dO r b i t a l a T w o - S t a g eS o l i dC r y o g e nC o o l e r .J o u r n a l of S p a c e c r a f t O p e r a t i o no f No. 2 , pp.85-91, Mar-Apr 1978. a n dR o c k e t s ,V o l .1 5 ,
5 .S t e y a r t , 2K.
andUltra-LowTemperatureCryogenicCooler I S A T r a n s a c t i o n s ,V o l .1 9 , No. 1, pp.3-14.
W. A . : R o t a t i n gC a r n o tC y c l eM a g n e t i cR e f r i g e r a t o rf o r J o u r n a lA p p l i e dP h y s i c s ,V o l .4 9 , No. 3 , March1978
,
Use Near pp.1227-1231.
6.Urbach, A . R . ; V o r r e i t e r , J . ; andMason, P.: Designof a S u p e r f l u i d Helium D e w a r f o rt h e IRAS T e l e s c o p e .P r e s e n t e da t7 t hI n t e r n a t i o n a lC r y o g e n i c E n g i n e e r i n gC o n f e r e n c e ,J u l y4 - 7 ,1 9 7 8 .
163
I1 Il
"
TABLE I.- GENE-
N A S AM I S S I O N CATEGORIES R E Q U I R I N G LOW AND ULTRA-LOW TEMPERATURECOOLERS APPROXIMATE TEMPERATURE RANGE (KELVIN)
DISCIPLINE
APPROXIMATE RANGE OF REFRIGERATION LOAD ~
APPLICATIONS MISSIONS(WEATHER, EARTH RESOURCES, POLLUTION MONITORING, ETC.)
10-100
MILLIWATTS TO 10 WATTS
HIGH ENERGY AND GAMMA-RAY, ASTRONOMY
4-100
MILLIWATTS TO 10 WATTS
INFRARED ASTRONOMY
0.3-10
MICROWATTS TO 100 MILLIWATTS
RELATIVITY MISSIONS
0.001-1.5
SUPERCONDUCTING DEVICES
1-15
WIDE RANGE
BASIC RESEARCH EXPERIMENTS
1.10
