We select laser diode which emits wavelength of1550 nm operating temperature. ... wavelength of 1550nm, fo is angular width of beam in front of laser diode ...
Spectral Properties of the Laser Beam in Relation to the Laser Diode Operating Temperature Lucie DORDOVA', Otakar WILFERT' 'Dept. of Radio Electronics, Brno University of Technology, Purkyinova 118, 612 00 Brno, Czech Republic
xdordo00gstud.feec.vutbr.cz, wilfertgfeec.vutbr.cz Abstract. Present day optical wireless communication systems work with wavelength of 1550 nm but it is considering whether optical communication at 850 nm offers more benefits. Currently we don't know answer on the uesionwhic opica winow s mre rasoabl to use in free space optical links but we try to solve this problem. This paper is focusing on the relation between laser diode operating temperature and spectral linewidth. d laser diode which emits wavelength of 1550 nm We select (RLTJ550-15G). The point of the research is to examine spectral properties of this laser diode in relation to increasing laser diode operating temperature.
consider if interferential filter can decrease received optical signal. For spectral linewidth measurement in range of temperatures (20°C - 45°C) Fabry - Perot interferometer was chosen. This interferometer is designed for spectral line measuring at wavelength 1550 nm and can exactly determinate spectral linewidth. We are going to observe spectral linewidth changes due to increasing laser diode operating temperature. Outcome will be displayed on oscilloscope. Workstation scheme is depicted at the followin figure.
Q
Keywords Optical wireless communication, 1550 nm telecom
band, laser operating temperature, spectral linewidth.
1. Introduction We investigate whether operating temperature of laser
deformation. [1].
F-P
interferometer
oscilloscope
2. Spectral linewidth measuring
We determinate spectral by finding spectral distance optical intensity value is ¢!_ _ maximum. For testing laser
>0
PD
LD lens
lens
Fig. 2. Workstation scheme.
diode can cause spectral linewidth variation or spectral line
Transmitting
LD
Fresnel
receiving lens
Transmittig
IF filter
Fig. 1. Scheme of optical wireless link. LD is laser diode, PD is photodetector, IF is interferential filter which transmits wavelength of 1550nm, fo is angular width of beam in front of laser diode and 0 is beam divergence behind transmitting lens.
In range of temperature changes we tested spectral line amplitude variances. In case of significant spectral line decrease together with spectral line expansion we have to
1-4244-0822-9/07/$20.00 ©)2007 IEEE.
linewidth of the laser diode between two points which half in comparison with diode RLT1550-15G with resonator Fabry-Perot emitting in 1550 nm band was chosen. The spectral distribution of this laser diode is then by Fabry-Perot resonator working conditions governed. Major advantage is the dirigible measuring sensitivity. lOur measurement was also focusing on the spectral line shape changes. We try to detect if spectral line shape can remodel with increasing temperature.
1~ ~ ~ ~ ~ ~ . . . . . . . M-ean tChi 18S. 09mV
I~~~~~~~~~~~~~~~~~~~~~~~~~
X.
''
...\
2'.Z.5ms
2V
Ext f
32
At 35°C laser diode is emitting light with wider spectral line in comparison with spectral line at 20°C.
3. Experimental results Chosen laser diode has absolute maximum ratings of characteristics at 25°C. Recommended operating temperature ranges are from -20°C to +40°C. We were testing which effect had occurred if recommended laser diode operating temperature overranged. We were measuring spectral linewidth for operating temperature 20°C, 35°C, 40°C and 45°C.
.: .
2
Fig. 5. Operating temperature 35°C.
Fig. 3. Spectral linewidth FWHM [2].
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20V
1
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Cli
ean
.l .1
.
.
/ \
I I
I
|
j S
-
.
..| .......-.
Chi
:16 5mV 5I:-V
2
I
50mV
2V
/
M 2.5ms
Ext
f
72mV.
Fig. 6. Operating temperature 40°C.
/ < ] At 40°C we observed spectral line peak deformation (for this measurement the measuring
sensitivity was changed).
2
V
2s2
NV
spetra
' .2.
M.'.m. 2V--:.
linewdt of 2 nm.d
t
i..
2.5ms -Ext
Ch M2..
2
i~
Fig. 7. Operating temperature 45°C.
....f...
mV
At this temperature spectral line splitting is
4. Conclusion
Valusc fspe
On the basis of measured values we can say that increasing operating temperature of laser diode causes spectral linewidth expansion and we also observe spectral line splitting at value of 45°C. This means that operating temperature over 40°C of chosen laser diode is improper
detectable. Values of spectral linewidths determinated by depicted above are showed in the following oscilloscopeo depictdabove chart:
lineshowedths dete:rminatedi
operating temperature T [0C] 20 35
linewidth AA [nm]
40
2,5
2,0
2,4
2,8
45
for free space optical links.
To sum up laser diode temperature stabilization is essential to avoid unsolicited spectral linewidth expansion and spectral line deformation.
Acknowledgements
Tab. 1. Laser diode operating temperature vs. spectral linewidth.
Visual demonstration relation between laser diode operating temperature and spectral linewidth is proper shown in the following graph: 3,0
This research has been supported by the Research programs of Brno University of Technology MSM21630513, by the grant project of FRVS No. 854/2007 and by the Grant Agency of the Czech under contracts No. 102/05/0571, Republic No. 102/06/1358, and No. 102/03/H109.
2,8 --2,6
2,4
References
2,2
[1] WILFERT, O.: Optoelektronika, UREL FEKT VUT v ISBN: 80-214-2264-5.
2,0
25
45
30
Tprq Fig. 8. Laser diode operating temperature vs. spectral linewidth.
[2]
Bahaa
Brne, 2002.
E. A. Saleh, Malvin Carl Teich: Fundamentals of
Photonics,
Wiley - Interscience, ISBN: 0471839655. [3] FRANZ, J.H., JAIN, V. K., Optical Communications Components and Systems. Narosa Publishing House, 2000
