Highly selective evanescent modal filter for two-mode optical fibers ...

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This work was supported by Litton Systems Inc. * Present address, Hewlett Packard Laboratories,. 1501 Page Mill Road, Palo Alto, California 94304. References.
September 1986 / Vol. 11, No. 9 / OPTICS LETTERS

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Highly selective evanescent modal filter for two-mode optical fibers W. V. Sorin,* B. Y. Kim, and H. J. Shaw Edward L. Ginzton Laboratory, W. W. Hansen Laboratoriesof Physics, Stanford University, Stanford, California 94305 Received April 24, 1986; accepted June 24, 1986

Efficient and selective coupling between a single-mode fiber and the LP 11 mode of a double-mode fiber is demonstrated in an evanescent directional coupler. With greater than 90%coupling to the LP 11 mode, suppression of the coupled power to the lower-order LP 01 mode was measured to be at least 24 dB. This device has application as a

modal filter for the construction of in-line all-fiber optical components.

The use of optical fibers in the low-mode-number regime has many potential applications. One possible use consists of modal multiplexing, in which each spatial mode comprises an individual information channel.' Another application is for the development of various in-line fiber devices. For example, an all-fiber frequency shifter has recently been demonstrated that uses a guided copropagating acoustic wave to couple between the LP0 1 and LP,1 modes in a double-mode fiber.2 Static coupling between the LP01 and LP11 modes has also been reported using periodic stresses or microbending.3 - 5 The feasibility of developing the potential uses of low-mode-number fibers depends on being able to couple efficiently and selectively into and out of the individual modes. Although a LPo1 mode selector is readily available (e.g., a tightly wound fiber coil), an efficient modal selector for the LP1, mode has not yet been reported. In this Letter a device is demonstrated that uses phase-matched evanescent coupling to transfer energy efficiently between a single-mode fiber and the LP1, mode of a double-mode fiber. Coupling between fibers is accomplished by employing a grinding and polishing technique first used to make tunable singlemode directional couplers.6 This arrangement provides separately guided output channels for the two modes of the double-mode fiber. Coupling between low-mode-number fibers has previously been reported,1' 7' 8 but little attention has been paid to achieving both large coupling efficiency and high modal selectivity. In a tunable directional coupler, evanescent coupling between a single-mode and a multimode fiber has been demonstrated,9 but coupling was not restricted between only two modes.

In referring to low-mode-number fibers it is assumed that the index difference between core and cladding is small so that the linearly polarized (LP) mode approximation can be used.1 0 The operation of the modal filter is based on phase-matched evanescent coupling between two dissimilar fibers. Proper index profiles for the two fibers must be chosen to ensure phase matching and therefore complete power transfer. Development of an efficient modal filter requires 0146-9592/86/090581-03$2.00/0

that the device maximize power transfer between the two desired modes while suppressing any coupling to the unwanted modes. In the device demonstrated here, transfer of power to the unwanted mode is minimized by making use of both the phase mismatch and the smaller modal overlap between the two lowestorder modes. To obtain an estimate on the upper bound for the coupling between the two lowest-order modes, a result from coupled-mode theory is used. The power coupled between two modes is given by1 ' sin [K2 + (A132) 2 ] 2L ( C

=

2 ~1+(A#//2K)

(1

where Pc is the coupled power, Po is the initial power in

the input mode, K is the coupling coefficient per unit length, L is the coupling length, and A# is the phase mismatch between the two modes. This equation is not strictly valid for the case of the modal filter since not only two but three modes are being coupled. Its use is intended only to give a rough estimate for the modal extinctions that are possible. Using Eq. (1) we can obtain an estimate for the coupling selectivity between the two modes of the double-mode fiber [LP(2) and LP(21)] when coupled to the lowest-order mode of the single-mode fiber [LPgl)]. Assume that the LP11 mode is phase matched to the LP(1)mode and 100%coupling occurs in a length of 1 mm (i.e., A: = 0 and

K =

7r/2 mm-').

Owing to the

difference in the extent of their evanescent fields the LP(21)mode can have a coupling coefficient to the single-mode fiber that is more than 10 times larger than that for the LP(21)mode. With this smaller coupling coefficient and a beat length (LB) of 300 Am,the maximum coupling between the LP(21)and LP(1) modes is

lessthan -35 dB (i.e.,K =

,r/20

mm-' and A\ = 27r/LB

21 mm-'). When a proper coupling length is chosen to take advantage of the sinusoidal coupling dependence between these two lowest-order modes I[K2 + (A13/2) 2 ]1/2 L

= Nr, N = integer), isolations may be as

large as -50 to -60 dB. This isolation will determine the extinction ratio between the LP(2l)and LP(2l)modes when the above arrangement is used as a modal filter. In order to demonstrate efficient modal coupling experimentally it was necessary to obtain two differ© 1986, Optical Society of America

OPTICS LETTERS / Vol. 11, No. 9 / September 1986

582

INDEX

4

DMF

SMF

LP0 1 LPI I ___

I.....~-

Fig. 1.

.

_

____

X

Relative index profiles for single-mode fiber (SMF)

and double-mode fiber (DMF) showing phase-velocity matching between LP 0 , and LP11 modes. Dashed lines indicate values of effective indices for the guided modes.

the two modes as they traveled along the fiber. The outputs of the directional modal coupler were then projected onto a screen where their far-field patterns could be observed. Figure 3 shows the far-field output patterns from the single-mode fiber (SMF) and double-mode fiber (DMF) with the coupling adjusted to approximately 50%. When the distance between the parallel fibers in the direction coupler was varied by translating one coupler half with respect to the other, the output from the double-mode fiber rotated and changed shape (i.e., became doughnutlike in shape), but the central portion of the pattern remained dark, indicating very little coupling to the lower-order LP(2) mode. By introducing a relatively large angle between the two coupling fibers it was possible to excite the LP(') mode, but owing to the decreased coupling length the power transferred to this mode was very small. The maximum coupling to the LP(') mode was measured to be 97%. This coupling efficiency was calcu-

ent fibers with the correct index profiles. Two almost compatible fibers were selected from several commercially available fibers by using a technique that permitted measurement of individual modal phase velocities. This phase-velocity measurement technique consisted of placing a prism on top of a polished fiber to couple out energy radiatively from the guided modes.'2 By measuring the angle of the radiated light, modal phase velocities could be determined to an accuracy of approximately 0.01%.

Figure 1 illustrates the relative index profiles of the two fibers that were used to demonstrate the evanescent modal coupler experimentally. Both fibers had silica cores but differed in their core diameters and the amount of doping in their claddings. The doublemode fiber had a core diameter of about 5.6 ,Amwith a numerical aperture approximately equal to 0.08. The

lated by dividing the coupled power by the sum of the coupled and uncoupled powers. The large coupling efficiency implies good phase matching between the two modes. At the expense of decreasing the coupling length the phase-matching condition could be tuned by angling the fibers with respect to each other in the directional coupler. Figure 4 shows outputs from the double-mode fiber when coupling efficiencies were greater than 90%. The mode patterns shown in photos (a) through (c) were consecutively generated by slowly translating the two parallel fibers with respect to each other through their maximum coupling position. The different intensity patterns of Fig. 4 can be obtained with little change in total coupled power. Similar changes in

single-mode fiber had a core diameter of 3.8 Am and a

numerical aperture of 0.12. Both fibers had cutoff wavelengths for the LP11mode of about 580 nm. To obtain double-mode operation the visible lines of an argon-ion laser were used. Owing to the constraints imposed by the available fibers, the fiber that was to act as a single-mode fiber allowed two modes to propagate. To ensure only single-mode operation a modal stripper was employed to remove any of the unwanted

LV') mode.

MODE STRIPPER PC

SMF

LPO

-

INPUT DMF SCREEN

Fig. 2. Experimental arrangement used in measuring parameters of the evanescent modal filter. Input LP0 1mode is coupled to output LP11 mode.

Figure 2 shows the experimental arrangement used to measure the coupling characteristics between the two dissimilar fibers. The fibers had a radius of curvature of 25 cm, which resulted in an effective coupling

length of about 1 mm. An oil having an index slightly lower than the cladding indices was placed between the two coupler halves. An argon-ion laser provided input signals at wavelengths of 457.9, 488.0, and 514.5

nm. The top fiber in Fig. 2 acted as a single-mode input to the modal coupler. The mode stripper was LP~11mode. used to eliminate any of the unwanted An in-line polarization controller'3 (PC) was placed after the mode stripper so that the polarization of the incoming LP(1)mode could be adjusted. The bottom fiber in Fig. 2 was double moded, supporting both the LP 01)and LP11 modes. The output section of this fiber was kept straight to prevent coupling between

SMF

DMF

Fig. 3. Coupled (DMF) and uncoupled (SMF) outputs from the modal coupler. Coupling is adjusted to approximately 50%; wavelength is 488.0 nm.

September 1986 / Vol. 11, No; 9 / OPTICS LETTERS

u

~~~~~(a)

4 |/ (b)

(C)

Fig. 4.

Double-mode fiber outputs for coupling efficiencies

greater than 90%. Mode shapes (a) through (c) resulted as coupling fibers were translated with respect to each other.

intensity patterns could be obtained by varying the input polarization while the fiber positions in the directional coupler remained constant. In its present form, the mode pattern sensitivity to coupling and polarization may limit the efficiency of the device as a modal filter since coupling coefficients will be affected by lobe positions. One method for avoiding this would be to use a double-mode fiber with an elliptical core to increase the separation between the propagation constants of the LP11 mode patterns. Insertion losses were relatively high, with the best measured result being a 5% loss at a coupling of 88%.

Scattering due to the short wavelength and any residual second-order mode not removed by the LP(1!)mode stripper could contribute to these measured losses. Losses for conventional polished single-mode couplers can be well under 1%, and similar losses should be

possible with the evanescent modal coupler. Accurate measurements of the extinction ratio for the powers coupled to the LP(2) and LP(2)modes were difficult to obtain. Owing to the W-index profile of the double-mode fiber it was difficult to strip away the LP(2) mode by fiber bending without inducing significant coupling to the lowest-order LP(') mode. The best measurement for the modal extinction ratio was obtained by placing an apertured detector (area 1 mm2 ) at the center location of the far-field LP11mode pattern (fiber-detector distance _ 1 m). Assuming

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that the major contribution to the power reaching the detector is from the LP01 mode, this signal was calibrated to determine the total power in this mode. The detector location was determined by finding the minimum power reading while adjusting the polarization controller to rotate the LP(2l)mode pattern. Using this method, a ratio of -24 dB was measured when coupling to the LP(') mode was greater than 90%. The value of -24 dB is poorer than theoretically expected. Possible reasons for this could be the scattering in the coupling region or limitations in the measurement scheme. With more carefully chosen fibers and longer wavelengths the extinction of the LP(2l) mode is ex-

pected to be much better. Although coupling was demonstrated with a doublemode fiber, this technique can be extended to allow coupling to fibers that possess several modes. For this case a different single-mode fiber will be needed to phase match to each individual mode. Such devices could act as modal multiplexers/demultiplexers to enhance the information capacity of optical fibers. In summary, a highly selective modal coupler has been demonstrated that evanescently couples between the LP01 and LP11modes in two different fibers. Coupling efficiencies as high as 97% were obtained.

Cou-

pling purity of the excited LP11mode was measured to be at least 24 dB. Theoretically this purity can be much better, and the above measurement may have been limited by experimental difficulties. This modal filter is currently being used in the development of a new fiber-optic single-sideband frequency shifter.2 Other applications of such a device could be for modal multiplexing/demultiplexing and the development of in-line optical-fiber devices. This work was supported by Litton Systems Inc. * Present address, Hewlett Packard Laboratories, 1501 Page Mill Road, Palo Alto, California 94304. References 1. R. G. Lamont, K. 0. Hill, and D. C. Johnson, Opt. Lett. 10,46 (1985).

2. B. Y. Kim, J. N. Blake, H. E. Engan, and H. J. Shaw, Opt. Lett. 11, 389 (1986). 3. R. C. Youngquist, J. L. Brooks, and H. J. Shaw, Opt. Lett. 9, 177 (1984).

4. H. F. Taylor, IEEE J. Lightwave Technol. LT-2, 617 (1984). 5. J. N. Blake, B. Y. Kim, and H. J. Shaw, Opt. Lett. 11, 177 (1986).

6. R. A. Bergh, G. Kotler, and H. J. Shaw, Electron. Lett. 16, 260 (1980). 7. K. 0. Hill, B. S. Kawasaki, and D. C. Johnson, Appl. Phys. Lett. 31, 740 (1977). 8. P. Bassi, G. Falciasecca, and M. Zoboli, "Experimental

results on coupling between optical fibers operating in low multimode regime," presented at the Fifth National Reunion of Applied Electromagnetism, St. Vincent, Ita-

ly, 1984. 9. M. S. Whalen and T. H. Wood, Electron. Lett. 21, 175 (1985). 10. D. Gloge, Appl. Opt. 10, 2252 (1971).

11. A. Yariv, IEEE J. Quantum Electron. QE-9, 919 (1973). 12. W. V. Sorin, B. Y. Kim, and H. J. Shaw, Opt. Lett. 11, 106 (1986). 13. H. C. Lefevre, Electron. Lett. 16, 778 (1980).