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sensitivity (0-55 A/W at 820 nm). The measured sensitivity of0 1 7 A/W leads to nex, = 25-7%. However, it must be men-tioned that the efficiency for short-pulse excitation is usuallysmaller than the value measured by long excitation.Conclusion: We have shown that photodetectors with risetimes less than 10 ps can be made from GaAs by laser-st imula ted MOCVD. The detectors show satisfactory fall-timecharacteristics and exhibit 25% quantum efficiency.W. ROTH 6th January 1983H . S C H U M A C H E RH . BEN EK I N GInstitute of Semiconductor E lectronicsSommerfeldstrafie, D-5100 Aachen, W. GermanyReferences1 GAMMEL, J. G., OHN'O, H., and BALLANTYNE, J. M.: 'High speed photo-conductive detectors using GalnAs1, IEEE J. Quantum Electron.,1981,QE-17, pp. 269-2722 KLEIN, H. J., BIMBERG, D., an d BENEKING, H.: 'Ultrafast thin filmsGaAs photoconductive detectors' , Thin Solid Films, 1982, 92, pp.273-2793 BENEKING, H.: 'On the response behaviour of fast photoconductiveoptical planar and coaxial semiconductor detectors' , IEEE Trans.,1982, ED-29, pp. 1431-14414 ROTH, w. , KRAUTLE, H. , KRINGS, A., an d BENEKING, H.: Talk given atthe MRS Annual Meeting 1-5 Nov. 1982, Boston5 KLEIN, H. J., BIMBERG, D. , BENEKING, H ., KUHL, J., a n d GOBEL, E. O.:'High peak power picosecond light pulses from a directly modula-ted semiconductor laser' , Appl. Phys. Lett., 1982, 41, pp. 394-3966 AUSTON, D. H., JOHNSON, A. M., SMITH, P. R., an d BEAN, J. G. : ' P i cOS e -cond optoelectronic detection, sampling and correlation measure-ments in amorphous semiconductors' , Appl. Phys. Lett., 1980, 37,pp . 371-3730013-5194/83/040142-02$!. 50/0

ENHANCEMENT OF BIREFRINGENCE INPOLARISATION-MAINTAINING FIBRESBY THERMAL ANNEALINGIndexing terms: Optical fibres, BirefringenceThe thermal behaviour of highly birefringent polarisation-maintaining optical fibres is studied, and the thermal hyster-esis of the birefringence is related to the quenching of thefibre. Suitable 'on-line' thermal annealing during fibre draw-ing is shown to be a method for the full development of thehigh anisotropic stresses potentially available in high bi -refringence structures using borosilicate stress-producingparts.

Introduction: Highly birefringent fibres attempt to overcomethe effect of perturbations, for example, bends an d twists bydeliberately introducing levels of birefringence in excess of tha tproduced by external factors, thereby rendering th e outputpolarisation state relatively immune to all but the most majordisturbances. Such fibres are characterised by their birefrin-gence B, defined as nx ny = A/?(//27r), where nx and ny referto th e refractive indices for light polarised along th e fibreprincipal axes x and y, respectively. A commonly quotedfigure of merit is the so-called beat length Lp = /./B and it isdesirable to achieve the smallest possible beat length to ensurelack of sensitivity to external perturbations.Very small values of beat length1 can be obtained by usingthe anisotropic thermal stress produced by incorpora t ing

high-expansion doped regions within the fibre. A variety ofsuch structures have been reported in the l i terature.1 3 Thesmallest values of beat length (highest birefringence) reportedto date have been achieved in our labora tory by means of theso-called 'bow-tie' side-pit structure; beat lengths of 0-55 mmat a wavelength of 633 nm have been successfully produced.

The temperature-dependent behaviour of a number of ellip-tically clad high-birefringence fibres was studied in Reference3, where a thermal hysteresis of the beat length was reported.However, no firm conclusions w ere reached regard ing theorigin of the effect, and it was found that the improvements inbirefringence obtained by thermal cycling decayed in a matterof days. We report here a simple heat-treatment cycle wherebysubstantial and stable improvements in beat length can beobtained. The technique ha s also been applied during the fibredrawing process.Experimental: In order to determine the thermal behaviour ofthe birefringence, linearly polarised light (/ = 633 nm) waslaunched into the test fibre at 45= to the principal axes, withmost of the fibre enclosed in a furnace consisting of a smallsilica tube and resistance wire wrapping. A photoelastic mo-dulator was used to yield sin A and cos A of the output light,where A is the retardance (see, for example, Reference 4). Thecharge in A, uniquely determined by the combination of sin Aan d cos A, was recorded as a function of temperature and timein order to moni tor th e fibre birefringence, or equivalently,beat length. The use of the photoelastic modulator providesan exceedingly sensitive and fast means of monitoring the fibrebirefringence. 4

'On-line' annealing was achieved by pulling the fibre fromthe preform in the normal way, but passing it through a four-zone annealing furnace prior to winding the fibre on the drum.The total length of the furnace was 1 m, and the zone tem-peratures were 55OC, 500C, 450C and 400C, with the fibreentering th e hottest zone first. The fibre was effectively atroom temperature on entering th e annealing furnace. Thenormal silicone elastomeric coating was found to survive theanneal satisfactorily, but allowance had to be made for itsthermal conductivity. The plot of Fig. 3 refers to an uncoatedfibre.

stress-producingborosilicate

silica substrate

Fig. 1 Micrograph (a) and sketch (b) of the cross-sectional structure ofthe bow-tie fibreResults: Fig. 1 shows th e cross-section of a bow-tie fibremanufactured in our laboratory. The boron-doped 'side-pit 'regions produ cing the stress are clearly visible. Fig. 2 is a plotof the relative change in the beat length Lp as a function oftemperature, obtained by monitoring th e retardance for alength of test fibre using the photoelastic modulator (seeabove). Curve (a) shows the behaviour of the 'as-received' fibreupon heating. Initially an increase in the beat length occurs,which is expected, since the differential contraction of thebow-tie sections and hence the internal stress anisotropy, i.e.th e net thermal stress, decreases. Subsequently the beat lengthundergoes a dramatic reduction, indicating an increase in thestress anisotropy. This major change begins at about 400 'C.

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The rapid reduction in Lp, however, ceases at about 750C.Upon relatively slow cooling of the fibre, the beat length re-turns to an altogether and always smaller value, whose exactmagnitude depends on the temperature treatment the fibrehasreceived. A second heating and slow cooling of the fibre resultsin only a small degree of hysteresis as indicated by curve {b). Ifthe fibre is now heated and cooled rapidly by switching off thefurnace, the reverse hysteresis is observed, producing a beatlength longer than the end point of curve (/>), although it stillis approximately 40% smaller than the as-received value.Similar hysteresis phenomena are observed in the axial stressin the preform and also other fibre structures, i.e. ellipticalcores and claddings which utilise boron-doped silica to prod-uce anisotro pic stress.*30r

Fig . 2 Plot of beat length against temperature for the bow-tie fibreCurves (a) and (b) refer to successive thermal cyclesAttenuation measurements indicate no significant change inthe fibre loss upon thermal cycling. Furthermore, once anelliptical or bow-tie fibre has been annealed, we have observedno relaxation of the beat length over periods of up to tenweeks. This is in direct contrast to the behaviour described inReference 3, where the beat length was reported to revert tothe original value within a few days.Our experiments show that rapid cooling of the fibre (orpreform) is responsible for the thermal hysteresis of the beatlength. It would appear that quenching the fibre during thedraw prevents the development of the full anisotropic stresspotentially available in the stress-producing sections of the

structure. Consequently the possibility exists for significantlyenhancing the birefringence by 'on-line' annealing of the fibre.

10 14 18 22 26fibre pulling speed . m /m in |3i5/3|Fig . 3 Plot of beat length against fibre pulling speed through 'on-line'annealing furnace

Beat length is normalised to the unannealed valueFig. 3 shows a plot of the beat length against fibre pullingspeed when the fibre is annealed 'on -line' by passing through afour-zone annealing furnace prior to being wound on the

drum, as described above. Even at relatively fast pullingspeeds (and hence cooling rates), significant reductions in the* O u r m a z d , A., Varnham, M. P., Burch, R. D ., and Payne, D. N.'Thermal properties of highly birefringent optical fibres and preforms'(to be published)

beat length are observed compared to the 'unannealed' value.This is consistent with our earlier annealing experiments,where it was found that fairly rapid cooling rates still producesubstantial improvements in birefringence. Again we have ob-served no relaxation of beat length to the original 'as-received'value over periods of many days.Conclusions: Quenching has been established as being respon-sible for the thermal hysteresis behaviour of highly birefrin-gent fibres utilising borosilicate glass for stress production.Annealing of lengths of test fibre , as well as 'on -line' annealingof the fibre during the drawing process, have been shown toproduce substantial increases in the already high values ofbirefringence achieved. 'On-line' annealing of highly birefrin-gent fibres therefore appears to be an essential process forutilising levels of stress potentially available in structures withborosilicate stress-producing sections.Acknowledgments: The authors express their gratitude to A. J.Barlow and R. B. Mears of this laboratory for their kindassistance and interest. A Research Fellowship was providedby the Pirelli General Cable Company (DNP) and a ResearchStudentship by British Aerospace PLC (MPV). The work wasfunded by the UK Science & Engineering Research C ouncil.A . O U R M A Z D 17th January 1983R. D. BIRCHM . P. V A RN H A MD. N . PAYNEE. J. TARBOXDepartment of ElectronicsUniversity of SouthamptonSouthampton, SO9 5NH, EnglandReferences1 BIRCH, R. D., PAYNE, D. N., and VARNHAM, M. P.: 'Fab rica tion ofpolarisation maintaining fibres using gas-phase etching', Electron.Lett., 1982,18, pp. 1036-10382 KATSUYAMA, T., MATSUMURA, H., an d SUGANUMA, T.: ' L oW - l o S Ssingle-polarisation fibres', ibid., 1981, 17, pp. 473-4743 RAMASWAMY, V., STOLEN, R. H., DIVINO, M. D. , a n d PLIEBEL, W .:'Birefringence in elliptically clad borosilicate single-mode fibres',Appl. Opt., 1979,18, pp. 4080-40844 BARLOW, A. J., an d PAYNE, D. N .: 'Measurement of fibre polarisa-tion properties using a photo-elastic modulator' . Technical Digestof Symposium on optical fibre m easurements, 1982, pp. 101-1040013-5194/83/040143-02$!.50/0

PSK HOMODYNE RECEIVER SENSITIVITYMEASUREMENTS AT 1-5

Indexing terms: Optical communication, Optical receiversMeasurements of the receiver sensitivity for an experimentaloptical PSK homodyne system, operating at 140 Mbit/s andat a wavelength of 1-52 nm , have given a 19 dB sensitivityimprovement over direct detection, and at 64 dBm isapproximately 2 dB away from th e theoretically predictedvalue.

Introduction: Significant advances in receiver sensitivity forhigh-speed long-span fibre systems have recently beenachieved at a wavelength of 1-5 /im 1 using the coherent recei-ver principle.2 The adoption of PSK modulation with coher-ent homodyne detection should give the best receiversensitivity and in addition suppresses the effects of Brillouinscattering thus allowing higher transmitted power to be used.3This combination should lead to the longest unrepeateredland and undersea fibre systems.This letter reports on a PSK homodyne experimentalsystem, operating at 1-52 ^m and at a data rate of 140 Mbit/s,which gives a receiver sensitivity 2 dB away from the 66-4dBm shot-noise-limited sensitivity, assuming a QE of 70%.

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