'Characterization of single mode optical fiber filters', Appl. Opt., 1981, 20, pp. 440-445. 10 YARIV, A.: 'Optical electronics' (Saunders College Publishing, Phil-.
G., MOREV, W. W., and GLENN, W. H.: ’Formation Of Bragg gratings in optical fibers by a transverse holographic method‘, Opr. Lett., 1989. 14, pp. 823-825 3 HAND, D. P., and RUSSELL, P. ST. 1.: ‘Photoinduced refractive-index changes in germanosilicatefibers’,O p f . Lerr.. 1990, 15, pp. 102-104 4 FIORI,c., and DEVINE, R. A.E . : ‘Ultraviolet irradiation induced compaction and photoetching in amorphous, thermal SiO,‘. Mat. Res. Soc. Symp. Proc., 1985,61, pp. 187-195 5 ROTHSCHILU, D., FHRLICH, U. J., and SHAVER, 0 C.: ‘Etlects of excimer laser irradiation on the transmission, index of refraction, density of ultragrade fused silica’, J. Appl Phys. Lett., 1989, 55, pp. 1276-1278 6 WILLIAMS, 0. L., AINSLIE, B. I., ARMITAGE, J . R., and KASHYAP, R : ‘Enhanced photosensitivity in germania doped silica fibres for future optical networks’. Proc. ECOC ‘92. 18th European Conf, on Optical Communication, 1992,pp. 425-428 7 WILLIAMS, 0. L., AINSLIE, 8. J., KASHYAP, R., CAMPBELL, R., and ARMITAGE, J. R . : ‘Broad bandwidth highly reflecting gratings formed in photosensitive Boron codoped fibres’. Proc. ECOC ’92, 18th European Conf. on Optical Communication 1992, pp. 923-926 8 FERTEIN, E., L E W U B I N , S., DOUAY, M., CANON, S., BtRNACiE, P.. NIAY. P., BAYON, F., and GEORGES, T.: ‘Shifts in resonance wavelengths of Bragg gratings during writing or bleaching experiments by U V illumination within germanosilicate optical fibre’. Electron. Lerr.. 1991.27, pp. 1838-1839 9 LAM, D. K., and GARSIDE, E. K ‘Characterization of single mode optical fiber filters’, Appl. Opt., 1981, 20, pp. 440-445 10 YARIV, A.:‘Optical electronics’ (Saunders College Publishing, Philadelphia, 1991).4th edn. I 1 PHILIPS Depressed Cladding Type (DLPC5) Product Code: 215E 12 KASHVAP, R., ARMITAGE, J . R., WYATT. R.,UAVEY. s. T., and WILLIAMS, D. L.: ‘All-fibre narrowband reflection gratings at l500nm’, Electron. Lett., 1990, 26, ( I I), pp. 730-732 13 TSAI, T. E., ASKINS, r. G., and FRIEnELE. E. 1.: ‘Pulse energy dependence in Bragg grating optical fiber materials‘ Mat Res Soc. Symp., 1992,244, pp. 47-52 2
MELTZ,
preserving fibre with a core diameter of 6 pm was used. The fibre length in the SRS amplifier was 22m. The SRS generator receiver spectral filter attenuation of Stokes 30dB
dichroic
Nd VAG n p u llOOMHz m o o pps Fig. 1 Schrmoric diagram ofrecirculating loop
and fibre delay line were made of an 840 m long fibre. The first 30-50m of the fibre participated effectively in the SRS generation. Attenuation of the Stokes and pump radiation by the filter was 30 and 2 dB, respectively. An Nd : YAG mode-locked laser with wavelength 1.064 pm, a pulse repetition frequency of IWMHz, and pulse duration of 70-~lOopswas used as a pump source. The average power of the laser pump radiation launched into the SRS amplifier in the experiment was 2-2.5 W. This value is somewhat lower than the SRS threshold power in the 22m long optical fibre. The average pump power launched into the SRS generator 1.5 W, which is somewhat higher than the SRS threshwas old power in a 50m long fibre. The SRS amplifier, SRS generator, and the filter form an SRS inverter. The Stokes signal of a sufficiently high intensity (logic one) fed into the SRS amplifier simultaneously with the pump pulse is amplified and causes its depletion. The resulting high Stokes signal is suppressed by the filter, and the depleted pump pulse is unable to generate a sufficiently intense Stokes signal in the SRS generator. As a result, a logic zero (a lowintensity Stokes pulse) is formed at the output of the SRS generator. If there is no input Stokes pulse, the pump radiation travels to the SRS generator almost unattenuated and gives rise to a high intensity Stokes pulse corresponding to a logic one at the output of the SRS generator. The dependence of the output Stokes pulse intensity on the input intensity can be rather sharp, and the ratio between a logic one and logic zero at the output can be higher than at the input. The Stokes pulses recirculating through the loop with the inverter are transformed from logic one to logic zero after every passage through the inverter. After several round trips the intensities of logic ones and logic zeros reach a high and low stable level, respectively. As a result, once launched into the loop, the information in the form of a pulse sequence with low and high intensities will be preserved, and occasional deviations of amplitudes from the stable levels will vanish.
-
DEMONSTRATION OF OVER 100 MILLION ROUND TRIPS IN RECIRCULATING FIBRE LOOP WITH ALL-OPTICAL REGENERATION
V. I. Belotitskii, E. A. Kuzin, M. P. Petrov and V. V. Spirin Indexinu terms. Optical fibre?,Nonlinear U W I C S More than 100 million round tnps of l00ps optical pulses with a pulse repetition frequency of IOOMHz through a fibre recirculating loop are demonstrated. The signal is regenerated by a NOR gate operating via the SRS effect. The recirculating time is more than 10min. This indicates that such a loop can be used to develop an all-optical memory with a long storage time. Introduction: Recently C1-41 we have reported the use of stimulated Raman scattering (SRS) to develop a logic NOR gate. The theoretical estimates show that the SRS NOR gate can provide a pulse repetition rate up to 10ii-1012pulse/s and pump pulse energy of 1 0 ~ i i - 1 0 ~ i 2 J / p m 2The . SRS NOR gate has a high gain coefficient and a sharp dependence of the output intensity on the input intensity. The properties of the SRS NOR gate make it most appropriate for the regeneration of optical signals. In this work we demonstrate experimentally over 100 million round trips of optical pulses through a recirculating fibre loop using an SRS NOR gate for the all-optical regeneration of optical signals. The recirculation time of the optical pulses was more than IOmin. This means that such a system can be used for the development of an all-optical loop fibre memory with a long storage time. Exerimentaf setup and principles: The optical recirculating loop is shown in Fig. 1. It includes an SRS amplifier, a filter that suppresses radiation at the Stokes wavelength, an SRS generator, and a fibre delay line. A singlemode polarisationELECTRONICS LETTERS 7th January 1993 Vol. 29
No 1
E.xperrmenral results and discussion: After the pump radiation is launched into the fibre loop. a sequence of logic ones is formed at the output of the inverter. When this sequence passes through the loop and reaches the input of the inverter, the sequence of logic zeros begins to be formed at the output and so on. As a result, the simplest signal sequence of logic zeros and ones that change from one into the other at each round trip arises in the loop. Fig. 2a depicts the envelope of the pulse train at the Stokes wavelength at the output of the SRS amplifier just after switching on the pump radiation for the case when the information is conserved during circulations. We observed circulations of the sequence for more than 10min. The situation when the sequence is destroyed is presented in Fig. 2a. In this
49
case the amplitudes of logic ones and zeros tend to one level and after several circulations become indistinguishable. The type of operation that will be achieved depends on the intensity of the pump pulses, the delay in the loop, and the parameters of the spectral filter. The effect of the pump power is demonstrated in Fig. 3. It shows the average intensities of logic zeros and ones in the
A c k n o w l e d g m e n t : The authors wish to express their sincere gratitude to V. S. Burtsev for this support of the investigation and useful discussions. 20th October 1992 V. I. Belotitskii, E. A. Kuzin, M. P. Petrov and V. V. Spirin ( A . F. Ioffe Physical Technical Institute of Russian Academy of Sciences, St. Petersburg, 194021, Russia)
References 1 KUZiN, E. A., PETROV, M. P.. and SPIRIN,v. v.: ‘Stimulated-Raman fiber-optic logical invertor’, Sou. Tech. Phys. Lett., 1986, 12, (4).pp. 185-186 [in Russian: Pisma Zh. Teckh. Fiz., 1986, 12, (7), pp.
406-4091 2 BELOTITSKII, v. I.,
KUZIN, E. A., PETROV, M. P., and SPIRIN, v. v.: ’Bistable dynamic ring structure based on stimulated light scattering’, Soc.. Phys. Tech. Phys., 1988, 33, (7). pp. 789-792 [in Russian’ Zh. Tekh. Fiz., 1988.58, (7),pp. 1325-13301 3 KUZIN, E. A., MAKSUTENKO, M. A., PETROV, M. P., and SPIRIN, v. v.: ‘All-optical fiber logic NOR gate using stimulated Raman scattering’, Optical Comp. and Proc., 1991, 1, (2). pp. 163-167 4 PETROV, M.P., and KUZIN, E. A.: ‘Nonlinear phenomena in optical fibres and the feasibility of their application in optical computers’, in ARSENAULT, H. H., SZOPLIK, T., and MACKOW (Eds.): ‘Optical processing and computing’ (Academic Press, San Diego, 1989), p.
33
1737121
CHIRP AND FEEDBACK CONTROL IN SEMICONDUCTOR PIGTAILED DFB LASERS WITHOUT INTEGRATED ISOLATOR
H lous
Fig. 2 Oscilloscope traces oJenuelopes ofStokes pulse train at output oJ SRS amplifier a Information conserved in loop b Information destroyed
logic one
08 09 10 1 1 pump intensity I SRS t h r e s h o l d intensity
1 2
1717131
Fig. 3 Intensities of logic ones and logic zeros against pump intensities launched into SRS amplifier
loop as functions of the pump power launched into the SRS generator. It is seen that for the pump power in the SRS generator below 0.8 of the threshold, ones and zeros do not differ from each other. As the pump intensity grows, the intensity of ones increases and the intensity of zeros decreases. The intensity of ones reaches a level of 0.5 of the pump intensity. The ratio between the intensities of the trains of ones and zeros can be higher than 50. Conclusion: We have experimentally demonstrated the use of the SRS NOR gate to regenerate optical signals in a recirculating fibre loop. The circulation time of the optical pulses was more than 10min. We believe that such a long circulation of optical pulses supported by all optical regeneration has been demonstrated for the first time. The recirculating loop with SRS NOR gate can be used for the development of an alloptical loop fibre memory with a long storage time.
50
lndexinq terms Semiconductor lasers, Lasers
The feedback generated by reflections at FCjPC connectors of pigtailed DFB laser diodes is controlled by varying the polarisation of the reflected light via induced birefringence in the fibre pigtail. It is shown that strong linewidth and mode hopping reductions can be obtained with an external double cavity coupled to the laser diode.
$ 0 8 $ 0 7
J. P. von d e r Weid, R. Passy a n d N. Gisin
Narrow spectral linewidth and frequency tunable optical sources are extremely important for a wide variety of applications ranging from coherent communications to interferometric sensors and metrology. Semiconductor laser diodes have been used as tunable sources, driven by a current ramp in order to generate the desired frequency sweep [l, 21 but the experiments were limited to short ranges due to the source linewidth, so that semiconductor lasers are still far from achieving the results achieved with tuned Nd : YAG lasers [3]. In any case, chirp linearity and mode hopping are still major problems ir. coherent reflectometry, together with the laser linewidth. Sharp spectral lines are usually obtained by coupling a semiconductor laser to an external cavity, depending on the feedback level coupled to the diode laser [4]. For feedback levels between -40 and -60dB the laser line splits into external cavity modes and the mechanism of mode selection and stability has been investigated both theoretically and experimentally [5, 61. We present results on the measurement and control of chirp and mode hopping in external cavity coupled DFB lasers. The laser source used in our experiments was an InGaAsP 1.55 pm DFB laser diode in which the separation between the fibre pigtail and the laser facet was increased to reduce the coupling of the laser to the external cavity. The mode rejection ratio was 20dB and the overall coupling efficiency 10%. between the diode laser and the singlemode fibre was Some experiments were also performed with a Fujitsu 1.3 pm commercial DFB laser. Both lasers gave similar experimental results.
-
ELECTRONICS LETTERS
7th January 1993 Vol. 29
No. 1
