- Email: [email protected]
read capability
15 September 1998
Optics Communications 154 Ž1998. 277–281
All-optical regenerative memory with full writerread capability A.J. Poustie ) , A.E. Kelly, R.J. Manning, K.J. Blow BT Laboratories, Martlesham Heath, Ipswich IP5 3RE, UK Received 26 January 1998; revised 25 March 1998; accepted 9 June 1998
Abstract We demonstrate an all-optical regenerative memory with the capability to write and read optical pulses at any time slot in an optical fibre delay line. The memory architecture comprises three TOAD optical switching gates. q 1998 Elsevier Science B.V. All rights reserved.
Future ultrahigh-speed photonic networks carrying information in the form of ultrashort optical pulses will either be based on high-speed optical time-division multiplexed ŽOTDM. channels w1x or on optical data packets w2x. In such networks, some form of information processing will be required directly at the optical layer to add functionality without introducing unnecessary latency. The function of ‘memory’ is one of the key building blocks that can be used to implement a variety of all-optical processing applications, including optical packet storage. Several types of serial optical memory suitable for storing optical packets have been demonstrated using recirculating optical fibre loops. These memory designs are either regenerative, where the pulses are replaced after some number of loop circulations w3–7x, or pulse-preserving, where the same optical pulses propagate on each circulation of the storage loop w8–10x. The regenerative designs have stability advantages over pulse-preservation memories since there is minimal degradation of the stored optical pulses by propagation effects w11x. Also, certain designs of pulse-preserving memories are essentially mode-locked lasers and the stored packets suffer undesired large-amplitude fluctuations due to relaxation oscillations w12x. We have previously described a novel all-optical regenerative memory architecture which not only provides stable storage of optical packets for ) 10 10 circulations of the stored data pattern w7x but also has the ability to equalise the ampli-
)
E-mail: [email protected]
tudes of the stored pulses w13x. The storage threshold of this memory can also be varied so that it can act as an all-optical discriminator for optical pulses of varying amplitude w13x. In all of these types of memories, the stored optical binary data can only be sampled and not modified within the memory itself w4x. Here we demonstrate an important extension to the regenerative memory design which not only allows optical pulses to be written to the memory but also permits optical pulses to be selectively read out from storage. This additional capability greatly extends the usefulness of the memory as a sub-system in more complex processing operations w14x. The experimental system is shown in Fig. 1. Each TOADrSLALOM w15,16x is a type of non-linear loop mirror ŽNOLM. w17x comprising a 50:50 fused-fiber coupler, two wavelength division multiplexer ŽWDM. couplers to introduce and reject the switching pulses, polarisation controllers to bias the loop and a semiconductor optical amplifier ŽSOA. as the non-linear element. The SOA is placed slightly asymmetrically in the loop to create a temporal switching window w15x. TOAD1 and TOAD2 form the wavelength-switched regenerative memory as described previously w7,13x. The 1 Gbitrs ‘write’ data with wavelength l1 s 1551 nm is entered into the memory through a switching input of TOAD1. This copies the data pattern onto the clock wavelength l2 s 1533 nm and into the optical fibre loop memory. It is then stored for long periods of time by amplifying the stored pulses to switch TOAD1 after each circulation of the optical fibre loop and achieve full optical regeneration of the data pattern from
0030-4018r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 3 3 1 - 9
278
A.J. Poustie et al.r Optics Communications 154 (1998) 277–281
Fig. 1. Schematic diagram of the all-optical regenerative memory with full writerread capability. DFB, distributed feedback semiconductor laser; IrP, input; OrP, output.
the clock source. TOAD2 effectively acts as a wavelength converter to return the data pattern wavelength back to l1 for the regeneration in TOAD1. To achieve the ‘read’ function we now incorporate TOAD3 to act as an all-optical modulator for the data stored in the regenerative memory. The stored data passes through TOAD3, which is normally biased to reflection to direct the stored pulses back to the input port of TOAD3 and hence circulate around the optical fibre delay line as before via the circulator. In order to read out pulses from the memory, a different single-shot ‘read’ data pattern at wavelength l2 is created with an electroabsorption ŽEA. modulator. This data pattern is used to provide the switching pulses to TOAD3, which in the presence of these selectively redirects some of the stored pulses to the output port in TOAD3 and hence out of the memory. Adjustable delay lines ŽSantec ODL-300. were used to attain bit-level synchronisation throughout the experiment and erbium doped fibre amplifiers ŽEDFAs. were used to amplify the optical pulses. Typically, the required switching energy of the optical pulses is extremely low Ž- 1 pJ w7,13,15x. which allows these more complex all-optical circuits to be practically realised. Fig. 2a–2c shows experimental results obtained when writing and reading data from the memory with different ‘read’ patterns. In this case, the pulses which are read out are not directly accessed and only the remaining stored pulses in the memory are shown. Each upper oscilloscope trace shows the 40 bit data sequence stored originally in the memory by inputting the ‘write’ data only once. In principle, the data pattern could fill the whole of the memory frame capacity Ž; 1000 bits. w7x but this short data pattern was only chosen to clearly resolve the stored binary data sequence with the real-time oscilloscope. The
middle oscilloscope traces show a section of the different data sequences used for the ‘read’ operation, which were again input only once. Since there was only synchronisation of the stored data with the ‘read’ data sequence at the bit level but no absolute synchronisation at the ‘memory frame’ level, the length of the ‘read’ data sequence was arranged to occupy the whole of the memory frame capacity. This guaranteed that the 40 bits of stored data were read with the first occurrence of the ‘read’ data. In principle, generating a memory frame synchronisation signal should allow any absolute time slot to be accessed directly. The lower oscilloscope trace shows the final stored data after pulses have been selectively read. Each figure is arranged vertically in bit-level synchronisation, so that the occurrence of a ‘read’ pulse leads to the removal of the corresponding pulse originally stored in the memory. From careful inspection, it can be seen that the three different final stored data patterns all correctly follow from the ‘read’ data being applied. In general, direct access to the optical pulses which are read out is desirable since they can then be redirected to other parts of a more complex processing system. If it is only desired to delete selective pulses within the stored pattern, as described above, then an electro-optic modulator could be used in place of TOAD3. To demonstrate that the optical pulses are actually read out from the memory, the output of TOAD3 was connected to the ‘write’ input of TOAD1. In this case, when the ‘read’ data pattern is applied to TOAD3, the optical pulses which are read out are re-written back into the same optical memory at a different temporal position in the memory frame. Fig. 3 shows the results for this memory configuration. The oscilloscope trace in Fig. 3a shows the detail of the originally stored 40 bit optical packet. Fig. 3b shows this
A.J. Poustie et al.r Optics Communications 154 (1998) 277–281
279
Fig. 2. Oscilloscope traces Ž5 nsrdiv. of original stored data pulses Župper., ‘read’ data sequence Žmiddle. and final stored data pulses Žlower. for ‘read’ patterns of: Ža. 10101010101 . . . ; Žb. 1111000011110000 . . . ; Žc. 100000000010000000001 . . . .
stored packet on the timescale of the memory frame where the packet only occupies 40 of the possible 1000 storage time slots. The packet can be seen to repeat approximately every microsecond corresponding to one round trip in the memory. The upper trace in Fig. 3c shows the data packet that remains in the memory when the 101010 . . . single shot ‘read’ data sequence is applied to TOAD3. This reads-out every second stored pulse Žcompare with lower trace of Fig. 2a.. However, the optical pulses which have been read out are now re-written back into the memory and the lower trace of Fig. 3c shows the detail of the additional stored pulse packet. The presence of two stored packets can clearly be seen at the memory frame level on the oscilloscope trace in Fig. 3d. In essence, we have performed an ‘optical cut and paste’ operation on the stored data. Again, the detailed traces of the stored pulses
in Fig. 3c are vertically arranged in bit synchronisation and it can be seen that the final combination of the two stored packets corresponds exactly to the originally stored packet in Fig. 3a. This demonstrates that no optical pulses were lost or created in the full writerreadrre-write operation. If TOAD3 was configured as an addrdrop multiplexer w18x it would be possible to create an ‘optical copy and paste’ operation in time, where the optical pulses which are read out would automatically be replaced from the ‘add’ channel to TOAD3 as well as being re-written into the memory at a different time. In common with other time-of-flight designed processing systems w19x, the memory architecture and functionality has the advantage of being speed scalable to much higher repetition rates than experimentally demonstrated here. As discussed in Ref. w13x, an ultimate switching
280
A.J. Poustie et al.r Optics Communications 154 (1998) 277–281
Fig. 3. Oscilloscope traces of stored data patterns with writerreadrre-write: Ža. detail of the individual pulses in the originally stored optical packet Ž5 nsrdiv., Žb. originally stored optical packet at the memory frame timescale Ž200 nsrdiv., Žc. detail of the two stored packets in the memory after the readrre-write operation Ž5 nsrdiv., Žd. two stored packets at the memory frame timescale Ž200 nsrdiv..
A.J. Poustie et al.r Optics Communications 154 (1998) 277–281
speed of ; 100 Gbitrs might be attainable in SOA-based all-optical switches Ž40 Gbitrs having been already demonstrated w20–22x. and the functionality could be implemented at speeds of ) 1 Tbitrs by using all-fibre non-linear loop mirrors at the expense of higher switching energies and latency. In addition, the full integration of SOA based all-optical switches w18,23–25x could greatly reduce the current latency in the optical memory system and allow bit-serial all-optical processing to be realised at high data rates w14x. In conclusion, we have demonstrated a novel all-optical regenerative memory with full writerread capability. The ability to manipulate individual bits within the optical memory offers the possibility of creating more complex functionality in the future. This all-optical architecture should be scaleable to ultrafast data rates beyond 100 Gbitrs and to low-latency applications using integrated SOA-based devices.
Acknowledgements The authors would like to thank Colin Ford and Dave Moodie at BT Laboratories for the supply of packaged SOAs and EA modulators.
References w1x S.W. Seo, K. Bergman, P.R. Prucnal, IEEE J. Sel. Areas Comm. 14 Ž1996. 1039. w2x D. Cotter, M.C. Tatham, J.K. Lucek, M. Shabeer, K. Smith, D. Nesset, D.C. Rogers, P. Gunning, in: G. Prati ŽEd.., Photonic Networks: Advances in Optical Communications, Springer, Berlin, 1997, p. 401. w3x T.J. Soukup, R.J. Feuerstein, V.P. Heuring, Appl. Optics 31 Ž1992. 3233. w4x H. Avramopoulos, N.A. Whitaker Jr., Optics Lett. 18 Ž1993. 22. w5x V.I. Belotitskii, E.A. Kuzin, M.P. Petrov, V.V. Spirin, Electron. Lett. 29 Ž1993. 49.
281
w6x S.L. Danielsen, B. Mikkelsen, C. Joergensen, T. Durhuus, K.E. Stubkjaer, IEEE Phot. Technol. Lett. 8 Ž1996. 434. w7x A.J. Poustie, K.J. Blow, R.J. Manning, Optics Comm. 140 Ž1997. 184. w8x M. Eiselt, W. Pieper, G. Groskopf, R. Ludwig, H.G. Weber, IEEE Phot. Technol. Lett. 4 Ž1993. 422. w9x C.R. Doerr, W.S. Wong, H.A. Haus, E.P. Ippen, Optics Lett. 19 Ž1994. 1747. w10x K.L. Hall, J.D. Moores, K.A. Rauschenbach, W.S. Wong, E.P. Ippen, H.A. Haus, IEEE Phot. Technol. Lett. 7 Ž1995. 1093. w11x J.D. Moores, W.S. Wong, H.A. Haus, Optics Comm. 113 Ž1994. 153. w12x K.L. Hall, in: Optical Fibre Communication, Vol. 6, 1997 OSA Technical Digest Series, Optical Society of America, Washington, DC, 1997, p. 251. w13x A.J. Poustie, K.J. Blow, R.J. Manning, Optics Comm. 146 Ž1998. 262. w14x V.P. Heuring, H.F. Jordan, J.P. Pratt, Appl. Optics 31 Ž1992. 3213. w15x J.P. Sokoloff, P.R. Prucnal, I. Glesk, M. Kane, IEEE Phot. Technol. Lett. 5 Ž1993. 787. w16x M. Eiselt, W. Peiper, H.G. Weber, IEEE J. Light. Technol. 13 Ž1995. 2099. w17x N.J. Doran, D. Wood, Optics Lett. 13 Ž1988. 56. w18x E. Jahn, N. Agrawal, H.J. Ehrke, R. Ludwig, W. Peiper and, H.G. Weber, Electron. Lett. 32 Ž1996. 216. w19x H.F. Jordan, V.P. Heuring, R. Feuerstein, Proc. IEE 82 Ž1994. 1678. w20x A.D. Ellis, D.A.O. Davies, A. Kelly, W.A. Pender, Electron. Lett. 31 Ž1995. 1245. w21x N.S. Patel, K.A. Rauschenbach, K.A. Hall, IEEE Phot. Technol. Lett. 8 Ž1996. 1695. w22x K.L. Hall, J.P. Donnelly, S.H. Groves, C.I. Fennelly, R.J. Bailey, A. Napoleone, Optics Lett. 22 Ž1997. 1479. w23x E. Jahn, N. Agrawal, M. Arbert, H.J. Ehrke, D. Franke, R. Ludwig, W. Peiper, H.G. Weber, C.M. Weinert, Electron. Lett. 31 Ž1995. 1857. w24x E. Jahn, N. Agrawal, W. Peiper, H.J. Ehrke, D. Franke, W. Furst, C.M. Weinert, Electron. Lett. 32 Ž1996. 782. w25x B. Mikkelsen, T. Durhuus, C. Joergensen, R.J.S. Pedersen, S.L. Danielsen, K.E. Stubkjaer, M. Gustavsson, W. Van Berlo, M. Janson, Postdeadline paper, Proceedings ECOC 1994, Florence, 1994.
Optics Communications 154 Ž1998. 277–281
All-optical regenerative memory with full writerread capability A.J. Poustie ) , A.E. Kelly, R.J. Manning, K.J. Blow BT Laboratories, Martlesham Heath, Ipswich IP5 3RE, UK Received 26 January 1998; revised 25 March 1998; accepted 9 June 1998
Abstract We demonstrate an all-optical regenerative memory with the capability to write and read optical pulses at any time slot in an optical fibre delay line. The memory architecture comprises three TOAD optical switching gates. q 1998 Elsevier Science B.V. All rights reserved.
Future ultrahigh-speed photonic networks carrying information in the form of ultrashort optical pulses will either be based on high-speed optical time-division multiplexed ŽOTDM. channels w1x or on optical data packets w2x. In such networks, some form of information processing will be required directly at the optical layer to add functionality without introducing unnecessary latency. The function of ‘memory’ is one of the key building blocks that can be used to implement a variety of all-optical processing applications, including optical packet storage. Several types of serial optical memory suitable for storing optical packets have been demonstrated using recirculating optical fibre loops. These memory designs are either regenerative, where the pulses are replaced after some number of loop circulations w3–7x, or pulse-preserving, where the same optical pulses propagate on each circulation of the storage loop w8–10x. The regenerative designs have stability advantages over pulse-preservation memories since there is minimal degradation of the stored optical pulses by propagation effects w11x. Also, certain designs of pulse-preserving memories are essentially mode-locked lasers and the stored packets suffer undesired large-amplitude fluctuations due to relaxation oscillations w12x. We have previously described a novel all-optical regenerative memory architecture which not only provides stable storage of optical packets for ) 10 10 circulations of the stored data pattern w7x but also has the ability to equalise the ampli-
)
E-mail: [email protected]
tudes of the stored pulses w13x. The storage threshold of this memory can also be varied so that it can act as an all-optical discriminator for optical pulses of varying amplitude w13x. In all of these types of memories, the stored optical binary data can only be sampled and not modified within the memory itself w4x. Here we demonstrate an important extension to the regenerative memory design which not only allows optical pulses to be written to the memory but also permits optical pulses to be selectively read out from storage. This additional capability greatly extends the usefulness of the memory as a sub-system in more complex processing operations w14x. The experimental system is shown in Fig. 1. Each TOADrSLALOM w15,16x is a type of non-linear loop mirror ŽNOLM. w17x comprising a 50:50 fused-fiber coupler, two wavelength division multiplexer ŽWDM. couplers to introduce and reject the switching pulses, polarisation controllers to bias the loop and a semiconductor optical amplifier ŽSOA. as the non-linear element. The SOA is placed slightly asymmetrically in the loop to create a temporal switching window w15x. TOAD1 and TOAD2 form the wavelength-switched regenerative memory as described previously w7,13x. The 1 Gbitrs ‘write’ data with wavelength l1 s 1551 nm is entered into the memory through a switching input of TOAD1. This copies the data pattern onto the clock wavelength l2 s 1533 nm and into the optical fibre loop memory. It is then stored for long periods of time by amplifying the stored pulses to switch TOAD1 after each circulation of the optical fibre loop and achieve full optical regeneration of the data pattern from
0030-4018r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 3 3 1 - 9
278
A.J. Poustie et al.r Optics Communications 154 (1998) 277–281
Fig. 1. Schematic diagram of the all-optical regenerative memory with full writerread capability. DFB, distributed feedback semiconductor laser; IrP, input; OrP, output.
the clock source. TOAD2 effectively acts as a wavelength converter to return the data pattern wavelength back to l1 for the regeneration in TOAD1. To achieve the ‘read’ function we now incorporate TOAD3 to act as an all-optical modulator for the data stored in the regenerative memory. The stored data passes through TOAD3, which is normally biased to reflection to direct the stored pulses back to the input port of TOAD3 and hence circulate around the optical fibre delay line as before via the circulator. In order to read out pulses from the memory, a different single-shot ‘read’ data pattern at wavelength l2 is created with an electroabsorption ŽEA. modulator. This data pattern is used to provide the switching pulses to TOAD3, which in the presence of these selectively redirects some of the stored pulses to the output port in TOAD3 and hence out of the memory. Adjustable delay lines ŽSantec ODL-300. were used to attain bit-level synchronisation throughout the experiment and erbium doped fibre amplifiers ŽEDFAs. were used to amplify the optical pulses. Typically, the required switching energy of the optical pulses is extremely low Ž- 1 pJ w7,13,15x. which allows these more complex all-optical circuits to be practically realised. Fig. 2a–2c shows experimental results obtained when writing and reading data from the memory with different ‘read’ patterns. In this case, the pulses which are read out are not directly accessed and only the remaining stored pulses in the memory are shown. Each upper oscilloscope trace shows the 40 bit data sequence stored originally in the memory by inputting the ‘write’ data only once. In principle, the data pattern could fill the whole of the memory frame capacity Ž; 1000 bits. w7x but this short data pattern was only chosen to clearly resolve the stored binary data sequence with the real-time oscilloscope. The
middle oscilloscope traces show a section of the different data sequences used for the ‘read’ operation, which were again input only once. Since there was only synchronisation of the stored data with the ‘read’ data sequence at the bit level but no absolute synchronisation at the ‘memory frame’ level, the length of the ‘read’ data sequence was arranged to occupy the whole of the memory frame capacity. This guaranteed that the 40 bits of stored data were read with the first occurrence of the ‘read’ data. In principle, generating a memory frame synchronisation signal should allow any absolute time slot to be accessed directly. The lower oscilloscope trace shows the final stored data after pulses have been selectively read. Each figure is arranged vertically in bit-level synchronisation, so that the occurrence of a ‘read’ pulse leads to the removal of the corresponding pulse originally stored in the memory. From careful inspection, it can be seen that the three different final stored data patterns all correctly follow from the ‘read’ data being applied. In general, direct access to the optical pulses which are read out is desirable since they can then be redirected to other parts of a more complex processing system. If it is only desired to delete selective pulses within the stored pattern, as described above, then an electro-optic modulator could be used in place of TOAD3. To demonstrate that the optical pulses are actually read out from the memory, the output of TOAD3 was connected to the ‘write’ input of TOAD1. In this case, when the ‘read’ data pattern is applied to TOAD3, the optical pulses which are read out are re-written back into the same optical memory at a different temporal position in the memory frame. Fig. 3 shows the results for this memory configuration. The oscilloscope trace in Fig. 3a shows the detail of the originally stored 40 bit optical packet. Fig. 3b shows this
A.J. Poustie et al.r Optics Communications 154 (1998) 277–281
279
Fig. 2. Oscilloscope traces Ž5 nsrdiv. of original stored data pulses Župper., ‘read’ data sequence Žmiddle. and final stored data pulses Žlower. for ‘read’ patterns of: Ža. 10101010101 . . . ; Žb. 1111000011110000 . . . ; Žc. 100000000010000000001 . . . .
stored packet on the timescale of the memory frame where the packet only occupies 40 of the possible 1000 storage time slots. The packet can be seen to repeat approximately every microsecond corresponding to one round trip in the memory. The upper trace in Fig. 3c shows the data packet that remains in the memory when the 101010 . . . single shot ‘read’ data sequence is applied to TOAD3. This reads-out every second stored pulse Žcompare with lower trace of Fig. 2a.. However, the optical pulses which have been read out are now re-written back into the memory and the lower trace of Fig. 3c shows the detail of the additional stored pulse packet. The presence of two stored packets can clearly be seen at the memory frame level on the oscilloscope trace in Fig. 3d. In essence, we have performed an ‘optical cut and paste’ operation on the stored data. Again, the detailed traces of the stored pulses
in Fig. 3c are vertically arranged in bit synchronisation and it can be seen that the final combination of the two stored packets corresponds exactly to the originally stored packet in Fig. 3a. This demonstrates that no optical pulses were lost or created in the full writerreadrre-write operation. If TOAD3 was configured as an addrdrop multiplexer w18x it would be possible to create an ‘optical copy and paste’ operation in time, where the optical pulses which are read out would automatically be replaced from the ‘add’ channel to TOAD3 as well as being re-written into the memory at a different time. In common with other time-of-flight designed processing systems w19x, the memory architecture and functionality has the advantage of being speed scalable to much higher repetition rates than experimentally demonstrated here. As discussed in Ref. w13x, an ultimate switching
280
A.J. Poustie et al.r Optics Communications 154 (1998) 277–281
Fig. 3. Oscilloscope traces of stored data patterns with writerreadrre-write: Ža. detail of the individual pulses in the originally stored optical packet Ž5 nsrdiv., Žb. originally stored optical packet at the memory frame timescale Ž200 nsrdiv., Žc. detail of the two stored packets in the memory after the readrre-write operation Ž5 nsrdiv., Žd. two stored packets at the memory frame timescale Ž200 nsrdiv..
A.J. Poustie et al.r Optics Communications 154 (1998) 277–281
speed of ; 100 Gbitrs might be attainable in SOA-based all-optical switches Ž40 Gbitrs having been already demonstrated w20–22x. and the functionality could be implemented at speeds of ) 1 Tbitrs by using all-fibre non-linear loop mirrors at the expense of higher switching energies and latency. In addition, the full integration of SOA based all-optical switches w18,23–25x could greatly reduce the current latency in the optical memory system and allow bit-serial all-optical processing to be realised at high data rates w14x. In conclusion, we have demonstrated a novel all-optical regenerative memory with full writerread capability. The ability to manipulate individual bits within the optical memory offers the possibility of creating more complex functionality in the future. This all-optical architecture should be scaleable to ultrafast data rates beyond 100 Gbitrs and to low-latency applications using integrated SOA-based devices.
Acknowledgements The authors would like to thank Colin Ford and Dave Moodie at BT Laboratories for the supply of packaged SOAs and EA modulators.
References w1x S.W. Seo, K. Bergman, P.R. Prucnal, IEEE J. Sel. Areas Comm. 14 Ž1996. 1039. w2x D. Cotter, M.C. Tatham, J.K. Lucek, M. Shabeer, K. Smith, D. Nesset, D.C. Rogers, P. Gunning, in: G. Prati ŽEd.., Photonic Networks: Advances in Optical Communications, Springer, Berlin, 1997, p. 401. w3x T.J. Soukup, R.J. Feuerstein, V.P. Heuring, Appl. Optics 31 Ž1992. 3233. w4x H. Avramopoulos, N.A. Whitaker Jr., Optics Lett. 18 Ž1993. 22. w5x V.I. Belotitskii, E.A. Kuzin, M.P. Petrov, V.V. Spirin, Electron. Lett. 29 Ž1993. 49.
281
w6x S.L. Danielsen, B. Mikkelsen, C. Joergensen, T. Durhuus, K.E. Stubkjaer, IEEE Phot. Technol. Lett. 8 Ž1996. 434. w7x A.J. Poustie, K.J. Blow, R.J. Manning, Optics Comm. 140 Ž1997. 184. w8x M. Eiselt, W. Pieper, G. Groskopf, R. Ludwig, H.G. Weber, IEEE Phot. Technol. Lett. 4 Ž1993. 422. w9x C.R. Doerr, W.S. Wong, H.A. Haus, E.P. Ippen, Optics Lett. 19 Ž1994. 1747. w10x K.L. Hall, J.D. Moores, K.A. Rauschenbach, W.S. Wong, E.P. Ippen, H.A. Haus, IEEE Phot. Technol. Lett. 7 Ž1995. 1093. w11x J.D. Moores, W.S. Wong, H.A. Haus, Optics Comm. 113 Ž1994. 153. w12x K.L. Hall, in: Optical Fibre Communication, Vol. 6, 1997 OSA Technical Digest Series, Optical Society of America, Washington, DC, 1997, p. 251. w13x A.J. Poustie, K.J. Blow, R.J. Manning, Optics Comm. 146 Ž1998. 262. w14x V.P. Heuring, H.F. Jordan, J.P. Pratt, Appl. Optics 31 Ž1992. 3213. w15x J.P. Sokoloff, P.R. Prucnal, I. Glesk, M. Kane, IEEE Phot. Technol. Lett. 5 Ž1993. 787. w16x M. Eiselt, W. Peiper, H.G. Weber, IEEE J. Light. Technol. 13 Ž1995. 2099. w17x N.J. Doran, D. Wood, Optics Lett. 13 Ž1988. 56. w18x E. Jahn, N. Agrawal, H.J. Ehrke, R. Ludwig, W. Peiper and, H.G. Weber, Electron. Lett. 32 Ž1996. 216. w19x H.F. Jordan, V.P. Heuring, R. Feuerstein, Proc. IEE 82 Ž1994. 1678. w20x A.D. Ellis, D.A.O. Davies, A. Kelly, W.A. Pender, Electron. Lett. 31 Ž1995. 1245. w21x N.S. Patel, K.A. Rauschenbach, K.A. Hall, IEEE Phot. Technol. Lett. 8 Ž1996. 1695. w22x K.L. Hall, J.P. Donnelly, S.H. Groves, C.I. Fennelly, R.J. Bailey, A. Napoleone, Optics Lett. 22 Ž1997. 1479. w23x E. Jahn, N. Agrawal, M. Arbert, H.J. Ehrke, D. Franke, R. Ludwig, W. Peiper, H.G. Weber, C.M. Weinert, Electron. Lett. 31 Ž1995. 1857. w24x E. Jahn, N. Agrawal, W. Peiper, H.J. Ehrke, D. Franke, W. Furst, C.M. Weinert, Electron. Lett. 32 Ž1996. 782. w25x B. Mikkelsen, T. Durhuus, C. Joergensen, R.J.S. Pedersen, S.L. Danielsen, K.E. Stubkjaer, M. Gustavsson, W. Van Berlo, M. Janson, Postdeadline paper, Proceedings ECOC 1994, Florence, 1994.