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s WDM transmission using NOLM and novel filtering technique
Optics Communications 217 (2003) 227–232 www.elsevier.com/locate/optcom
All-optical passive 2R regeneration for N 40 Gbit/s WDM transmission using NOLM and novel filtering technique Sonia Boscolo *, Sergei K. Turitsyn, Keith J. Blow Photonics Research Group, School of Engineering and Applied Science, Aston University, Birmingham B4 7ET, UK Received 25 October 2002; received in revised form 18 December 2002; accepted 7 January 2003
Abstract We propose an all-optical passive 2R regeneration method for WDM (N 40 Gbit/s) dispersion-managed RZ transmission based on specially designed WDM guiding filters and in-line nonlinear optical loop mirrors. By system optimisation, the feasibility of 150 GHz-spaced l6 channel transmission over 25,000 km of standard fibre is numerically demonstrated. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.79.Sz; 42.79.Yd; 42.81.Dp; 42.65.Re
1. Introduction New solutions are required to increase both the error-free transmission distance and the link capacity in wavelength-division multiplexed (WDM) systems operating at high bit-rates over the standard monomode fibre (SMF) most commonly installed in current networks. High capacity 20 and 40 Gbit/s-based WDM experiments over a few thousand km through SMF-based lines without active control have been reported [1–3]. Ultralong, high capacity transmission over SMF is still a challenge to be resolved. It has been shown recently that appropriate optical filtering is a
*
Corresponding author. Tel.: +44-121-359-3621x4961; fax: +44-121-359-0156. E-mail address: [email protected] (S. Boscolo).
promising technique that enables substantial advance in improving the performance of 40 Gbit/s WDM systems [4]. In this paper we propose a novel technique of all-optical passive 2R regeneration for 40 Gbit/s WDM return-to-zero (RZ) transmission based on specially designed WDM guiding filters and in-line nonlinear optical loop mirrors (NOLMs). Previous studies have addressed the use of NOLMs to reshape and stabilize pulses in uniform dispersion fibre transmission systems [5–7] and in a similar problem of stabilization of fibre soliton lasers [8]. Recently, we have demonstrated the feasibility and investigated the tolerance of distance-unlimited transmission of a 40 Gbit/s RZ single-channel data signal over dispersion-managed (DM) SMF without active control [9,10]. The key to success was the loop mirror saturable absorption action that allowed for regeneration of pulse amplitude and
0030-4018/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0030-4018(03)01123-4
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S. Boscolo et al. / Optics Communications 217 (2003) 227–232
shape. Here, we show that quasi-regeneration of signals in WDM systems can be achieved by combining NOLMs with proper optical filtering. The suggested system configuration is designed to ensure periodical cleaning up of different WDM channels during transmission, which leads to significant enhancement of the system performance. Indeed, by applying numerical modelling to system optimisation, we demonstrate the feasibility of 150 GHz-spaced l6 channel transmission over 25,000 km of SMF.
2. System description The WDM transmission scheme used for demonstration of the technique is depicted in Fig. 1. This configuration is similar to our previously reported single-channel transmission scheme [9], except that we use here one NOLM for each channel. Such a per-channel scheme avoids crossphase modulation (XPM)-induced wave distortion inside the loop mirror. The transmission line is composed of an equal number of 32.3 km-long SMF and 6.8 km-long dispersion compensating fibre (DCF). The dispersion map consists of an alternation of a SMF–DCF and a DCF–SMF block. The dispersion is 15.0 ps/(nm km) for the SMF and )71.2 ps/(nm km) for the DCF at 1550 nm, giving an average dispersion of 0.09 ps/(nm km). The effective area is 70 lm2 for the SMF and 30 lm2 for the DCF. The attenuation is 0.22 dB/ km in the SMF and 0.65 dB/km in the DCF. An erbium-doped fibre amplifier with a noise figure of 4.5 dB and a power gain of 11.6 dB follows each of the two blocks. No filter is added at the amplifier. An identical NOLM for each channel is placed into the transmission line every five periods of the
dispersion map. Each NOLM incorporates a 50:50 coupler, and a loop of dispersion-shifted fibre with zero dispersion at 1550 nm, an attenuation of 0.3 dB/km, and an effective area of 25 lm2 . Unbalancing of the NOLM is achieved with an asymmetrically placed attenuator close to the loop coupler. Pre-amplification is included, by adding an extra-gain G to the amplifier immediately preceding the NOLM. The continuous-wave input–output power mapping of the NOLM–preamplifier combination is given by GD rGð1 DÞLeff Pout ¼ Pin 1 cos Pin : ð1Þ 2g 2 Here Pout and Pin are the output and the input powers, respectively; D is the power loss of the loop attenuator; r is the nonlinear coefficient of the loop fibre; g ¼ expð2CLÞ, where L is the loop length, and C is the loss coefficient (1/km) of the loop fibre; and Leff ¼ ð1 1=gÞ=ð2CÞ. Stable system operation of the NOLM is obtained by requiring that the NOLM operates in the region just after the first peak of the switching curve, and by demanding the output power to equal the input one. After fixing a suitable value for the loop length, L ¼ 1:5 km throughout this paper, the two conditions above uniquely determine D and G for a given Pin . The demultiplexer and multiplexer placed before and after the NOLM are realized with specially designed optical filters with an amplitude transfer function m mk 2m m mk 2 f ðmÞ ¼ exp ; m > 1; Dm0 Dm00 ð2Þ where k is the channel number. Such a design enables filters to perform simultaneously two func-
Fig. 1. One element of the periodic WDM transmission system.
S. Boscolo et al. / Optics Communications 217 (2003) 227–232
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Fig. 2. The WDM filtering.
tions: WDM channel separation and channel guiding. Indeed, the super-Gaussian part of the transfer function provides for sharp edges, that enable a clean separation between the channels, while the Gaussian part allows for a guiding top (see Fig. 2). This filter transfer function can be easily implemented with existing technologies. For instance, one may use a fibre Bragg grating with the prescribed apodisation profile. We use here a value of 7 for the parameter m controlling the degree of edge sharpness in (2), which is chosen after performing a number of simulations, where the full-width at half-maximum (FWHM) bandwidth of the super-Gaussian part of the function is kept at a fixed value. We remark that, unlike filters that are commonly placed into a transmission line to limit the bandwidth of the noise, the WDM filters considered here do affect pulse propagation. This implies that good system performance depends on the proper balance between the edgecutting and the guiding effects of the WDM filters. Therefore, an optimisation of the ratio Dm0 =Dm00 has been performed.
3. Transmission simulations and results Fig. 3 shows an example of single pulse propagation in the WDM system. This example corresponds to Dm0 =Dm00 ¼ 0:8333. The pulse profile is taken at the starting point of the DM cycle. It can be seen that the pulse settles to a steady state after a
Fig. 3. Single pulse propagation in the WDM system. Top, pulse stabilization at the starting point of the DM cycle. Bottom, evolution of the stationary RMS pulse width (dashed line) and bandwidth (solid line) over the distance between filters – NOLM blocks.
short initial transition distance, which demonstrates the effectiveness of the NOLM in stabilizing the pulses. The stationary pulse parameters at the beginning of each cycle are 1.7 mW peak power, 9.1 ps FWHM pulse width, and )0.006 THz2 chirp. We point out that the strength of the dispersion map used here, S ’ 15, is beyond the range where stable DM solitons exist [11]. Fig. 3 also shows the evolution of the stationary root-mean-square (RMS) pulse width and bandwidth over the distance between two consecutive filters – NOLM blocks (one period of the system). The starting point is the output of one regeneration block and the ending point is the input of the next block. The pulse bandwidth exhibits a net change of )33% within one period, and is restored to its starting value by the next regeneration block, thus experiencing a
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jump at each regeneration block position. Note that the large net change in bandwidth within one period is an indication that the stable pulse dynamics imposed by the periodic deployment of the regeneration elements in the system is nonlinear. Note also that the stationary pulse bandwidth or temporal width for the system is strictly fixed by the WDM filtering. Indeed, we have found that the pulse width, considered at a fixed point in each cycle, stabilizes always on the same value for any initial width and peak power. We have performed 40 Gbit/s WDM transmission simulations with a channel spacing of 150 GHz. Each channel carries a 26 1 pseudo-random binary sequence data stream using Gaussian pulses with the stationary peak power, width, and chirp reached during single pulse propagation. The launch point is the input of the amplifier preceding the NOLM. The system performance is examined in terms of maximum transmission distance corresponding to a linear Q-value of more than 6 (equivalent to a bit error rate of 109 ) for the worst channel. The Q-factor for each channel is evaluated just before the NOLM location. The bandwidth of the electrical filter at the receiver is set to 25 GHz which is chosen after performing a range of simulations. Fig. 4 shows the maximum error-free transmission distance for each channel as a function of the ratio Dm0 =Dm00 for four channel WDM transmission. Here Dm0 is set to 75 GHz, corresponding
to a FWHM bandwidth of 139 GHz which is slightly less than the channel spacing, and Dm00 is allowed to vary. It can be seen that a transmission distance of more than 30,000 km is possible for all channels when Dm0 =Dm00 is equal to 0.9375. Fig. 5 shows the Q-value evolution for four channel WDM and single-channel transmission, using the optimal WDM filtering. In this example input pulses with a peak power of 0.14 mW (corresponding to 1.7 mW at the starting point of the DM cycle), a FWHM of 9.4 ps, and a chirp of )0.005 THz2 are used. The extra-gain of the amplifier preceding the NOLM is G ¼ 29:0 dB, and the power loss of the loop attenuator is D ¼ 24:9 dB. Note that the pre-amplifier gain is slightly increased with respect to the value required by the operational regime of the NOLM, to provide partial compensation of the loss introduced by the WDM filters. One may see that single-channel transmission over practically unlimited distances is possible in the optimised WDM system. Unlimited transmission means here that after some stage, the accumulation of detrimental effects through the transmission system is stabilized, yielding nondecaying Q-factor > 6. This result corresponds to our recently reported result using a single-channel transmission scheme [9], and indicates that the WDM performance is only affected by interchannel effects. Fig. 6 shows the eye-diagram of one of the four channels at the maximum transmission distance and the single-channel eye-dia-
Fig. 4. Maximum error-free transmission distance versus Dm0 =Dm00 for 40 Gbit/s 4 WDM system.
Fig. 5. Q-values versus propagation distance for 40 Gbit/s 4 WDM and 40 Gbit/s single-channel transmission.
S. Boscolo et al. / Optics Communications 217 (2003) 227–232
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Fig. 7. Maximum error-free transmission distance versus number of channels.
Fig. 6. Eye-diagrams of the 40 Gbit/s 4 WDM and 40 Gbit/s single-channel data signals.
gram after 40,000 km transmission. It is seen that the WDM transmission performance is mainly degraded by inter-channel XPM-induced timing jitter. The single-channel eye-diagram is still open at 40,000 km, showing that the accumulation of intra-channel timing jitter through the transmission system is not enough at this stage to produce a significant degradation of the Q-factor. In both WDM and single-channel transmissions, in fact, intra-channel four-wave mixing is not a limiting factor, due to the loop mirror intensity filtering action in suppressing the background and reducing the amplitude jitter of ones. To demonstrate the flexibility of the proposed technique, we have increased the number of channels up to 16. Fig. 7 shows the maximum er-
ror-free transmission distance for the worst channel as a function of the number of channels. It can be seen that 16 channel WDM transmission over 25,000 km is achievable. So far, we have not included the third-order dispersion (TOD) of fibres in our calculations. We point out that the per-channel operation of the NOLM and the short length of the loop fibre make the effect of chromatic dispersion on the propagation of any WDM channel in the loop negligible, whereas, self-phase modulation is the dominant effect. On the other hand, an allowed dispersion in
Fig. 8. Maximum error-free transmission distance versus residual TOD for 40 Gbit/s 16 WDM system.
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the NOLM would obviously contribute to the dispersion of the transmission line. Now, to estimate the impact of TOD on the WDM transmission performance, we allow the presence of a residual (uncompensated) TOD in the fibre link. For operation in the anomalous average dispersion regime, the maximum amount of TOD permitted in the system is given by b3;cr ¼ hb2 i0 = ð2pDmÞ, where hb2 i0 is the average group-velocity dispersion at the reference frequency, and Dm is the detuning of the outermost channel from the reference frequency. Fig. 8 shows the maximum error-free transmission distance for the worst channel as a function of the TOD present in the system, for 16 channel WDM transmission. One may see that TOD affects the achievable transmission distance but does not destroy the system operation. As a final note, we anticipate that the periodic in-line pulse reshaping performed by NOLMs may allow waveform distortions caused by polarisation-mode dispersion to be reduced.
4. Conclusion We have proposed an all-optical passive regeneration technique based on specially designed
WDM guiding filters coupled with in-line NOLMs to enhance the transmission performance of 40 Gbit/s WDM RZ systems. Optimisation of the system allowed for 150 GHz-spaced 16 channel transmission over 25,000 km of SMF.
References [1] D. LeGuen, S. Del Burgo, M.L. Moulinard, D. Grot, M. Henry, F. Favre, T. Georges, Postdeadline Papers of OFC/ IOOCÕ99, PD4, San Diego, California, 1999. [2] T. Tsuritani, A. Agata, K. Imai, K. Tanaka, T. Miyakawa, N. Edagawa, M. Suzuki, Postdeadline Papers of ECOCÕ2000, PD1.5, Munick, Germany, 2000. [3] K. Suzuki, H. Kubota, A. Sahara, M. Nakazawa, Electron. Lett. 36 (2000) 443. [4] S. Bigo, Tech. Dig. of OFCÕ2002, WX3, Anaheim, California, 2002, pp. 362–364. [5] M. Matsumoto, H. Ikeda, A. Hasegawa, Opt. Lett. 19 (1994) 183. [6] N.J. Smith, N.J. Doran, J. Opt. Soc. Am. B 12 (1995) 1117. [7] I. Gabitov, D.D. Holm, B.P. Luce, J. Opt. Soc. Am. B 14 (1997) 1850. [8] I.N. Duling III, Opt. Lett. 16 (1991) 539. [9] S. Boscolo, S.K. Turitsyn, K.J. Blow, Electron. Lett. 37 (2001) 112. [10] S. Boscolo, S.K. Turitsyn, K.J. Blow, IEEE Photon. Technol. Lett. 14 (2002) 30. [11] J.H.B Nijhof, N.J. Doran, W. Forysiak, A. Berntson, Electron. Lett. 34 (1998) 481.
All-optical passive 2R regeneration for N 40 Gbit/s WDM transmission using NOLM and novel filtering technique Sonia Boscolo *, Sergei K. Turitsyn, Keith J. Blow Photonics Research Group, School of Engineering and Applied Science, Aston University, Birmingham B4 7ET, UK Received 25 October 2002; received in revised form 18 December 2002; accepted 7 January 2003
Abstract We propose an all-optical passive 2R regeneration method for WDM (N 40 Gbit/s) dispersion-managed RZ transmission based on specially designed WDM guiding filters and in-line nonlinear optical loop mirrors. By system optimisation, the feasibility of 150 GHz-spaced l6 channel transmission over 25,000 km of standard fibre is numerically demonstrated. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.79.Sz; 42.79.Yd; 42.81.Dp; 42.65.Re
1. Introduction New solutions are required to increase both the error-free transmission distance and the link capacity in wavelength-division multiplexed (WDM) systems operating at high bit-rates over the standard monomode fibre (SMF) most commonly installed in current networks. High capacity 20 and 40 Gbit/s-based WDM experiments over a few thousand km through SMF-based lines without active control have been reported [1–3]. Ultralong, high capacity transmission over SMF is still a challenge to be resolved. It has been shown recently that appropriate optical filtering is a
*
Corresponding author. Tel.: +44-121-359-3621x4961; fax: +44-121-359-0156. E-mail address: [email protected] (S. Boscolo).
promising technique that enables substantial advance in improving the performance of 40 Gbit/s WDM systems [4]. In this paper we propose a novel technique of all-optical passive 2R regeneration for 40 Gbit/s WDM return-to-zero (RZ) transmission based on specially designed WDM guiding filters and in-line nonlinear optical loop mirrors (NOLMs). Previous studies have addressed the use of NOLMs to reshape and stabilize pulses in uniform dispersion fibre transmission systems [5–7] and in a similar problem of stabilization of fibre soliton lasers [8]. Recently, we have demonstrated the feasibility and investigated the tolerance of distance-unlimited transmission of a 40 Gbit/s RZ single-channel data signal over dispersion-managed (DM) SMF without active control [9,10]. The key to success was the loop mirror saturable absorption action that allowed for regeneration of pulse amplitude and
0030-4018/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0030-4018(03)01123-4
228
S. Boscolo et al. / Optics Communications 217 (2003) 227–232
shape. Here, we show that quasi-regeneration of signals in WDM systems can be achieved by combining NOLMs with proper optical filtering. The suggested system configuration is designed to ensure periodical cleaning up of different WDM channels during transmission, which leads to significant enhancement of the system performance. Indeed, by applying numerical modelling to system optimisation, we demonstrate the feasibility of 150 GHz-spaced l6 channel transmission over 25,000 km of SMF.
2. System description The WDM transmission scheme used for demonstration of the technique is depicted in Fig. 1. This configuration is similar to our previously reported single-channel transmission scheme [9], except that we use here one NOLM for each channel. Such a per-channel scheme avoids crossphase modulation (XPM)-induced wave distortion inside the loop mirror. The transmission line is composed of an equal number of 32.3 km-long SMF and 6.8 km-long dispersion compensating fibre (DCF). The dispersion map consists of an alternation of a SMF–DCF and a DCF–SMF block. The dispersion is 15.0 ps/(nm km) for the SMF and )71.2 ps/(nm km) for the DCF at 1550 nm, giving an average dispersion of 0.09 ps/(nm km). The effective area is 70 lm2 for the SMF and 30 lm2 for the DCF. The attenuation is 0.22 dB/ km in the SMF and 0.65 dB/km in the DCF. An erbium-doped fibre amplifier with a noise figure of 4.5 dB and a power gain of 11.6 dB follows each of the two blocks. No filter is added at the amplifier. An identical NOLM for each channel is placed into the transmission line every five periods of the
dispersion map. Each NOLM incorporates a 50:50 coupler, and a loop of dispersion-shifted fibre with zero dispersion at 1550 nm, an attenuation of 0.3 dB/km, and an effective area of 25 lm2 . Unbalancing of the NOLM is achieved with an asymmetrically placed attenuator close to the loop coupler. Pre-amplification is included, by adding an extra-gain G to the amplifier immediately preceding the NOLM. The continuous-wave input–output power mapping of the NOLM–preamplifier combination is given by GD rGð1 DÞLeff Pout ¼ Pin 1 cos Pin : ð1Þ 2g 2 Here Pout and Pin are the output and the input powers, respectively; D is the power loss of the loop attenuator; r is the nonlinear coefficient of the loop fibre; g ¼ expð2CLÞ, where L is the loop length, and C is the loss coefficient (1/km) of the loop fibre; and Leff ¼ ð1 1=gÞ=ð2CÞ. Stable system operation of the NOLM is obtained by requiring that the NOLM operates in the region just after the first peak of the switching curve, and by demanding the output power to equal the input one. After fixing a suitable value for the loop length, L ¼ 1:5 km throughout this paper, the two conditions above uniquely determine D and G for a given Pin . The demultiplexer and multiplexer placed before and after the NOLM are realized with specially designed optical filters with an amplitude transfer function m mk 2m m mk 2 f ðmÞ ¼ exp ; m > 1; Dm0 Dm00 ð2Þ where k is the channel number. Such a design enables filters to perform simultaneously two func-
Fig. 1. One element of the periodic WDM transmission system.
S. Boscolo et al. / Optics Communications 217 (2003) 227–232
229
Fig. 2. The WDM filtering.
tions: WDM channel separation and channel guiding. Indeed, the super-Gaussian part of the transfer function provides for sharp edges, that enable a clean separation between the channels, while the Gaussian part allows for a guiding top (see Fig. 2). This filter transfer function can be easily implemented with existing technologies. For instance, one may use a fibre Bragg grating with the prescribed apodisation profile. We use here a value of 7 for the parameter m controlling the degree of edge sharpness in (2), which is chosen after performing a number of simulations, where the full-width at half-maximum (FWHM) bandwidth of the super-Gaussian part of the function is kept at a fixed value. We remark that, unlike filters that are commonly placed into a transmission line to limit the bandwidth of the noise, the WDM filters considered here do affect pulse propagation. This implies that good system performance depends on the proper balance between the edgecutting and the guiding effects of the WDM filters. Therefore, an optimisation of the ratio Dm0 =Dm00 has been performed.
3. Transmission simulations and results Fig. 3 shows an example of single pulse propagation in the WDM system. This example corresponds to Dm0 =Dm00 ¼ 0:8333. The pulse profile is taken at the starting point of the DM cycle. It can be seen that the pulse settles to a steady state after a
Fig. 3. Single pulse propagation in the WDM system. Top, pulse stabilization at the starting point of the DM cycle. Bottom, evolution of the stationary RMS pulse width (dashed line) and bandwidth (solid line) over the distance between filters – NOLM blocks.
short initial transition distance, which demonstrates the effectiveness of the NOLM in stabilizing the pulses. The stationary pulse parameters at the beginning of each cycle are 1.7 mW peak power, 9.1 ps FWHM pulse width, and )0.006 THz2 chirp. We point out that the strength of the dispersion map used here, S ’ 15, is beyond the range where stable DM solitons exist [11]. Fig. 3 also shows the evolution of the stationary root-mean-square (RMS) pulse width and bandwidth over the distance between two consecutive filters – NOLM blocks (one period of the system). The starting point is the output of one regeneration block and the ending point is the input of the next block. The pulse bandwidth exhibits a net change of )33% within one period, and is restored to its starting value by the next regeneration block, thus experiencing a
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jump at each regeneration block position. Note that the large net change in bandwidth within one period is an indication that the stable pulse dynamics imposed by the periodic deployment of the regeneration elements in the system is nonlinear. Note also that the stationary pulse bandwidth or temporal width for the system is strictly fixed by the WDM filtering. Indeed, we have found that the pulse width, considered at a fixed point in each cycle, stabilizes always on the same value for any initial width and peak power. We have performed 40 Gbit/s WDM transmission simulations with a channel spacing of 150 GHz. Each channel carries a 26 1 pseudo-random binary sequence data stream using Gaussian pulses with the stationary peak power, width, and chirp reached during single pulse propagation. The launch point is the input of the amplifier preceding the NOLM. The system performance is examined in terms of maximum transmission distance corresponding to a linear Q-value of more than 6 (equivalent to a bit error rate of 109 ) for the worst channel. The Q-factor for each channel is evaluated just before the NOLM location. The bandwidth of the electrical filter at the receiver is set to 25 GHz which is chosen after performing a range of simulations. Fig. 4 shows the maximum error-free transmission distance for each channel as a function of the ratio Dm0 =Dm00 for four channel WDM transmission. Here Dm0 is set to 75 GHz, corresponding
to a FWHM bandwidth of 139 GHz which is slightly less than the channel spacing, and Dm00 is allowed to vary. It can be seen that a transmission distance of more than 30,000 km is possible for all channels when Dm0 =Dm00 is equal to 0.9375. Fig. 5 shows the Q-value evolution for four channel WDM and single-channel transmission, using the optimal WDM filtering. In this example input pulses with a peak power of 0.14 mW (corresponding to 1.7 mW at the starting point of the DM cycle), a FWHM of 9.4 ps, and a chirp of )0.005 THz2 are used. The extra-gain of the amplifier preceding the NOLM is G ¼ 29:0 dB, and the power loss of the loop attenuator is D ¼ 24:9 dB. Note that the pre-amplifier gain is slightly increased with respect to the value required by the operational regime of the NOLM, to provide partial compensation of the loss introduced by the WDM filters. One may see that single-channel transmission over practically unlimited distances is possible in the optimised WDM system. Unlimited transmission means here that after some stage, the accumulation of detrimental effects through the transmission system is stabilized, yielding nondecaying Q-factor > 6. This result corresponds to our recently reported result using a single-channel transmission scheme [9], and indicates that the WDM performance is only affected by interchannel effects. Fig. 6 shows the eye-diagram of one of the four channels at the maximum transmission distance and the single-channel eye-dia-
Fig. 4. Maximum error-free transmission distance versus Dm0 =Dm00 for 40 Gbit/s 4 WDM system.
Fig. 5. Q-values versus propagation distance for 40 Gbit/s 4 WDM and 40 Gbit/s single-channel transmission.
S. Boscolo et al. / Optics Communications 217 (2003) 227–232
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Fig. 7. Maximum error-free transmission distance versus number of channels.
Fig. 6. Eye-diagrams of the 40 Gbit/s 4 WDM and 40 Gbit/s single-channel data signals.
gram after 40,000 km transmission. It is seen that the WDM transmission performance is mainly degraded by inter-channel XPM-induced timing jitter. The single-channel eye-diagram is still open at 40,000 km, showing that the accumulation of intra-channel timing jitter through the transmission system is not enough at this stage to produce a significant degradation of the Q-factor. In both WDM and single-channel transmissions, in fact, intra-channel four-wave mixing is not a limiting factor, due to the loop mirror intensity filtering action in suppressing the background and reducing the amplitude jitter of ones. To demonstrate the flexibility of the proposed technique, we have increased the number of channels up to 16. Fig. 7 shows the maximum er-
ror-free transmission distance for the worst channel as a function of the number of channels. It can be seen that 16 channel WDM transmission over 25,000 km is achievable. So far, we have not included the third-order dispersion (TOD) of fibres in our calculations. We point out that the per-channel operation of the NOLM and the short length of the loop fibre make the effect of chromatic dispersion on the propagation of any WDM channel in the loop negligible, whereas, self-phase modulation is the dominant effect. On the other hand, an allowed dispersion in
Fig. 8. Maximum error-free transmission distance versus residual TOD for 40 Gbit/s 16 WDM system.
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the NOLM would obviously contribute to the dispersion of the transmission line. Now, to estimate the impact of TOD on the WDM transmission performance, we allow the presence of a residual (uncompensated) TOD in the fibre link. For operation in the anomalous average dispersion regime, the maximum amount of TOD permitted in the system is given by b3;cr ¼ hb2 i0 = ð2pDmÞ, where hb2 i0 is the average group-velocity dispersion at the reference frequency, and Dm is the detuning of the outermost channel from the reference frequency. Fig. 8 shows the maximum error-free transmission distance for the worst channel as a function of the TOD present in the system, for 16 channel WDM transmission. One may see that TOD affects the achievable transmission distance but does not destroy the system operation. As a final note, we anticipate that the periodic in-line pulse reshaping performed by NOLMs may allow waveform distortions caused by polarisation-mode dispersion to be reduced.
4. Conclusion We have proposed an all-optical passive regeneration technique based on specially designed
WDM guiding filters coupled with in-line NOLMs to enhance the transmission performance of 40 Gbit/s WDM RZ systems. Optimisation of the system allowed for 150 GHz-spaced 16 channel transmission over 25,000 km of SMF.
References [1] D. LeGuen, S. Del Burgo, M.L. Moulinard, D. Grot, M. Henry, F. Favre, T. Georges, Postdeadline Papers of OFC/ IOOCÕ99, PD4, San Diego, California, 1999. [2] T. Tsuritani, A. Agata, K. Imai, K. Tanaka, T. Miyakawa, N. Edagawa, M. Suzuki, Postdeadline Papers of ECOCÕ2000, PD1.5, Munick, Germany, 2000. [3] K. Suzuki, H. Kubota, A. Sahara, M. Nakazawa, Electron. Lett. 36 (2000) 443. [4] S. Bigo, Tech. Dig. of OFCÕ2002, WX3, Anaheim, California, 2002, pp. 362–364. [5] M. Matsumoto, H. Ikeda, A. Hasegawa, Opt. Lett. 19 (1994) 183. [6] N.J. Smith, N.J. Doran, J. Opt. Soc. Am. B 12 (1995) 1117. [7] I. Gabitov, D.D. Holm, B.P. Luce, J. Opt. Soc. Am. B 14 (1997) 1850. [8] I.N. Duling III, Opt. Lett. 16 (1991) 539. [9] S. Boscolo, S.K. Turitsyn, K.J. Blow, Electron. Lett. 37 (2001) 112. [10] S. Boscolo, S.K. Turitsyn, K.J. Blow, IEEE Photon. Technol. Lett. 14 (2002) 30. [11] J.H.B Nijhof, N.J. Doran, W. Forysiak, A. Berntson, Electron. Lett. 34 (1998) 481.