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Polarization independent wavelength converter based on Kerr non-linearity in DS fibre
Optics Communications 229 (2004) 187–190 www.elsevier.com/locate/optcom
Polarization independent wavelength converter based on Kerr non-linearity in DS fibre Giorgio Maria Tosi-Beleffi *, Franco Curti, Davide M. Forin, Francesco Matera Fondazione Ugo Bordoni, Via Baldassarre Castiglione 59, 00142 Rome, Italy Received 8 August 2003; received in revised form 20 September 2003; accepted 21 September 2003
Abstract We experimentally demonstrate the possibility to achieve a polarization insensitive wavelength conversion, operating at 2.5 and 10 Gbit/s, based on multi-wavelength generation in dispersion shifted fibre. The wavelength converter, able to reshape and generate replicas of an incoming signal at several different lambdas by the optical Kerr effect, was placed, in the middle of a 100-km long generic point-to-point optical link obtained by using an installed cable between Rome and Pomezia (25 km). Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.79.S Keywords: Optical communications systems
1. Introduction It is well known that wavelength conversion is a fundamental corner stone for future networks [1]. In fact, the possibility to handle in a fast way the frequency shift of a signal carrier ensures dynamic allocation of resources and restoration necessary to achieve optical networks based on bandwidthon-demand. Furthermore, reshaping capability and polarization insensitive conversion are fundamental requirements to guarantee a useful behaviour of wavelength converters at networkÕs *
Corresponding author. Tel.: +390654802231; fax: +30965 4804402. E-mail address: [email protected] (G.M. Tosi-Beleffi).
nodes [2]. Recently we have proposed a Multi Wavelength Converter Reshaper (MWCR), based on Kerr non-linearity in DS fibre, able to simultaneously perform reshaping of an incoming signal and its frequency conversion, covering several ITU-Grid channels [3]. The power transfer function of converted signals analysed in that work, shows a non-zero threshold and a spread maximum useful for reshaper use. In this work, we show that this device can be modified and upgraded in order to obtain a polarization insensitive conversion. In our experiments, performed at 2.5 and 10 Gbit/s, a pump and a signal are injected in a DS fibre placed in the middle of a generic 100 km long point-to-point optical link obtained by an installed cable between Rome and Pomezia.
0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.10.030
188
G.M. Tosi-Beleffi et al. / Optics Communications 229 (2004) 187–190
2. Experimental set-up Experiments are carried out in a 10 km long DS fibre, with typical loss coefficient a ¼ 0:2 dB/km, non-linear coefficient c ¼ 2:2 [W km]1 , chromatic dispersion D ¼ 0:3 ps/nm km (at 1542.14 nm), dispersion slope S0 ¼ 0:063 ps/nm2 km and DGD ¼ 0.13 ps. The fibre is made by a single spinning; this feature limits the fluctuation of the zero-dispersion wavelength (around 1540 nm in our case), along the propagation, responsible for the reduction of parametric gain and gain bandwidth and also for troubles in maintenance of a right phase relation between the interacting waves [4]. Moreover working close to the zero dispersion area is a way to achieve a greater multi-wavelength generation using powers typical of available booster amplifiers (around 100 mW for signal and pump too). A DFB laser source and a tunable laser source (ECL) are used to generate, respectively, a signal, at k ¼ 1542:93 nm, and a pump at k ¼ 1542:14 nm mixed together as reported in Fig. 1. Polarization insensitive conversion requires a proper diversity scheme. The incoming optical signals are split into their polarization components by means of a polarization beam combiner (PBC) and launched into the fibre from opposite sides in a counterpropagating scheme.
Fig. 1. 2.5 and 10 Gbit/s polarization insensitive wavelength converter experimental set-up.
The peak signal power level is around 60 mW while the pump power level, around 70 mW, is distributed exactly at 50% into the output arms of the PBC just by moving the in-line polarization controller PC3. In order to restrict the depletion of the sources power due to Brillouin scattering phenomena, signal and pump are angular modulated at 140 Mbit/s by a PRBS generator, not shown in Fig. 1. The pump is modulated by an external phase modulator (PHM) while the signal coming out from the DFB laser is directly modulated. The 2.5 and 10 Gbit/s NRZ amplitude signal modulation is obtained trought a Mach–Zehnder device (IM), in order to limit the chirping phenomenon, driven by a PRBS generator. The deployed 23.5-km long cable, between Rome and Pomezia, contains spans of G.653 fibres. These fibres were looped back to obtain 2 50 km spans in order to emulate a node in a generic network link. The signal, after the amplitude modulation, passes through an erbium doped optical amplifier (S1) and goes into the first span covering 50 km of DS fibre. An optical attenuator (A1) and a 10% coupler (C10%), used to measure real optical input at the entrance of the MWCR, are placed before a block made by a power amplifier (S2), a 1 nm-band pass filter (BFS) and a second optical attenuator (A2), useful to set the value of signal power to optimize the optical reshaping function [3]. After the 30% coupler (C30%), signal and pump are injected, via a 90% coupler (C10%), into a polarization beam combiner (PBC) in order to split the orthogonal states inside the counterpropagating scheme. A polarization controller in the loop (PCL) is used to exactly recombine the polarization states at the PBC obtaining, from the 10% arm of C10%, the output spectrum of Fig. 2. The signal replica, after the Demux, is launched into the Rome–Pomezia cable covering the second 50-km long DS fibre span and arriving at the receiver for the BER measurement. BER measurements were performed by using a PRBS generator (2^31-1 bits) and adjusting the pump wavelength in order to fit the optical spectral components with the channels of the demux characterized by 16 channels 100 GHz spaced.
G.M. Tosi-Beleffi et al. / Optics Communications 229 (2004) 187–190
Fig. 2. Spectrum at the output of the loop.
3. Results The generation of the high number of components can be simply explained by the presence of the Kerr phase modulation process [5]. For this reason the envelope of the beating between pump and signal, near k0 (around 1540 nm in our case), determines a phase modulation on the pump at a frequency equal to xP xS , that generates new frequencies spaced by this value. In Fig. 3 we report the behaviour of the reshaping function Pout(Pin) for NRZ coded ch. 47 at 10 Gbit/s. As can be seen the transfer function of these channel, and in general of all the channels analysed in this work, shows a non-zero threshold and a spread maximum that we can exploit for reshaper use. By adjusting the signal and pump power, at the DS fibre input, it is possible to
Fig. 3. CH47 transfer function.
189
choose, in each different case, the better working point. In the case of 2.5 Gbit/s, we have tested the generated signals and we have achieved good results over a wavelength range (ITU-Grid) up to ch. 47 ðk ¼ 1539:7 nm) and also to ch. 41 (k ¼ 1544:5 nm) covering a whole conversion range of 4.8 nm. In Fig. 4 we report the BER versus the received power for these channels. About 10 Gbit/s experiments, we have achieved good results over a wavelength range between ch. 46 ðk ¼ 1540:5 nm) and ch. 41 (k ¼ 1544:5 nm) covering a whole conversion range of 4 nm. In Fig. 5 we report the BER versus the received power for these channels. The behaviour of the converted signals depend both on the SNR, due to the different generation efficiency, and to the chromatic dispersion regime seen by each channel along the device; furthermore
Fig. 4. 2.5 Gbit/s BER measurements.
Fig. 5. 10 Gbit/s BER measurements.
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the intrinsic physical mechanism at the basis of this kind of conversion adds a different amount of chirp to each channel modifying the shape of the pulses at the receiver input. For these reasons the propagated signals, covering 50+50 km of DS fibre, show better performances compared with the BTB one especially at 10 Gbit/s were the pulses are narrower respect to 2.5 Gbit/s one. In order to test the performances of the polarization diversity scheme, we have measured the difference between the minimum and maximum SNR value of converted signals obtained just by rotating the polarization controller placed inside the incoming signal-way (PC in Fig. 1). At 2.5 Gbit/s we have achieved DSNRCH47 ¼ 4 dB and DSNRCH41 ¼ 1 dB instead of DSNRCH46 ¼ 0:97 dB and DSNRCH41 ¼ 0:98 dB obtained at 10 Gbit/s.
4. Conclusions We have demonstrated that a counter-propagating polarization diversity scheme can be used to obtain a polarization insensitive conversion in a
Multi Wavelength Converter Reshaper both at 2.5 and 10 Gbit/s.
Acknowledgements This work has been carried out in the framework of Intern.it that is a project cooperation between FUB and Italian Communication Ministry. We thank Michele Guglielmucci (ISCTI – Istituto Superiore Comunicazioni e Tecnologie dellÕInformazione of Rome) supplying the instrumentations and the Rome–Pomezia sperimental deployed cable.
References [1] Govind P. Agrawal, Fiber Opt. Commun. Syst. 1997. [2] ATLAS deliverable D213, Availble from. [3] F. Curti, F. Matera, Giorgio Maria Tosi-Beleffi, Opt. Commun. 208 (2002) 85. [4] M. Karlsson, JOSA B 15 (1998) 2269. [5] R. Thompson, R. Roy, PRA 43 (1991) 4987.
Polarization independent wavelength converter based on Kerr non-linearity in DS fibre Giorgio Maria Tosi-Beleffi *, Franco Curti, Davide M. Forin, Francesco Matera Fondazione Ugo Bordoni, Via Baldassarre Castiglione 59, 00142 Rome, Italy Received 8 August 2003; received in revised form 20 September 2003; accepted 21 September 2003
Abstract We experimentally demonstrate the possibility to achieve a polarization insensitive wavelength conversion, operating at 2.5 and 10 Gbit/s, based on multi-wavelength generation in dispersion shifted fibre. The wavelength converter, able to reshape and generate replicas of an incoming signal at several different lambdas by the optical Kerr effect, was placed, in the middle of a 100-km long generic point-to-point optical link obtained by using an installed cable between Rome and Pomezia (25 km). Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.79.S Keywords: Optical communications systems
1. Introduction It is well known that wavelength conversion is a fundamental corner stone for future networks [1]. In fact, the possibility to handle in a fast way the frequency shift of a signal carrier ensures dynamic allocation of resources and restoration necessary to achieve optical networks based on bandwidthon-demand. Furthermore, reshaping capability and polarization insensitive conversion are fundamental requirements to guarantee a useful behaviour of wavelength converters at networkÕs *
Corresponding author. Tel.: +390654802231; fax: +30965 4804402. E-mail address: [email protected] (G.M. Tosi-Beleffi).
nodes [2]. Recently we have proposed a Multi Wavelength Converter Reshaper (MWCR), based on Kerr non-linearity in DS fibre, able to simultaneously perform reshaping of an incoming signal and its frequency conversion, covering several ITU-Grid channels [3]. The power transfer function of converted signals analysed in that work, shows a non-zero threshold and a spread maximum useful for reshaper use. In this work, we show that this device can be modified and upgraded in order to obtain a polarization insensitive conversion. In our experiments, performed at 2.5 and 10 Gbit/s, a pump and a signal are injected in a DS fibre placed in the middle of a generic 100 km long point-to-point optical link obtained by an installed cable between Rome and Pomezia.
0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.10.030
188
G.M. Tosi-Beleffi et al. / Optics Communications 229 (2004) 187–190
2. Experimental set-up Experiments are carried out in a 10 km long DS fibre, with typical loss coefficient a ¼ 0:2 dB/km, non-linear coefficient c ¼ 2:2 [W km]1 , chromatic dispersion D ¼ 0:3 ps/nm km (at 1542.14 nm), dispersion slope S0 ¼ 0:063 ps/nm2 km and DGD ¼ 0.13 ps. The fibre is made by a single spinning; this feature limits the fluctuation of the zero-dispersion wavelength (around 1540 nm in our case), along the propagation, responsible for the reduction of parametric gain and gain bandwidth and also for troubles in maintenance of a right phase relation between the interacting waves [4]. Moreover working close to the zero dispersion area is a way to achieve a greater multi-wavelength generation using powers typical of available booster amplifiers (around 100 mW for signal and pump too). A DFB laser source and a tunable laser source (ECL) are used to generate, respectively, a signal, at k ¼ 1542:93 nm, and a pump at k ¼ 1542:14 nm mixed together as reported in Fig. 1. Polarization insensitive conversion requires a proper diversity scheme. The incoming optical signals are split into their polarization components by means of a polarization beam combiner (PBC) and launched into the fibre from opposite sides in a counterpropagating scheme.
Fig. 1. 2.5 and 10 Gbit/s polarization insensitive wavelength converter experimental set-up.
The peak signal power level is around 60 mW while the pump power level, around 70 mW, is distributed exactly at 50% into the output arms of the PBC just by moving the in-line polarization controller PC3. In order to restrict the depletion of the sources power due to Brillouin scattering phenomena, signal and pump are angular modulated at 140 Mbit/s by a PRBS generator, not shown in Fig. 1. The pump is modulated by an external phase modulator (PHM) while the signal coming out from the DFB laser is directly modulated. The 2.5 and 10 Gbit/s NRZ amplitude signal modulation is obtained trought a Mach–Zehnder device (IM), in order to limit the chirping phenomenon, driven by a PRBS generator. The deployed 23.5-km long cable, between Rome and Pomezia, contains spans of G.653 fibres. These fibres were looped back to obtain 2 50 km spans in order to emulate a node in a generic network link. The signal, after the amplitude modulation, passes through an erbium doped optical amplifier (S1) and goes into the first span covering 50 km of DS fibre. An optical attenuator (A1) and a 10% coupler (C10%), used to measure real optical input at the entrance of the MWCR, are placed before a block made by a power amplifier (S2), a 1 nm-band pass filter (BFS) and a second optical attenuator (A2), useful to set the value of signal power to optimize the optical reshaping function [3]. After the 30% coupler (C30%), signal and pump are injected, via a 90% coupler (C10%), into a polarization beam combiner (PBC) in order to split the orthogonal states inside the counterpropagating scheme. A polarization controller in the loop (PCL) is used to exactly recombine the polarization states at the PBC obtaining, from the 10% arm of C10%, the output spectrum of Fig. 2. The signal replica, after the Demux, is launched into the Rome–Pomezia cable covering the second 50-km long DS fibre span and arriving at the receiver for the BER measurement. BER measurements were performed by using a PRBS generator (2^31-1 bits) and adjusting the pump wavelength in order to fit the optical spectral components with the channels of the demux characterized by 16 channels 100 GHz spaced.
G.M. Tosi-Beleffi et al. / Optics Communications 229 (2004) 187–190
Fig. 2. Spectrum at the output of the loop.
3. Results The generation of the high number of components can be simply explained by the presence of the Kerr phase modulation process [5]. For this reason the envelope of the beating between pump and signal, near k0 (around 1540 nm in our case), determines a phase modulation on the pump at a frequency equal to xP xS , that generates new frequencies spaced by this value. In Fig. 3 we report the behaviour of the reshaping function Pout(Pin) for NRZ coded ch. 47 at 10 Gbit/s. As can be seen the transfer function of these channel, and in general of all the channels analysed in this work, shows a non-zero threshold and a spread maximum that we can exploit for reshaper use. By adjusting the signal and pump power, at the DS fibre input, it is possible to
Fig. 3. CH47 transfer function.
189
choose, in each different case, the better working point. In the case of 2.5 Gbit/s, we have tested the generated signals and we have achieved good results over a wavelength range (ITU-Grid) up to ch. 47 ðk ¼ 1539:7 nm) and also to ch. 41 (k ¼ 1544:5 nm) covering a whole conversion range of 4.8 nm. In Fig. 4 we report the BER versus the received power for these channels. About 10 Gbit/s experiments, we have achieved good results over a wavelength range between ch. 46 ðk ¼ 1540:5 nm) and ch. 41 (k ¼ 1544:5 nm) covering a whole conversion range of 4 nm. In Fig. 5 we report the BER versus the received power for these channels. The behaviour of the converted signals depend both on the SNR, due to the different generation efficiency, and to the chromatic dispersion regime seen by each channel along the device; furthermore
Fig. 4. 2.5 Gbit/s BER measurements.
Fig. 5. 10 Gbit/s BER measurements.
190
G.M. Tosi-Beleffi et al. / Optics Communications 229 (2004) 187–190
the intrinsic physical mechanism at the basis of this kind of conversion adds a different amount of chirp to each channel modifying the shape of the pulses at the receiver input. For these reasons the propagated signals, covering 50+50 km of DS fibre, show better performances compared with the BTB one especially at 10 Gbit/s were the pulses are narrower respect to 2.5 Gbit/s one. In order to test the performances of the polarization diversity scheme, we have measured the difference between the minimum and maximum SNR value of converted signals obtained just by rotating the polarization controller placed inside the incoming signal-way (PC in Fig. 1). At 2.5 Gbit/s we have achieved DSNRCH47 ¼ 4 dB and DSNRCH41 ¼ 1 dB instead of DSNRCH46 ¼ 0:97 dB and DSNRCH41 ¼ 0:98 dB obtained at 10 Gbit/s.
4. Conclusions We have demonstrated that a counter-propagating polarization diversity scheme can be used to obtain a polarization insensitive conversion in a
Multi Wavelength Converter Reshaper both at 2.5 and 10 Gbit/s.
Acknowledgements This work has been carried out in the framework of Intern.it that is a project cooperation between FUB and Italian Communication Ministry. We thank Michele Guglielmucci (ISCTI – Istituto Superiore Comunicazioni e Tecnologie dellÕInformazione of Rome) supplying the instrumentations and the Rome–Pomezia sperimental deployed cable.
References [1] Govind P. Agrawal, Fiber Opt. Commun. Syst. 1997. [2] ATLAS deliverable D213, Availble from