Optoelectronics Research Centre University of Southampton Southampton SO17 1BJ, UK Telephone: +44 1703 593150 e-mail: [email protected] Straight Line 10 Gb/s Soliton Transmission over 1000 km of Standard Fibre with In-Line Chirped Fibre Grating for Partial Dispersion Compensation A. B. Grudinin, M. Durkin, M. Ibsen, R. I. Laming, *A. Schiffini, *P. Franco, †E. Grandi and ’M. Romagnoli *Pirelli Cavi s.p.a., viale Sarca 222, 20146 Milano, Italy †Dipartimento di Elettronica, Universita di Pavia, via Ferrata 1, 27100 Pavia, Italy ’Fondazione Ugo Bordoni, via B Castiglione 59, 00142 Roma, Italy Abstract We demonstrate a highly practical and robust soliton transmission system with partial dispersion compensation. Error free transmission was obtained for 10 Gb/s data over 1000km of standard fibre with nominal dispersion of 17 ps/nm@@km. Despite variations in the dispersion compensation ratio and the relatively high UV-induced birefringence of the fibre gratings the system exhibits BERs below 10-10 for any state of input polarisation provided that the average intra-span dispersion is negative. The majority of the installed fibre base has a low dispersion at wavelengths in the region of 1300nm. Upgrading to higher bit rates using erbium-doped fibre amplifiers operating around 1550nm is attractive but then the fibre dispersion imposes a severe limitation on transmission distances. For some years there were two main approaches considered to compensate fibre dispersion: (i) linear, based on straightforward dispersion compensation using compensating fibres or chirped fibre gratings and (ii) non-linear, based on the use of optical solitons. Both methods have their peculiar advantages and disadvantages. In particular linear systems suffer from fibre nonlinearity which deteriorates the system performance at distances exceeding just 500km while the major problems with non-linear systems are the short amplifier spacings required and interactions between the transmitting pulses. Fig.1 Experimental set-up. Recent experiments have revealed that the best performance is offered by a combination of the two methods where the first part of the intra-span distance operates in the non-linear regime whilst the dispersion of the second (linear) part is compensated by a dispersion compensating element [1-4]. Furthermore such hybrid systems are effective in reducing both noise- and collision-induced timing jitters [5,6], while the Gaussian shape of the propagating pulse wings allows higher mark-space ratios without a significant increase in soliton interaction strength [7]. 1 In this Letter we present the first experimental results demonstrating straight line soliton transmission at 10 Gb/s over 1000km of standard (high dispersion) fibre with 100km amplifier spacing and chirped fibre gratings as the dispersion compensating elements. The experimental set-up is shown in Figure 1. The soliton transmitter is an actively modelocked fibre ring laser emitting bandwidth limited 30ps pulses at 1549.5nm. The average output power was 14dBm. Pseudorandom data is encoded on to the pulse stream by a Ti:LiNbO 3 Mach-Zender modulator. The transmission line comprises ten ~100km spans of standard telecommunications fibre each followed by an erbium-doped fibre amplifier and a chirped fibre grating. The linearly chirped fibre Fig.2 Typical reflection (a) and time delay (b) spectra gratings were each 75 cm long of linearly chirped fibre grating. with a 4.5 nm bandwidth centred at 1549.5nm and 90% ± 3% reflectivity. Typical reflection and time delay spectra are shown in Figure 2. All the gratings had the same dispersion of 1600ps/nm ± 10ps/nm. The gratings were spliced to 3-port optical circulators with an average loss of the assembled compensators of about 4.4dB, including ~ 1.7dB insertion loss of the circulators. Due to variations in the transmission fibre dispersion the intraspan dispersion compensation ratio varied between 89% and 99%. Quasi-random variations in average intra-span dispersion models a practical system reasonably well. The average output power from each amplifier (prior to the grating) was ~ 14 dBm. Polarisation mode dispersion of the transmission fibre was less than 0.1ps/km1/2. Input pulses were pre-chirped with 2.1km of dispersion compensating fibre with D = 60ps/nm@km. The measured bit error rate performance (BER) is given in Figure 3 with the abscissa of the chart being received power. The open circles show BER before and closed circles after the 1000km of transmission. The results confirm successful transmission over 1000km with an observed power penalty of only 3dB and there was no indication of the existence of an error floor to measured error rates below 1@10-11. This penalty was caused mainly by signal-to-noise ration degradation due to amplifier noise and the slightly low extinction ratio of the LiNbO3 modulator. The inset of Fig.3 is an eye diagram at 10 Gb/s after 1000km. 2 In the course of the experiments we were able to observe the influence of the fibre gratings UV induced birefringence [8] on the system performance. Measurements of polarisation-dependent group velocity delay of the gratings used in the experiment indicated a value of 8 ± 3 ps. After the first amplifier we have clearly seen polarisation dependent ~10ps variations in the pulse arrival time, which however did not result in degradation of the BER. More significantly, the temporal variations due to the grating polarisation mode delay (PMD) remains approximately constant at 10ps up to 500km. BER measurements at 500km, shown in Fig.2 revealed a very small power penalty. After 1000km of propagation variations in the pulse arrival time increased to ~ 20ps and BER measurements indicated an ~ 1dB polarisation dependence in power penalty, but the system performance was still essentially error-free. However Fig.3 Error rate performance against received signal this effect requires more power experimental and theoretical Ž - back-to-back study in order to establish the • - after 500km impact of PMD on the • - after 1000km transmission limit of soliton Inset shows measured eye diagram after 1000km systems with partial dispersion transmission compensation and further experiments are currently in progress. In conclusion we have demonstrated highly practical and robust soliton transmission system with partial dispersion compensation. Error free transmission was obtained for 10 Gb/s data over 1000km of standard fibre with nominal dispersion of 17ps/nm@km. Despite variations in the dispersion compensation ratio and the relatively high polarisation-dependent group velocity delay of the fibre gratings the system exhibits BERs below 10-10 for any state of input polarisation provided that the average intra-span dispersion was negative. It was observed that overcompensation of intra-span dispersion results in a significant increase in polarisation sensitivity and deterioration of the BER. Acknowledgements The authors wish to thank Prof. D.N.Payne for his continued encouragement and helpful discussions, M.J.Cole for contribution to the grating fabrication system and F.Vaninetti for the gratings PMD measurements. The work has been partially funded by the ESTHER contract of 3 EEC/ACTS program. The work of M.R. was done in the framework of the agreement between FUB and the Italian PT Administration. References 1. Kubota, H. and Nakazawa, M., “Partial soliton communication system”, Opt. 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