Straight line 10 Gb/s soliton transmission over 1000 km of standard fibre with in-line chirped fibre grating for partial dispersion compensation

We demonstrate a highly practical and robust soliton transmission system with partial dispersion compensation. Error free transmission was obtained for 10 Gb/s &ta; over 1000 km of standard fibre with nominal dispersion of 17 ps/nrm km. Despite variations in the dispersion compensation ratio and the relatively high UV-induced birefiingence of the fibre gratings the system exhibits BERs below lo”” for any state of input polarisation provided that the average ha-span dispersion is negative. The majority of the installed fibre base has a low dispersion at wavelengths in the region of 1300 nm. L’pgrading to higher bit rates using erbium-doped fibre amplifiers operating around 1550 nm 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 500 km while the major problems with non-linear systems are the short amplifier spacings required and interactions between the transmitting pulses. 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 [l-4]. Furthermore such hybrid systems are effective in reducing both noiseand 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 [n. In this Letter we present the frost experimental results demonstrating straight line soliton transmission at 10 Gb/s 100 km amplifier spacing and elements. over 1000 km of standard (high dispersion) fibre with chirped fibre gratings as the dispersion compensating The experimental set-up is shown in Figure 1. The soliton transmitter is an actively mode-locked fibre ring laser emitting bandwidth limited 30 ps pulses at 1549.5 run. The average output power was 14 dBm. Pseudorandom data is encoded on to the pulse stream by a Ti:LiNbO, Mach-Zender modulator. The transmission line comprises ten -100 km spans of standard telecommunications fibre each followed by an erbiumdoped fibre amplifier and a chirped fibre grating. The linearly chirped fibre gratings were each 75 cm long with a 4.5 nm bandwidth centred at 1549.5 run and 90% f 3% reflectivity. Typical reflection and time delay spectra are shown in Figure 2. All the gratings had the same dispersion of 1600 ps/nm + lOps/nm. The gratings were spliced to 3-port optical circulators with an average loss of the assembled compensators of about 4.4 dB, including 1.7 dB insertion loss of the circulators. Due to variations in the transmission fibre dispersion the i&a-span; 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 0.1 p&m;’“. Input pulses were pre-chirped with 2.1 km of dispersion compensating fibre with D = 60 ps/nmkm. 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 1000 km of transmission. The results confii successful transmission over 1000 km with an observed power penalty of only 3 dB and there was’ no indication of the existence of an error floor to measured error rates below 1 *lo”‘. This penalty was caused mainly by signal-to-noise ration degradation due to amplifier noise and the slightly low extinction ratio of the LiNbOJ modulator. The inset of Fig.3 is an eye diagram at 10 Gb/s after 1000 km. 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 f 3 ps. After the first amplifier we have clearly seen polarisation dependent 10 ps 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 10 ps up to 500 km. BER measurements at 500 km, shown in Fig.2 revealed a very small power penalty. After 1000 km of propagation variations in the pulse arrival time increased to 20 ps and BER measurements indicated an 1dB polarisation dependence in power penalty, but the system performance was still essentially error-free. However this effect requires more experimental and theoretical study in order to establish the impact of PMD on the transmission limit of soliton systems with partial dispersion 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 1000 km of standard fibre with nominal dispersion of 17 p&m; 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 lo”’ for any state of input polarisation provided that the average &a-span;dispersion was negative. It was observed that over-compensation of intra-span dispersionresults in a significant increase in polarisation sensitivity and deterioration of the BER.Acknowledgemenfs. The authors wish to thank Prof. D. N. Payne for his continuedencouragement and helpful discussions, M. J. Cole for contribution to the gratingfabrication system and F. Vaninetti for the gratings PMD measurements. The work hasbeen partially funded by the ESTHER contract of EEC/ACTS program. The work ofM.R. was done in the Wework of the agreement between FUB and the Italian PTAdministration.*? References1. Kubota, H. and Nakazawa, M., “Partial soliton communication system”, Opt.Commun., 1992,87. pp.15-18 :2. Suzuki, M., Morita, I., Yamamoto, S., Edagawa, N., Taga, H., and Akiba, S., “Timingjitter reduction by periodic dispersion compensation in soliton transmission”, OpticalFibre Communications (OFC’95), 1995, San-Diego, USA, Paper PD203. Edagawa, N., Morita, I., Suzuki, M., Yamamoto, S., Taga, H., and Akiba, S.,“20 Gb/s,8 100 km straight line single channel soliton based RZ transmission experiment usingperiodic dispersion compensation “, Proc. 21’ European Conference on Optical Communications (ECOC’95), Brussels, 1995, pp.983-9864. Le Guen, D., Favre, F., Moulinard, M. L., Henry, M., Michaud, G., Mace, L., Devaux,F. Charbonnier, B., Georges, T.,“200 Gb/s 100 km-span soliton WDM transmissionover 1000 km of standard fibre with dispersion compensation and pre-chirping”,Optical Fibre Communications (OFC’97), 1997, Dallas, USA, Paper PD-175. Grudinin, A. B., and Goncharenko I, A., “Increased amplifier spacing in solitonsystem with partial dispersion compensation”, Electron. Left., 32, pp. 1602-1604,19966. Smith, N. J. Forysiak, W., and Doran, N., “Reduced Gordon-Haus jitter due toenhanced power solitons in strongly dispersion-managed systems”, EZectron. Lett. 32, pp.2085-2086, 19967. Smith, N.J., Knox, F.M., Doran, N. J., Blow, K. J., and Bennion, I., “Enhanced powersolitons in optical fibres with periodic dispersion management “, Electron Lett. 32,