Abstract

The differential phase-shift keying (DPSK) modulation format allows for longer transmission distances, thanks to the associated higher receiver sensitivity and to the possibility of reduced penalty from fibre nonlinearities, owing to the constant signal intensity and pseudo-random signal phase distribution [1]. Such benefits may nevertheless be lost, unless the management of the nonlinear effects and of chromatic dispersion are properly taken into account [2]. For this reason, it may be necessary to adopt substantially different optimal system configurations (i.e., amplification scheme, dispersion map, signal power) when choosing DPSK instead of on-off keying formats. In order to explore the impact of the DPSK format on best system configuration, and to cross-check our simulation tools, first we performed transmission experiments involving 16x40 Gbit/s WDM channels on a recirculating loop including a symmetric Ultrawave™ fiber dispersion map. In these experiments we used the RZ-DPSK format, and we brought each 100 km span to optical transparency with 75%(25%) of backward (forward) Raman gain. We experimentally varied the pre-compensation, and we observed a significant transmission improvement after 4000 km for the pre-compensation of -350 ps/nm. Subsequently, we fixed the best prechirp value and we obtained an optimal signal input power to -3dBm, with ldBm resolution grid. Next, we numerically determined, we believe for the first time, what is the optimal system configuration for all-Raman 40 Gbit/s WDM transmission links when using the RZ-DPSK format. We considered both symmetric and non-symmetric dispersion maps (see Fig. 1), and we could identify the relative merit and benefit of each configuration. In our massive simulations, we used multiple long pseudo-random patterns of 2¹⁰-1 bits in a comb of 5, RZ-DPSK channels (with 100 GHz spacing). In order to estimate the optimal system performance, we employed the DPSK Q-factor [3], which is defined as the smallest of the amplitude Q_A and the optical phase Q_AO ("differential phase") of the received signal. We monitored the worst channel performance in terms of its associated maximum transmission distance. The longest distance of 6300 km is achieved for a symmetric map (see fig. 2), with 90% backward pumping, no pre-compensation, a path-averaged dispersion of -0.1 ps/nm/km, and the input signal average power of -2.74 dBm. On the other hand, with a non-symmetric map best performance is achieved with equal forward and backward Raman gain. Indeed, in this case a maximum transmission distance of 5750 km is predicted for a signal average power of -6.74 dBm, a span average dispersion of -0.2 ps/nm/km, and a total dispersion pre-compensation of-75 ps/nm. In conclusion, maximum transmission distances are predicted with a symmetric dispersion map and with nearly backward Raman pumping. On the other hand, using a non-symmetric dispersion map may be advantageous in practical links, since such configuration is more tolerant to dispersion fluctuations. Fig. 1. Dispersion maps 1 (top) and 2 (bottom). . 2: Max. transmission distance vs. path average dispersion and pre- compensation, with a 50% pump split, map 1 (left) and with 90% backward pumping, map 2 (right).

© 2007 IEEE