Numerical investigation of signal degradation due to four-wave mixing in a 21-channel 2.5-Gb/s coherent heterodyne DPSK-system

The impact of strong four-wave mixing on an 8-bit repetitive differential phase shift keying (DPSK) signal is investigated numerically assuming a 21-channel frequency multiplexed 2.5-Gb/s coherent system with either a conventional or a dispersion shifted fiber. This investigation includes the effect of group-velocity dispersion and a depletable pump. The worst case with all carriers and all signals initially in phase is compared with a case of quasirandom phase relation between the channels. The results are presented in terms of the power penalty and error rate performance as a function of input power and fiber length. It is found that in a conventional fiber 21 channels separated by five times the bitrate can be launched with as much as 1 mW/channel to render a power penalty at 10^-9 below 1 dB. Under the same conditions a dispersion shifted fiber allows less than 0.25 mW/channel