Power limitations due to four-wave mixing effects in frequency division multiplexed coherent systems using cascaded optical amplifiers

The impact of four-wave-mixing (FWM) effects on phase modulated coherent multichannel systems is investigated numerically for long-haul communications systems using optical amplifiers. Assuming ideal amplifiers spaced at 100 km and compensation of fiber dispersion at the receiver end, the performances of a 2.5-Gb/s 15-channel system are compared for conventional and dispersion-shifted fibers. For 1 mW per channel and 12.5-GHz channel separation, FWM leads to a channel averaged power penalty of 3.3 dB (up to 7 dB in particular channels) after 800 km in a conventional fiber, and 8 dB after only 300 km in the dispersion-shifted fiber. As expected, an increase of channel separation from 12.5 to to 37.5 GHz leads to improved system performance for both types of fibers. The improvement is, however, limited by modulation instability occurring in the individual channels so that for very large channel separations a better system performance is achieved using dispersion-shifted fibers