Impact of channel-spacing on the 100 Gb/s 40-km Ethernet PMD

A numerical analysis that uses the Q-factor to investigate the impact of multi-channel semiconductor optical preamplifier (SOA) nonlinearities on the 4 times 25 Gb/s 40-km Ethernet link is presented for channel-spacing values in the 200-800 GHz range. The analysis, based on advanced methods to differentiate SOA nonlinear effects, shows that the main deleterious impact is produced by cross-gain modulation (XGM). However, four-wave mixing (FWM) is responsible for the system performance difference as a function of channel-spacing. In this respect, the 800 GHz channel-spacing plan should be preferred. For fiber lengths greater than 30 km, FWM is practically inconsequential and XGM is less severe. Although this work represents and aid in the design of next-generation Ethernet products, its general nature also applies to similar WDM transmission systems.