Minimum Channel Spacing in an OFDM CPFSK Optical Coherent System

Channel spacing in optical frequency-division multiplexed (OFDM) coherent optical systems with continuous-phase frequency-shift-keying (CPFSK) is obtained both theoretically and experimentally. The bit error ratio is derived taking into account the non-Gaussian interference statistics, expressed in terms of the Marcum Q-function, and the sensitivity penalty is reported as a function of the signal-to-interference ratio and the channel spacing for different IF filters. Laboratory demonstration of a state-of-the-art fully engineered two-channel 2.5 Gb s-1 CPFSK coherent system at 1550 nm has been performed, showing an inter-channel crosstalk measurement in agreement with the theoretical analysis. A 0.1 nm channel spacing is experimentally confirmed.