Impact of phase noise in weakly coherent systems: a new and accurate approach

Coherent optical systems for future broadband local loops may use lasers with significant phase noise, manifest as broad linewidths. This phase noise can be accommodated if the receiver is correctly designed, i.e. if nonsynchronous (envelope or square-law) IF demodulation is used and sufficient IF bandwidth is provided. It is difficult to analyze the performance of a coherent optical receiver when the signals are corrupted by phase noise. The central theoretical problem arising from filtering a signal with phase noise is defined in a particular form which permits the derivation of the forward or Fokker-Planck partial differential equation for probability density of the output voltage of the receiver. The results are used to discuss the IF bandwidth required for optical heterodyne receivers for amplitude-shift-keying (ASK) signals