IMPACT OF PHASE NOISE IN WEAKLY COHERENT SYSTEMS - A NEW AND ACCURATE APPROACH

Coherent optical systems for future broad-band local loops may use lasers with significant phase noise, manifest as broad line-widths. 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 this paper in a particular form which permits the derivation of the forward or Fokker-Planck partial differential equation for the probability density of the output voltage of the receiver. We have developed a method for accurate numerical solution of this differential equation. Although the characteristic function of this probability density has been derived analytically in the limit of low phase noise, and the probability density has been simulated numerically, this is the first time that this problem has been solved accurately in an applicable form. The results are used to discuss the IF bandwidth required for optical heterodyne receivers for ASK signals for different threshold settings. We show that more phase noise can be tolerated than was earlier thought possible. © 1990 IEEE