Influence of laser phase noise on 400Mb/s coherent optical DPSK system.

The influence of laser phase noise on a 400MB/s optical DPSK system is experimentally investigated with linewidths ranging from 1.2MHz to 8MHz. This range corresponds to linewidth to bit rate ratios, "epsilon", of 0.33% to 2%. The system performance with these non-zero linewidths is evaluated against a zero linewidth performance baseline. The sensitivity degradation at a bit error rate of 10 sup -9 increases from 1.8dB to 7dB as "epsilon" is increased from 0.33% to 1%. When "epsilon" is increased beyond 1%, bit error rate floors higher than 10 sup -9 develop. These findings agree well with the existing theories and allow the generalization of these results to other bit rates as well as establishing practical criteria for lasers to be used in DPSK systems.