Bit error rate floors in coherent optical systems with delay demodulation

Reports the results of an accurate analysis of the effect of filtering on phase noise, appropriate to coherent optical receivers with delay demodulation. Based on a numerical solution of a Fokker-Planck equation, the authors show that the phase at the output of the IF filter is very closely a Gaussian random process. The variance of phase fluctuations over a time equal to the integration time of the filter is reduced by a factor of 0.363. They show that receivers with minimum IF bandwidth can therefore operate with about 1.36 times the laser linewidth predicted disregarding the filtering effect. They outline a route to analysing the performance of receivers with broader than optimum IF filters.