CPFSK coherent optical receivers: impact of IF bandwidth and laser phase noise

A detailed theoretical model for narrowband CPFSK receivers is presented. It accounts for the influence of IF bandwidth on the signal voltage, non-Gaussian statistics of the output noise, correlation between noise samples stemming from limited IF bandwidth, and the impact of laser phase noise. It is found that the IF bandwidth needed to avoid intersymbol interference is 2.2 times the bit rate for a modulation index of 1; it is higher for other modulation index values. The bit error ratio is within 1 dB of the shot noise limit for the modulation index of 1.