Optical power requirements for detecting OOK and FSK signals corrupted by phase noise

The decision of on-off keying (OOK) and frequency-shift keying (FSK) is considered for coherent lightwave communications in which the optical pulses are corrupted by the phase noise associated with nonzero laser linewidths. Heterodyne lightwave detection is assumed, followed by intermediate frequency filtering, envelope detection, and postdetection low-pass filtering. Using analytical results published previously, theoretical minimum values of the required average power are obtained in photons/bit, for achieving a bit error rate of 10^-9. Numerical results are given as functions of laser linewidth-to-bit-rate ratio for OOK, binary FSK (or 2-FSK), and 8-FSK. It is shown that heterodyne detection for lightwave signals of this kind can be made highly robust to phase noise, in contrast to the case of binary phase-shift keying