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A fundamental limit in the detection of angular modulation in coherent optical communications

In order for the system to be sensitive to phase modulation (PSK), heterodyne or homodyne detection must be used, the latter being the easiest to implement since no strict control of the phase of the local oscillator laser is needed; only central/frequency stability in an AFC scheme is required. Assuming operation in the quantum limit, the photodetector output processes consists of the PSK signal spectrally translated to the (electronic) intermediate frequency (i.f.), affected by both additive (shot) noise due to the photon nature of light signal and phase noise due to the unavoidable finite linewidth in the longitudinal spectrum of both transmitter and local oscillator semiconductor lasers.