In a coherent optical receiver of an optical communications system, methods and systems for receiving a data signal x(t) modulated on an optical signal. A linearly polarized LO light is generated, which has a frequency of f1=f0±Δf, where f0 is a frequency of a narrowband carrier of the optical signal, and Δf corresponds with a band-width fB of the data signal x(t). The LO light and a received light of the optical signal are heterodyned on a photodetector. An analog signal generated by the photodetector is low-pass filtered to generate a filtered signal, using a filter characteristic having a sharp cut-off at a frequency of Δf+nfB, where n is an integer multiple. An analog-to digital (A/D) converter samples the filtered signal at a sample rate of 2(Δf+nfB) to generate a corresponding multi-bit digital sample stream. The multi-bit digital sample stream is digitally processed to recover respective In-Phase and Quadrature components of the received light of the optical signal.
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