Phase-diversity homodyne detection of multilevel optical modulation with digital carrier phase estimation

This paper describes a phase-diversity homodyne receiver that which can cope with multilevel modulation formats. The carrier phase drift is estimated with digital signal processing (DSP) on the homodyne-detected signal, entirely restoring the complex amplitude of the incoming signal. Our DSP-based phase-estimation scheme consists of a simple and demultiplexable architecture that allows the system to reach significantly higher performance than conventional optical delay detection. Since the whole optical signal information is preserved with our receiver, various kinds of postprocessing of the received signal become possible. For example, we can demultiplex wavelength-division/optical time-division multiplexed channels and compensate for group velocity dispersion of fibers as well as the nonlinear phase noise in the electrical domain. We also experimentally evaluate the performance of our receiver. Our offline bit-error rate experiments show the feasibility of transmitting polarization-multiplexed 40-Gb/s quadrature phase-shift keying signals over 200 km with channel spacing of 16 GHz, leading to spectral efficiency of 2.5 b/s/Hz.