A Kennedy receiver for optical Quadrature Phase Shift Keying

The principles employed by Kennedy in the design of his binary receiver are extended to M-ary PSK signals. An implementable Kennedy receiver for Quadrature Phase Shift Keying signals, that uses polarization multiplexing to distribute the signal and the local oscillator to the various detectors while minimizing the need for path stabilization, is proposed and analyzed. It is shown that this Kennedy receiver can surpass the sensitivity of an ideal homodyne receiver by more than 0.8 dB for QPSK signals for Bit Error Rates used in standard communications. The performance of the QPSK receiver is analyzed in detail with the systemic impairments that would be expected in a real system, including imperfect detectors, phase noise, amplitude noise, polarization-related noise, and optical noise. It is shown that the use of photon-number resolving detectors in the receiver helps mitigate the effects of the impairments, and allow the Kennedy receiver for QPSK to operate at error rates lower than an ideal homodyne receiver even in the presence of impairments. Dolinar's receiver for binary signals is extended to M-ary PSK, showing that feedforward receivers can approach Helstroms bound, far exceeding the sensitivity of a homodyne receiver. Finally, the performance of the extended Dolinar receiver is considered in the presence of impairments, showing that it may be possible to implement a receiver that surpasses the sensitivity of a homodyne receiver using available optical components.