A polarization-insensitive FSK optical heterodyne receiver

A polarization-state-independent binary frequency-shift-keying (FSK) optical heterodyne receiver, realized by splitting the received signal between two orthogonal polarization axes and combining the signals after demodulation, is described. Various receiver configurations are considered using a discriminator with a limiter for applications in which received-signal-envelope fluctuations can not be ignored and, without a limiter, when the fluctuations are negligible. For applications that require the limiter, a diversity method or a variable-gain limiter that improves the performance of the limiter/discriminator receiver is proposed. Numerical results indicate that, with diversity and a limiter/discriminator structure, polarization independence can be achieved with a system performance which is nearly that of an ideal receiver. Using a discriminator without a limiter, when there is no source of envelope fluctuations other than those due to Gaussian noise, the receiver performance degradation compared to the ideal baseline receiver is 0.5 dB.<>