Theory for optical heterodyne DPSK receivers with post-detection filtering

We present a general theoretical model for optical heterodyne DPSK receivers for optical communications systems where transmitter and local oscillator lasers have significant linewidths. Quantum phase noise in the lasers is treated as such, but in contrast with previous models for DPSK, receiver noise and local oscillator shot noise are treated as additive noise on the receiver output voltage, as this allows a straightforward description of the effects of post-detection filtering. As a consequence of the detection scheme, a key issue is to account for the non-Gaussian statistics of the output voltage, and this is done using a Chernov bound formulation. Detailed numerical results for a typical 140 Mbit/s p-i-n-FET front end are presented. It is found that heterodyne DPSK gives improved receiver sensitivity compared to other heterodyne detection schemes for IF linewidths below 0.7 percent of the data rate.