Theory for heterodyne optical ASK receivers using square-law detection and postdetection filtering

We present an approximate but comprehensive theoretical model for heterodyne ASK receivers relevant to optical communication systems where the transmitter and local oscillator have significant linewidths. The ASK receiver modelled here uses square-law detection and post-detection filtering. As a consequence of this detection scheme a key issue in the theory is to account for the very different noise statistics associated with receiving ONE and ZERO symbols. The theory also takes into account receiver noise and nonideal intermediate frequency (IF) filter characteristics and permits an evaluation of optimum threshold setting. Detailed numerical results for receiver sensitivity for a typical 140 Mbit/s pin-FET front end are presented. The receiver sensitivity penalty for nonzero linewidth is found to be less than 2.3 dB for IF linewidth below the very large value of 0.5 of the bit rate for an optimum ASK receiver. This clearly indicates the advantage of postdetection filtering in coherent optical receivers using rather broad linewidth lasers compared with the system considered earlier where the noise filtering was performed at the IF stage.