Theoretical analysis of coherent optical FSK systems with limiter-discriminator detection

A detailed theoretical analysis, based upon Rice's theory of clicks, for evaluating the error performance of heterodyne optical FSK receivers with limiter-discriminator detection and pre- and integrate-and-dump (I&D) post-detection filtering is developed, accounting for laser phase noise, intersymbol interference (ISI) and intermediate-frequency (IF) filtered carrier envelope fluctuation effects. From the analysis it turns out that the envelope fluctuations induced by phase noise may produce, for operating IF bandwidths, a much smoother transition to the bit error rate floor than expected when considering IF infinite bandwidth, thereby generating performance degradation right in the operating signal-to-noise ratio (SNR) region. The contribution to this degradation due to a change in the click statistics generated by the envelope fluctuations is found to be negligible. From the analysis, it is also found that the floor gets lower and approached more rapidly as the modulation index and the bandwidth get larger, until the known floor for infinite IF bandwidth is reached. Finally, general curves are presented for system performance relating bit error probability to optimum IF bandwidth, optimum modulation index, laser linewidth and receiver sensitivity. These curves represent the ultimate limit to performance of a heterodyne optical FSK receiver with limiter-discriminator detection and pre- and I&D post-detection filtering.