In this talk we will present fundamental theoretical considerations affecting the performance of coherent lightwave communication systems. These systems include a single-mode laser and a modulator at the source and a coherent receiver which is the optical onalog of a superheterodyne radio set. Instead of detecting photons directly, the coherent receiver first converts the incoming signal from the optical regime down to the radio regime, and then uses conventional electronic circuitry to perform various signal processing operations, such as amplification and demodulation. In principle, this technique can yield large increases (~ 20 dB) in receiver sensitivity compared with direct detection using avalanche photodiodes. One of the prime causes of this degradation has been identified as laser phase noise, a phenomenon that is known to be particularly serious in semiconductor devices.
"Coherent Lightwave Communications", Proc. SPIE 0841, Fiber Optic Networks and Coherent Technology in Fiber Optic Systems II, (1 January 1987); doi: 10.1117/12.967602; https://doi.org/10.1117/12.967602