Sensitivity enhancement using coherent heterodyne detection

This paper focuses on some heretofore overlooked advantages of coherent detection (CD) over direct detection (DD) in optical fiber communication systems. The basis for these sensitivity improvements is the fact that the local laser in coherent fiber optic systems adds a large number of photons to the net received optical signal. Under these conditions, the Bohr correspondence principle enables us to treat the photodiode output signal classically. Specifically, the usual CD environment yields a photodiode output that can be regarded as a classical signal disturbed by Gaussian noise (central limit theorem). This detection process has been studied extensively, and there exist many powerful signal processing techniques that may be used to enhance system sensitivity. In particular, we utilize a sum of digitally modulated subcarriers to frequency modulate an optical carrier along with the aid of coherent detection to achieve 11.1 photons/b sensitivity as determined by computer simulation