Optical communication system employing coherent detection and method

Abstract

An optical communications system for providing wavelength division multiplexed communications with high spectral channel density and increased sensitivity and selectivity through coherent detection utilizes a fixed frequency source for introducing at least one fixed frequency carrier for system-wide propagation through a system bus. Terminal devices coupled to the system bus functioning as transmitters remove a portion of the system-wide carrier energy and effect a frequency translation and modulation of the translated carrier energy for return to the system bus as a modulated, frequency shifted information bearing signal. Terminal devices functioning as a receiver effect either synchronous homodyne or heterodyne coherent detection by removing a portion of the system-wide carrier energy, optionally effecting a frequency translation, and combining the frequency shifted carrier energy with that of the frequency shifted information bearing signal to effect coherent detection. Where the relative frequency translation of information bearing signal and the locally shifted demodulating carrier energy are the same, homodyne detection is achieved, and, where the relative frequency shift of the information bearing signal and the locally shifted carrier energy is different, an intermediate frequency is provided for subsequent processing.