Phase Compensation Receivers for Optical Communication

Phase compensation receivers are investigated in the context of optical communication, with particular emphasis on low visibility channels. It is shown that phase compensation can be viewed as a reduction of the number of spatial modes in the signal field so that an improvement in communication performance can be realized by a receiver which exploits the spatial coherence of the compressed signal mode. The phase compensation receiver structure separates nicely into configurations for measuring the phase of the signal field across the aperture and the required processors to obtain the phase compensation control from the measurements. The optimum phase control, which is chosen to spatially concentrate the signal power in the focal plane, is shown to be the minimum-mean-squared-error estimate of the aperture field phase, when the estimation error is "small enough." The phase observation-estimator structures are placed in an idealized phase compensated receiver. Minimum signal power requirements for adequate phase estimation performance are established. Communication performance, in terms of the parameters that describe the low visibility channel, is examined for both compenstated and uncompensated receivers. Depending on the system used, and for phase coherence times in the tens of milliseconds, phase compensation is most appealing for signaling rates from the middle kilobit to low megabit per second range; the resulting improvement in communication performance made possible by phase compensation is as much as 40-60 dB in optical power relative to an uncompensated receiver.