Optimum conditions for heterodyne detection of light*

The problem of detecting a coherent light beam in the presence of unwanted background radiation by the heterodyne method is examined. For a sufficiently strong local-oscillator field, the detectability of the signal is unaffected by the presence of the background radiation. It is shown that, in general, there exists an optimum receiver size that maximizes the signal-to-noise ratio. This result is illustrated by several examples. A procedure for the detection of a light signal of unknown direction is suggested.