An analysis is made of the problem of detecting a weak light beam from a distant source in the presence of a background of much greater intensity, by the photoelectric heterodyne technique. In this method the incident light is superposed on the light beam from a local laser, whose frequency can be adjusted by a feedback arrangement so as to maximize a certain “beat note” in the output of the detector. With the aid of plausible assumptions it is shown that the effectiveness of the method is largely independent of the intensity of the background light, and of the fluctuation properties of the incident light. The key parameter is the number of photoelectrons released by the signal beam in a time comparable with its coherence time.
L. MANDEL, "Heterodyne Detection of a Weak Light Beam," J. Opt. Soc. Am. 56, 1200-1204 (1966)