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The technique of focused heterodyne detection, wherein a diffraction-limited lens is used to focus the signal beam to an Airy pattern, is described as a means of greatly reducing the angular alignment requirement of the conventional optical heterodyne system. An expression for signal-to-noise ratio as a function of angular misalignment of the signal and local oscillator beams is derived analytically, and the results of an experiment conducted to test the accuracy of this expression are shown to agree fairly well with theoretically expected results. However, the advantage gained by focusing the signal beam (relaxed angular alignment requirement) is offset by a new requirement: the center of the focused spot must never deviate from the center of the aperture in the focal plane by more than a fraction of its diameter. This aperture must be used in order to exclude extraneous, noise-producing light from the photodetector, and should be about the same size as the Airy disk in order to maximize signal-to-noise ratio. To overcome this difficulty, a system utilizing an image dissector tube to provide a virtual aperture on the photo-cathode was constructed. This system provides automatic acquisition of the focused heterodyne signal as well as automatic tracking of the signal through reasonable spatial perturbations on the photocathode. The tracking capability was demonstrated to be about two resolution elements peak-to-peak for a 1000-cps signal position perturbation, increasing linearly to thirty elements at or below 60 cps. A resolution element is defined as the effective size of the virtual aperture (about 0.025 cm).
© 1965 Optical Society of America
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