Directional Characteristics in Optical Heterodyne Detection Processes

Because of the spatial variation of the electric fields and the finite size of the detector, the output current in a coherent optical detection process varies as a function of orientation. When two unfocused light beams impinge upon the detector, the output current is shown to be identical in form to the amplitude of the field generated at the origin of one of the sources by the other source when the detector is replaced by a reflector. If the effective aperture is increased by focusing the light beams with a single lens, then the directivity increases. However, if multiple apertures are used in order to maintain the images in the same position, the directivity is reduced. The detection of coangular beams, such as from the longitudinal modes of a laser, is relatively insensitive to angle of incidence.