Optical Heterodyne Experiments with Enclosed Transmission Paths

Experiments to determine the feasibility of using optical heterodyne receivers at the end of long and complicated transmission paths showed that nearly ideal performance is possible. The paths were enclosed, thus avoiding atmospheric turbulence. We set up two receivers, one coherent and the other noncoherent, and compared their signal-to-noise performances and stability. The degree of beam distortion resulting from transmission over a path was determined by comparing the path losses as measured by the two receivers. Heterodyne detection was nearly 80 per cent efficient for a four-mile path involving 63 reflections and transmission through a considerable number of other optical components. Stable heterodyne operation was observed.