Optical Heterodyne Experiments with Enclosed Transmission Paths

It is well known that efficient operation of a light heterodyne receiver requires a high degree of phase coherence between the received signal beam and the local oscillator beam.1 For example, if the angle between the received beam and the local beam is in error by as little as one second of arc there can be considerable loss of signal. Also it is obvious that if the beam were translated by a beam width the detection efficiency would be reduced to zero. These requirements are so critical that many declared successful operation of a heterodyne detector at the end of a long transmission line to be very doubtful. Experiments on the transmission of light beams through the open 161 162 TI-IE BELL SYSTEM TECHNICAL JOURNAL, FEBRUARY 1908 atmosphere have shown that atmospheric turbulence might render the beam unsuitable for heterodyne detection. 2 However, the medium which we visualize for long-distance optical transmission consists of a series of lenses or mirrors for guiding the beam through an enclosure which isolates it from its environment. It is quite likely that successful guided transmission will require some type of beam position servo control in some of the guiding elements. If we provide this control, the position of the received beam can be stabilized so that an efficient heterodyning adjustment, once made, will be maintained. Stabilizing the position of the received beam automatically stabilizes its angle of arrival. During some previous experiments 3 we observed that single-mode transmission appeared to be preserved even after many reflections.