WIDE-BAND HETERODYNE SPATIAL TRACKING FOR OPTICAL SPACE COMMUNICATIONS

A wideband optical heterodyne spatial tracker for use in diode-laser-based heterodyne optical space communications is described. The tracking error signals are formed by correlating the output of an intensity-noise-canceling communications receiver with the outputs of the tracking photodetectors. This correlation technique removes noise-induced offsets, which may significantly limit performance of other wideband tracking schemes and is an alternative to adding amplitude modulation in the track channel. The technique also enhances the SNR in the track channel relative to square-law-based demodulation of the track channel alone. The correlation concept is experimentally verified and closed-loop tracking performance is demonstrated.