Effects of mode conversion in acoustooptical modulation

The conversion of a Gaussian laser beam into higher order optical beam modes through acoustooptical modulation is treated. In particular the effects of this conversion in an optical communications system are described with emphasis placed on the actual signal received by a distant antenna rather than on the total power scattered. For the special case of pulse amplitude modulation, which is considered in detail, the received optical signal displays phase modulation as well as the impressed amplitude modulation. The relative importance of the two effects depends strongly on the location of the receiving aperture in the far field of the transmitted beam. An experiment in which 10.6-μ, laser light is scattered by a germanium acoustooptical modulator is described. The results of the experiment compare well with the predictions of the above analysis.