Acoustic Light Modulators Using Optical Heterodyne Mixing

Acoustic light modulators are discussed in which the optical carrier is reinserted in the diffracted, frequency-shifted light beam. Reinsertion is accomplished in a novel fashion using a Kösters prism. In contrast to the usual acoustic modulator, the diffracted light is power modulated at the acoustic frequency. Modulation bandwidth and depth are each calculated as a function of the optical and acoustic beam parameters, assuming a Gaussian optical beam and rectangular acoustic beam. It is shown that the modulation bandwidth increases proportional to the optical beam diffraction angle and equals the inverse of the transit time of the sound across the waist of the optical beam. The optimum modulation depth, for a given acoustic power, corresponds to approximate equality of the optical and acoustic diffraction angles. Confirming experiments in the frequency range 250/350 MHz are described.