We now consider the diffraction of light by acoustic waves in an optically transparent medium in which the acoustic wave is excited. An optical beam is incident onto the cell and travels through the acoustic beam. Via the elasto-optical effect, the traveling acoustic wave sets up a spatial modulation of the refractive index which, under proper conditions, will diffract the incident beam into one or more directions.
The AO interaction can be viewed as a parametric process where the incident optical wave mixes with the acoustic wave to generate a number of polarization waves. The polarization waves in turn generate new optical waves at various diffraction orders.