1. Theoretical basis

1.1 Fundamentals of acousto-optic interaction

The propagation of an elastic or acoustic wave in a transparent medium - for example, the succession of mechanical contractions and dilations accompanying a longitudinal wave - is associated with a local variation in the material's refractive index. The result is a spatio-temporal modulation of the index, and the zone traversed by the elastic wave then behaves like a mobile phase grating; this is the photo-elastic effect. The mathematical study can be carried out from two complementary points of view. The classical wave point of view leads to results similar to those of optical gratings, particularly with regard to the diffraction of light waves. Unlike fixed gratings, however, the moving grating propagates at the speed of the elastic...