Before elaborating on the principles of acousto-optics let me give you my personal impression on how this field has developed. It is now roughly fifty years ago that Brillouin predicted certain effects that should occur when a beam of light interacts with a beam of high frequency sound. (Refs.1,2). In particular he predicted that in such an interaction, if the light were incident at a certain critical angle, it would be split into various orders propagating into different directions. This can be under-stood intuitively if one considers the planes of compression and rarefaction caused by the sound wavefronts to act somewhat like the layers of atoms in a crystal. The effect should then be closely analogous to that of X-ray diffraction. Because, however, in the case of a sound wave the "atomic planes" are moving with sound velocity, the diffracted orders of light should not only propagate in different directions but also have slightly different frequencies caused by Doppler shift. Effects like this were indeed observed in 1932 by Debye and Sears(Ref. 3) and also by Lucas and Biquard. (Ref. 4) .