Scattering of light by acoustic waves was first investigated by Brillouin. The acoustic frequencies involved in Brillouin scattering fall in the ultrasonic and hypersonic regions. Hypersonic waves in a medium are caused by thermal excitation, whereas ultrasonic waves can be excited electronically using piezoelectric transducers. The acoustic waves used in acousto-optics are generally ultrasonic waves that have frequencies in the range between about 100 kHz and a few gigahertz. The basic principles of acousto-optic devices are based on the scattering of light by the periodic index variations generated by an acoustic wave in the supporting medium. These periodic index variations form a moving index grating, generated by a traveling acoustic wave, or a standing index grating, generated by a standing acousticwave. Because the speed of sound in dielectric media that are used for device applications typically falls in the range between 1 and 10 km s-1, the index gratings generated by ultrasonic acoustic waves have grating periods ranging from the order of 1 μm to a few centimeters. A unique property of the index grating created by an acoustic wave is that its period and modulation depth can be varied by varying the frequency and amplitude, respectively, of the acoustic wave through variation in the electronic signal applied to the transducer. Therefore, the operating parameters of an acousto-optic device can be controlled electronically. Practical acousto-optic devices include modulators, beam deflectors, frequency shifters, couplers, switches, and spectrum analyzers.

Elastic waves

An acoustic wave in a medium is an elastic wave of space- and time-dependent periodic deformation in the medium.