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The field of science and technology that is concerned with the diffraction of visible or infrared light (usually from a laser) by high-frequency sound in the frequency range of 50–2000 MHz. The term “acousto” is a historical misnomer; sound in this frequency range should properly be called ultrasonic. Such sound cannot be supported by air, but propagates as a mechanical wave disturbance in amorphous or crystalline solids, with a sound velocity ranging from 0.6 to 6 km/s (0.4 to 4 mi/s) and a wavelength from 3 to 100 μm. See Laser, Ultrasonics
The sound wave causes a displacement of the solid's molecules either in the direction of propagation (longitudinal wave) or perpendicular to it (shear wave). In either case, it sets up a corresponding wave of refractive-index variation through local dilatation or distortion of the solid medium. It is this wave that diffracts the light by acting as a three-dimensional grating, analogous to x-diffraction in crystals. The fact that the grating is moving is responsible for shifting the frequency of the diffracted light through the Doppler effect. See Diffraction grating, Doppler effect, Refraction of waves, Wave motion, X-ray diffraction
Since the 1960s, acoustooptics has moved from a scientific curiosity to a relevant technology. This evolution was initially driven by the need for fast modulation and deflection of light beams, and later by demands for more general optical processing. It was made possible by the invention of lasers, the development of efficient ultrasonic transducers, and the formulation of realistic models of sound-light interaction.