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The refractive index of an optical medium is altered by the presence of sound. Sound therefore modifies the effect of the medium on light; ., sound can contro2 light (Fig. ). Many useful devices make use of this acousto-optic | Fundamentals of Photonics Bahaa E. A. Saleh Malvin Carl Teich Copyright 1991 John Wiley Sons Inc. ISBNs 0-471-83965-5 Hardback 0-471-2-1374-8 Electronic CHAPTER 20 ACOUSTO-OPTICS INTERACTION OF LIGHT AND SOUND A. Bragg Diffraction B. Quantum Interpretation C. Coupled-Wave Theory D. Bragg Diffraction of Beams ACOUSTO-OPTIC DEVICES A. Modulators B. Scanners C. Interconnections D. Filters Frequency Shifters and Isolators ACOUSTO-OPTICS OF ANISOTROPIC MEDIA Sir William Henry Bragg 1862-1942 left and Sir William Lawrence Bragg 1890-1971 right a father-and-son team were awarded the Nobel Prize in 1915 for their studies of the diffraction of light from periodic structures such as those created by sound. 799 The refractive index of an optical medium is altered by the presence of sound. Sound therefore modifies the effect of the medium on light . sound can control light Fig. . Many useful devices make use of this acousto-optic effect these include optical modulators switches deflectors filters isolators frequency shifters and spectrum analyzers. Sound is a dynamic strain involving molecular vibrations that take the form of waves which travel at a velocity characteristic of the medium the velocity of sound . As an example a harmonic plane wave of compressions and rarefactions in a gas is pictured in Fig. . In those regions where the medium is compressed the density is higher and the refractive index is larger where the medium is rarefied its density and refractive index are smaller. In solids sound involves vibrations of the molecules about their equilibrium positions which alter the optical polarizability and consequently the refractive index. An acoustic wave creates a perturbation of the refractive index in the form of a wave. The medium becomes a dynamic graded-index medium an inhomogeneous medium with a time-varying stratified refractive index. The theory of acousto-optics deals with the perturbation of the refractive index caused by sound