In 1922, Leon Brillouin began to analyze thermal acoustic fluctuations in liquids and solids by the scattering of light or X-rays off of changes in the refractive index of the medium. In his paper, Brillouin gave the first look at the theoretical explanation of what has become known as acousto-optic diffraction. He used an analogy to diffraction gratings to determine binding equations on the output light. From his theory he was able to predict some of the common phenomena known today regarding acousto-optic (AO) devices, namely the deflection of light and the frequency shift of the deflected light. An acousto-optic effect is produced by generating an ultrasonic wave in an optically transparent material. As the acoustic wave travels through the medium, it causes periodic variations in the index of refraction. For instance, in a crystalline solid, the wave produces compressions and refractions in the crystal lattice. Because the changes in the refractive index are periodic, the material behaves much like a diffraction grating, with site spacing equal to the wavelength of the ultrasonic wave.The Acousto-Optic effect was first observed in 1932 by Lucas, Biquard, Debije and Sears. The effect became extremely important after the invention of laser light by Theodore Maiman in 1960, and found its way in electronic communication systems and other fields of interest, very rapidly. The early experiments have first been theoretically explained by Raman and Nagendra Nath in 1935.Typically the acoustic wave is launched into the medium by a piezo-electric transducer. Application of an electric field to the transducer has the effect of deforming the material, producing an internal strain. This strain is transmitted to the acousto-optic medium which is mechanically coupled to the transducer. The acousto-optic medium can be liquid (such as water), solid (such as lead molybdate), or even gas (such as water vapor). Any strain in the material should produce a change in the refractive index. Incident light on the acousto-optic medium will scatter off variations in the refractive index. The scattering of light can be tuned to create destructive interference between certain wavelengths, thereby constructing an optical filter. Other common uses of acousto-optic media include devices for modulating light for communication, deflecting light, convolving or correlating signals, optical matrix processing, analyzing the spectrum of signals, optical sources, laser mode lockers, Q-switchers, delay lines, image processing, general and adaptive signal processing, tomographic transformations, optical switches, neural networks, optical computing, and much more.In Raman-Nath scattering, also known as Debye-Sears scattering, the thickness, L, of the cell is much less than a quarter wavelength. It is so thin that there is no significant interaction between the even and odd waveguide modes. In this case, the normally incident electromagnetic field will be spread out into a number of grating modes, each of which is shifted in frequency either up or down by an integer number of the acoustic wave frequency. This case is more complicated to analyze since it involves all of the Fourier modes. In general sidebands of like order are equal in amplitude and decrease as a function of n.\Bragg scattering can be accomplished by having a very wide slab of material, the diffraction is said to be in the Bragg regime. The main characteristic of Bragg diffraction is two output beams: the undiffracted beam or carrier beam, and the principal diffracted beam or the sideband beam. The sideband beam will be frequency shifted either up or down depending on the direction of the incoming carrier beam. This case is similar to Bragg diffraction of X-rays in a crystal lattice. It turns out that other orders are produced within the acousto-optic device, but these orders destructively interfere since they are not phase matched. The only order which is phase matched throughout the lattice is the principal diffracted order. Besides many AO devices, it is also possible to apply Acousto-Optic for nondestructive testing. This can be done by Schlieren photography or by means of Bragg imaging.

Bragg diffraction of x-rays occurs when the rays interact with a crystalline lattice at the appropriate angle. Bragg Diffraction of visible light occurs when the light interacts at the Bragg angle with an ultrasonic field of the appropriate frequency. (The spacing between acoustic condensations and rarefactions acts like the planes in an atomic lattice.) If a beam of light is Bragg diffracted by an ultrasonic beam that previously has passed through an object, an image of the structure of the object is made visible in the diffraction field of the optical beam since there is a one-to-one mapping of the ultrasonic field onto the diffraction order. In many acoustic Bragg imaging applications, the sound field must pass through the object which is to be imaged. Ultrasonic attenuation at the very high acoustic frequencies needed for Bragg imaging (typically ~ 25 – 30 MHz) severely limits the nondestructive testing (NDT) applications of traditional acoustic Bragg imaging. Ina recent paper, a reflection-based application of acoustic Bragg imaging is discussed which may have useful industrial and biomedical NDT applications.