A Guide to Acousto-Optic Modulators

Sound waves travelling through a crystal can be modelled as crests of increased refractive index alternating with troughs of decreased refractive index. Light incident on gradients in refractive index is scattered, therefore the light scatters from the acoustic wavefronts. In an AOM the light scattered from successive wavefronts interferes constructively. Figure 1 shows two rays of light impinging on two consecutive wavefronts in a crystal. Note that only some of the light is scattered from these wavefronts. The optical and acoustic wavelengths are denoted by λL and Λ respectively, while θi and θd are the angles the incident and scattered light rays make with the acoustic wavefronts respectively. The condition for constructive interference of the scattered light is