Photoelasticity and acousto-optics

The change in refractive indices with mechanical stress, the photoelastic effect, is used in analyzing stress patterns in engineering components of complicated shape. It is also important in acousto-optic devices, optical switches, modulators, and scanners in which ultrasonic waves modulate the refractive index, producing an optical grating. Materials with sizable photoelastic coefficients (p) are required to enhance the interaction between mechanical strain x and refractive index n. Changes in the indicatrix are given by These quantities are actually tensors but are treated as scalars in the following discussion which is concerned with the magnitudes of the photoelastic coefficients p, and not their variation with direction. Unlike the linear electro-optic effect, photoelasticity occurs in all symmetry classes and is not a null property. Photoelastic coefficients are dimensionless because strain and refractive index are dimensionless. For most oxides and halides, pmax ≅ 0.2. The maximum values of p measured for other materials range from 0.1 to 0.6. To gain a clearer understanding of the effects of stress on refractive index, consider the effect of hydrostatic pressure on a cubic crystal.