Abstract
Perturbation theory for the plane-wave eigenvector equation of crystal optics is developed. General expressions for the refractive-index changes that result from a general homogeneous dielectric perturbation are found for isotropic solids, uniaxial crystals, and biaxial crystals. The expressions may be applied directly to the electro-optic effect for any direction of light propagation, for either state of light polarization, and for any direction of electric-field application. They are also useful for the photoelastic effect when the elastic perturbation is homogeneous.