Optical beam diffraction tensor in birefringent crystals

We demonstrate that anisotropy of Fresnel diffraction in a birefringent medium can be quantitatively characterized by a planar tensor. Eigenvectors of the tensor correspond to directions of minimum and maximum beam divergence. Zero eigenvalues exist for the fast eigenmode near the optic axis of biaxial crystals and correspond to autocollimated beam propagation. Applications of the beam diffraction tensor are demonstrated for analysis of noncritical phase matching in acousto-optics and three-wave mixing.