  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.
