Simple expressions for calculation of directions of collinear phase matching in second harmonic generation in biaxial crystals

Expressions appreciably simpler than those used in the literature were derived for approximate calculations of the directions of collinear phase matching under conditions of optical second harmonic generation in biaxial crystals. This simplification was made possible by the use of expressions for the refractive indices obtained from the optical indicatrix rather than those determined from an analysis of the surface of the Fresnel normals.