Birefringent prism couplers for thin-film optical waveguides

The coupling from a laser to a thin-film optical waveguide by a prism coupler composed of a birefringent material can be strongly dependent on the orientation of the optic axis. It is shown that when the effective index of the wave guided by the film lies between the ordinary and extraordinary refractive indices of the prism, the coupling depends strongly on the orientation of the optic axis. Differences in orientation of only a few degrees separate ranges of orientation in which strong coupling occurs from ranges in which there is no coupling. The orientation dependence of the coupling is considered both for the pure mode case wherein the optic axis lies in the plane of incidence and for the case when the optic axis is rotated out of the plane of incidence, so that mode coupling occurs. When the effective refractive index of the guided wave is less than both the ordinary and extraordinary refractive indices of the prism, the coupling properties are found to be similar to those obtained with an isotropic prism.