Birefringent Coupler for Integrated Optics

An optical coupler has been developed whereby a birefringent prism or substrate is utilized to couple light into thin film optical waveguides. The coupler is designed so that the incident light, polarized as an extraordinary wave, passes through the birefringent material at such an angle that the index of refraction in the input medium is higher than that of the waveguide. In addition, the orientation of the birefringent material is chosen so that the reflected waves within the waveguide are incident on the input medium at such an angle that the medium index of refraction appears lower than that of the waveguide. In this way, the light is trapped within the waveguide and, when the light is incident at the appropriate angle, this light propagates in the waveguide as a single mode. Coupling takes place over the whole area common to the waveguide and input medium; thus, neither a sharp edge nor a trapped evanescent region are required to prevent the light coupled into the waveguide from leaking back into the coupling prism. The absence of a boundary makes it practical to couple wide beams of light into planar guides. Couplers made of calcite have been used to couple TM He-Ne laser light into multimode waveguides formed of liquids and optical cement.