Coupling to optical waveguide modes was investigated by depositing two thin film layers on the hypotenuse of a rutile prism. Coupling efficiency was determined as a function of input beam angle for the three lowest order TE and TM modes. The coupling lineshape was found to agree with the predictions of a lossless, ray-optics analysis for all but the lowest order TM modes. This result is explained in terms of the relative effects of loss on the coupling efficiencies of the various modes.
T. A. Galantowicz, "Lineshape Measurement for Coupling to Waveguide Modes in the Prism—Film Coupler," Appl. Opt. 13, 2525-2528 (1974)