A prism–film coupler has been discussed recently by Tien, Ulrich, and Martin as a device to couple efficiently a laser beam into thin-film dielectric light guides. This coupler also allows an accurate measurement of the spectrum of propagating modes from which the refractive index and the thickness of the film can be determined. We present here a theory of the prism–film coupler. The physical principles involved are illustrated by a method that combines wave and ray optics. We study the modes in the thin-film light guide and their modification by the effect of coupling. We also calculate the field distributions in the prism and the film, the power transfer between the prism and the film, and derive a condition of optimum operation. In one example, 81% of the laser power can be fed into any desired mode of propagation in the film.
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