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The theory of the prism–film coupler is generalized to include cases in which the thicknesses of both the coupling gap and of the light-guiding film vary along the direction of propagation. The profiles of gap and film are optimized with respect to the input beam, resulting in a theoretical coupling efficiency of practically unity. It is shown that the problem of optimum coupling is equivalent to the problem of matching the spatial amplitude distribution of the input beam to the leakage field of the coupler. This theory is applied to a coupler with a linearly tapered gap, yielding a theoretical coupling efficiency of 0.96. Experimentally, an efficiency of 0.88 has been achieved.
R. Ulrich (1971) studied this question.