Theory of the Prism–Film Coupler by Plane-Wave Analysis

TLDR

A laser beam can be coupled with high efficiency into a light‑guiding thin film by means of a prism–film coupler, which consists of a totally reflecting prism separated from the film by a narrow low‑index gap. The authors analyze this coupling scheme using plane‑wave expansion to model the interaction between the prism, gap, and film. They show that coupling efficiency depends on the balance between coupling and leakage, derive a condition for undistorted beam propagation, and demonstrate that a Gaussian beam can achieve up to 80 % efficiency.

Abstract

A laser beam can be coupled with high efficiency into a light-guiding thin film by means of a prism–film coupler. Basically this device is a totally reflecting prism, the light-guiding film being separated from the reflecting prism face by a narrow gap of reduced refractive index. This coupling scheme is analyzed in detail by the method of plane-wave expansion. It is shown how the coupling efficiency is determined by the competition between the desired coupling effect and the reverse effect of leakage. A general condition is derived under which the transverse profile of the input beam continues undistorted into the guide. The theory is illustrated for a gaussian beam, which allows a maximum coupling efficiency of 0.80.