Abstract

We have developed the formalism based on the Goss-Hânchen shift to calculate the m-line profiles of the reflected light from a coupling prism pressed on a multilayer guide. The m-line profiles correspond to the coupling and decoupling energy in and out of the guide. If the light intensity is high enough, it will disturb guide index, and two phenomena can be involved: thermal and photorefractive effects. Due to the rather large thermal diffusion length, the thermal effect gives mostly a shift to the considered m-lines. The photorefractive effect is proportional to the local intensity. We compute the energy distribution under the prism, in the guide, both in the propagating direction and in the direction perpendicular to dioptres. Then we introduce non linear coefficient value for each layer of the guide to obtain the refractive index profile from energy distribution. After sampling it in 2 dimensions, we are able to calculate the modification of the m-line profile induced by this third order nonlinearity. Our results show that this m-line profile is then distorted from the linear one. Thus, thermal effect appears to be easily distinguishable from that of photorefractive effect and this method must be useful to measure very weak non linear coefficients which we expect to have with dielectric thin films.

© 1990 Optical Society of America