MODELING ION-EXCHANGE IN GLASS WITH CONCENTRATION-DEPENDENT DIFFUSION-COEFFICIENTS AND MOBILITIES

Multimode buried waveguides made in silicate glass by fieldassisted io n exchange present very asymmetric profiles. We show how this phenomen on originates in the large dependence of the kinetics on the local ion concentrations. For this purpose, we derive an interdiffusion equatio n that includes the effects of concentration-dependent diffusion coeff icients and mobilities. We show how to deduce this dependence from mea surements on ion-diffused samples. The maximum concentration of the in coming ions is computed from surface equilibrium conditions and is use d in the interdiffusion equation as a limiting parameter for coefficie nt variations. To control the model accuracy for surface as well as bu ried waveguides, we measure ion profiles with three independent method s: M-lines, scanning electron microscopy, and near-field refractometry . When applied to Ag+-Na+ exchange in silicate glass, the model yields theoretical estimations in good agreement with experiments. This appr oach underlines the fundamentally nonlinear process that takes place d uring ion exchange and is also valuable to properly model single-mode waveguide fabrication. (C) 1996 Society of Photo-Optical Instrumentati on Engineers.