Accurate modeling of y-junctions and couplers made by ion-exchange techniques

The ion exchange in a glass substrate is probably the most widespread and inexpensive technology used to fabricate integrated optical circuits. In this paper we present a detailed modeling of the refractive index profile evolution in the case of closely spaced diffusion windows where the superposition principle for single isolated guides is no longer valid. This is the typical situation occurring in the geometrically simple yet critical case of a Y-junction branching or of a coupler. The model is based on the solution of the nonlinear parabolic differential equation describing the ion diffusion kinetics by a finite difference approach and has been validated experimentally in the planar waveguide case. Examples of device behaviors in case the refractive index profiles are determined using this coupled diffusion window model are given and compared with those obtained when the superposition of single diffusion windows is used.