Antoine Kevorkian

Modeling ion exchange in glass with concentration-dependent diffusion coefficients and mobilities

Multimode buried waveguides made in silicate glass by field- assisted ion exchange present very asymmetric profiles. We show how this phenomenon originates in the large dependence of the kinetics on the local ion concentrations. For this purpose, we derive an interdiffusion equation that includes the effects of concentration-dependent diffusion coefficients and mobilities. We show how to deduce this dependence from measurements on ion-diffused samples. The maximum concentra- tion of the incoming ions is computed from surface equilibrium conditions and is used in the interdiffusion equation as a limiting parameter for coefficient variations. To control the model accuracy for surface as well as buried waveguides, we measure ion profiles with three independent methods: M-lines, scanning electron microscopy, and near-field refrac- tometry. When applied to Ag 1 -Na 1 exchange in silicate glass, the model yields theoretical estimations in good agreement with experiments. This approach underlines the fundamentally nonlinear process that takes place during ion exchange and is also valuable to properly model single- mode waveguide fabrication.

Superimposed grating wavelength division multiplexing in Ge-doped SiO2/Si planar waveguides

An improved model of wavelength division multiplexing (WDM) by superimposed gratings in planar waveguides has been developed. Based on this theory, principal design rules of N-channel WDM are established and a fanout capacity of 100 is estimated. Finally, fourchannel WDM at 840 nm has been demonstrated on a Ge:SiO2 /SiO2 /Si planar waveguide, which was hydrogenated to enhance its photosensitivity. The superimposed gratings were written using a narrow linewidth KrF excimer laser in an interferometric setup used in inscribing fiber Bragg gratings.

Modelization of a semi-leaky waveguide: application to a polarizer

We present the analysis of a semi-leaky waveguide with the help of 3 different methods. Since the resulting numerical values still show some differences, the validity of these methods is discussed. Both theoretical and experimental studies of a semi-leaky waveguide show a maximum of the losses for an index of the superstate slightly greater than the index of the waveguide. We finally apply these results to design and characterize a polarizer based on the deposition of an anisotropic layer on an otherwise lossless waveguide.

Local electric fields in optical glasses during field-assisted ionic exchanges

We study the phenomena connected with high concentration of incoming ions emerging during field-assisted migration in otpical glass. We find that ion dynamics are very different at concentrations higher and lower than a certain parameter called the transition concentration. To explain anomalies at high ionic concentrations, we introduce a supplementary local electric field. This field opposes to the field E0 existing in the glass at all concentrations of incoming ions and is connected with a local space charge. We investigate its dependence on concentration and on E0. These effects are studied using a model with concentration-dependent diffusion coefficients and mobilities. We present a method to obtain the concentration dependence of the relevant quantities. Theoretical curves are compared with experimental results measured in usual silicate glasses, during Ag+-Na+ exchanges.