Optical waveguides are formed in glass when dopant ions such as Ag+, K+, Tl+, Cs+ replace alcalin ions already present in the glass (typically Na+ or K+). Part of the refractive index increase is due to the higher polarizability of dopant ions. However this substitution leads to a mechanical stress field, as the exchanged species have different ionic radii (Table 1) and a corresponding elasto-optically induced index change (which can be either positive or negative and can therefore modify the index profile). In certain applications, this stress induced birefringence can be useful (i.e. to allow polarization separation in a proximity coupler, to provide strong polarization maintenance on an optical chip), in others, it can be a nuisance (polarization sensitivity in proximity coupler based on wavelength demultiplexers, multiplexers).