Commercially available glasses are not optimized for ion exchange in order to fabricate multimode optical waveguides. For most of the glass types the refractive index increase and the waveguide thickness are too small and the loss is increased. Therefore in the last years special glass types have been developed for thallium and silver ion exchange, respectively. By this type of exchange waveguides can be fabricated with refractive index profiles sufficiently matched to that one of 50 μm-graded-index fibers. However, thallium is a highly toxic ion and for silver-sodium exchange a yellow cnuloring of the glasses appears due to reducing impurities as, e. g., As3+, Fe2+ and Sb3+./1/.
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