Influence of dehydration/sintering conditions on the distribution of impurities in sol-gel derived silica glass.

Use of commercial fumed silica as the starting material for preparation of optical waveguides by sol-gel techniques has been viewed with skepticism. Impurity levels in this material, especially those of transition metal oxides, are too high for waveguide applications. However, we find that during dehydration/ consolidation in an atmosphere of C1(2) and He from which O (2) is excluded that most impurities will be removed as a volatile chloride. This is demonstrated in the case of sol to which 1 wt% hematite as added. After dehydration/consolidation this level was reduced to 24 ppb's.