Effects of OH-group and Cl on the distribution of refractive index in silica glass.

Ways of manufacturing silica glass blanks with a homogeneous refractive index (n) distribution by appropriately distributing the OH-group and Cl were discussed. First, the effect of fictive temperature, which was considered to be set in the cooling process of stress relief annealing, was estimated. Then, the effects of OH-group and Cl were examined in terms of the distribution profiles of the relative n for silica glasses without other impurities. The literature formulas for the effects of OH-group and Cl on n, ΔnOH=−1×10−7⁄OH (mass ppm) and ΔnCl=−1×10−7⁄Cl (mass ppm), were shown to be reasonable.Calculations show that there were three patterns, in which the concentration of OH-group and Cl were distributed in the ingot so as to offset the effect of the fictive temperature, for manufacturing silica glass blanks with homogeneous n distribution.