Effects of OH-Group on Distribution of Refractive Index in Silica Glass

To study the effects of the fictive temperature, Tf, and the OH-group on the refractive index, n, of silica glass, several samples were prepared containing from 680 to 850wt ppm of OH-group in different distribution patterns. Trimethoxymethylsilane was used as a raw material and hence the samples contained no detectable chlorine, whose inhomogeneous distribution was an important factor affecting local change of n for commercial silica glasses. For a sample with homogeneous OH-group distribution, the distribution pattern in n after stress free annealing was determined by measuring the local deviations in n, Δn, with a laser interferometer. The distribution pattern was considered to be due to the distribution in Tf. The Δn distributions observed for the annealed samples with various OH-group distribution patterns agreed well with those calculated as a simple summation of the Δn due to the local deviation in OH-group, i.e. Δn=-1×10-7/ΔOH (wt ppm), and the Δn due to the local deviation in Tf. A way to manufacture silica glass with sufficiently homogeneous refractive index distribution was discussed.