Influence of carrier gas on the optical homogeneity of synthetic silica glass

In this paper, synthetic silica glass was prepared by chemical vapor deposition using hydrogen and oxygen as carrier gases, respectively. Influences of different carrier gases on the distribution of hydroxyl and chlorine content and optical homogeneity in synthetic silica glass were investigated by ultraviolet-visible-near infrared spectrophotometer, X-ray fluorescence spectrometer and laser interferometer. The experimental results indicated that the hydroxyl content of silica glass prepared with hydrogen carrier gas is like a parabolic distribution, and the hydroxyl content decreases from 1270 ppm to 1020 ppm along the center to the edge of the silica glass in the direction of the transparent surface. The chlorine content distribution is the highest in the middle part of the glass, reaching about 60 ppm, while the other parts are basically the same and about 40 ppm. The change of refractive index caused by the distribution of hydroxyl and chlorine content is contrary and counteracts each other, which promotes the optical homogeneity of silica glass to be better. The hydroxyl content of silica glass prepared with oxygen carrier gas presents an inverse parabolic distribution, and the hydroxyl content increases from 880 ppm to 1010 ppm along the center to the edge of the silica glass in the direction of the transparent surface. The chlorine content increases from 60 ppm to 150 ppm and then decreases to about 40 ppm along the center to the edge of the silica glass. Because the distribution of hydroxyl content and chlorine content is extremely uneven, the change of refractive index caused by the distribution of hydroxyl and chlorine content can't offset each other, which leads to the optical homogeneity of silica glass to be poor. Therefore, synthetic silica glass with high optical homogeneity can be prepared by using hydrogen as the carrier gas.