| 摘要 |
PURPOSE:To obtain the titled preform having an optimum refractive index distribution shape and low loss with a low residual OH group content with good reproducibility, by calcining a specific fine glass particle aggregate in an O2 gas-containing atmosphere, heating the calcined aggregate in a gas atmosphere having dehydrating action to reduce the OH groups and heating the aggregate at a high temperature. CONSTITUTION:A glass raw material gas and flame gas are respectively fed from glass fine particle synthesizing torches 6 and 7 of a glass fine particle aggregate preparation apparatus and heated in a protective vessel 5 to carry out thermal oxidation reaction and synthesize oxide glass fine particles, which are deposited and grown on the tip of a rotated and pulled up starting member 4 to give a glass fine particle aggregate 1. The resultant aggregate 1 is then put in a quartz furnace core tube 10 provided in a carbon heater resistance furnace 9 having an uniform temperature distribution in the longitudinal direction and an O2-containing gas is then introduced from a gas feed port 13 to calcine the aggregate 1 at 1,000-1,300 deg.C. The temperature is then reduced to 900-1,300 deg.C in the next process and the atmosphere is converted into a gas atmosphere, e.g. a halogen gas, etc., and, as necessary, O2, etc., having dehydration action to carry out heat treatment. The residual OH content is reduced to <=0.05ppm and the aggregate 1 is then heated to 1,450-1,600 deg.C and transparently vitrified. |
