| 摘要 |
PURPOSE:To form a glass preform having distinct boundary between the core layer and the clad layer, and free from the abnormal refractive index distribution, by carrying out the heat-treatment of a porous quartz glass preform within a specific temperature range in a specific atmosphere. CONSTITUTION:The porous glass preform 1 composed of the core-forming glass layer 2 and the clad-forming glass layer 3 is lowered into the heating furnace 5 (the sign 11 is electrical heater) under rotation, and is converted to transparent glass gradually from its lower end. In the above process, He gas containing about <=2.5vol% thionyl chloride vapor is introduced through the inlet 6 and discharged from the outlet 8, and the heat-treatment is carried out at or above the temperature to convert the preform to a nearly transparent glass (i.e. about >=1,250 deg.C). |
