| 摘要 |
PURPOSE:To obtain the subject parent material of the silica glass having uniformly doped F of low concentration in radial direction by heat-treating the porous silica parent material having a bulk density lower than specific value at a temperature higher than specific temperature in a gaseous atmosphere containing low F-content of lower than fixed value and sufficiently diffusing F. CONSTITUTION:The porous silica parent material having <=0.3g/cm<3> bulk density obtained by hydrolyzation of Si compound in well-known oxyhydrogen flame is heat-treated under following conditions: i.e., said porous silica is heat-treated at a temperature higher than 1300 deg.C and for a time longer than 30 min in a gas containing F-raw material lower than 1vol% of total fed gas, e.g., in a gas diluted with inert gas such as He gas and F is sufficiently diffused. Next, the porous silica in which F is diffused is melted and vitrified to afford a silica glass for optical fiber, in which F having <=0.2% refractive index difference based on silica is uniformly doped in radial direction with <=0.0003 variation width of absolute refractive index, is obtained. |
