METHOD FOR MANUFACTURING OPTICAL FIBER BASE MATERIAL, AND OPTICAL FIBER

摘要

A method for making a high quality optical fiber preform, wherein the generation of foreign substance on the inner surface of a glass pipe can be suppressed at the process of carrying out thermal diffusion of an alkali metal element. The method comprises: (1) a thermal diffusion process for thermally defusing the alkali metal element into the inner side of a silica glass pipe by heating the glass pipe from the outside by a heat source while vapors of alkali metal salt generated by heating an alkali metal salt is supplied to the inside of the glass pipe from an end thereof; (2) a collapsing process for forming a core rod by collapsing the glass pipe after the thermal diffusion process; and (3) a process for adding a cladding part around the circumference of the core rod, wherein at the start of the thermal diffusion, the alkali metal salt is heated at a temperature for making the vapor pressure of the alkali metal salt to be 0.1 kPa or less, and thereafter the alkali metal salt is heated up to a temperature for making the vapor pressure of the alkali metal salt to be larger than 0.1 kPa.