Method for manufacturing optical fiber base material, and optical fiber
A method for making a high quality optical fiber preform includes: 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; collapsing the glass pipe for forming a core rod; and adding a cladding part around the circumference of the core rod. 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.
1. An optical fiber preform manufacturing method comprising:
a thermal diffusion process for oxidizing and thermally diffusing an alkali metal element into the inner side of a silica glass pipe, the glass pipe being heated with a heat source performing relative movement in the longitudinal direction of the glass pipe, while vapors of alkali metal salt generated by heating an alkali metal salt is supplied with a carrier gas to the inside of the glass pipe from an end thereof, the thermal diffusion process including:
heating up the glass pipe to a temperature of 1500° C. or more and 2200° C. or less while heating the alkali metal salt at a temperature for making the vapor pressure of the alkali metal salt to be 0.1 kPa or less, and
raising up the heating temperature of the alkali metal salt to a temperature for making the vapor pressure of the alkali metal salt to be larger than 0.1 kPa, while maintaining the heating temperature of the glass pipe at 1500° C. or more and 2200° C. or less;
a collapsing process for forming a core rod by collapsing the glass pipe after the thermal diffusion process; and
a cladding process for adding a cladding part around the circumference of the core rod.
2. An optical fiber preform manufacturing method according to claim 1 , wherein
at the start of the thermal diffusion of the alkali metal element into the inner side of the glass pipe in the thermal diffusion process, the vapor pressure of alkali metal salt is made 0.05 kPa or less.
3. An optical fiber preform manufacturing method according to claim 1 , wherein
at the time of heating the alkali metal salt up to a temperature for making the vapor pressure of alkali metal salt to be larger than 0.1 kPa, the temperature increase rate is 5° C./min or more and 15° C./min or less.
4. An optical fiber preform manufacturing method according to claim 1 , wherein
at the time of heating the alkali metal salt up to a temperature for making the vapor pressure of alkali metal salt to be larger than 0.1 kPa, the heating temperature is maintained at an intermediate temperature for a given time.
5. An optical fiber preform manufacturing method according to claim 1 , wherein
at the start of thermal diffusion of the alkali metal element into the inner side of the glass pipe, the heating temperature of alkali metal salt is the melting point of the alkali metal salt or less.
6. An optical fiber preform manufacturing method according to claim 1 , wherein
at the start of thermal diffusion of the alkali metal element into the inner side of the glass pipe, the heating temperature of alkali metal salt is 734° C. or less.
7. An optical fiber preform manufacturing method according to claim 1 , wherein
the alkali metal salt is potassium bromide or potassium iodide at the thermal diffusion process.
8. An optical fiber preform manufacturing method according to claim 1 , wherein
the carrier gas contains oxygen at the thermal diffusion process.