Method for producing optical fiber preform, and optical fiber preform

Abstract

A production method and others according to the present embodiment are provided with a structure for effectively preventing occurrence of accidental spiking during drawing of a preform. In order to control the residual He-concentration in the center part of the preform, a transparent glass rod that has a predetermined outer diameter and is already sintered but is not doped with an alkali metal yet is annealed in in the atmosphere not containing He gas for an annealing time determined by referring to result data in which the relationship between the annealing time and the residual He-concentration is previously recorded for each outer diameter. In the result data, actually measured data of the residual He-concentration in a produced optical fiber preform and the annealing time are accumulated as annealing treatment results.

1 . A method for producing an optical fiber preform including a core preform having a center rod with an alkali metal doped region formed along a longitudinal direction and mainly comprised of silica glass, and a cladding portion provided on an outer periphery of the core preform and mainly comprised of silica glass, the method comprising:
 (1) a core preform producing step including:
 a rod producing step of producing a transparent glass rod configured to be the center rod and having a predetermined diameter, the transparent glass rod being obtained by dehydration treatment of a porous rod with glass fine particles deposited along the longitudinal direction in an atmosphere containing Cl gas, and by sintering treatment of the porous rod after the dehydration treatment in an atmosphere containing He gas; 
 a doping step of doping the alkali metal into the transparent glass rod along the longitudinal direction; and 
 a diameter expanding step of forming a peripheral core portion on the outer periphery of the center rod obtained by the doping step; 
   (2) a cladding portion producing step of forming the cladding portion on the outer periphery of the core preform obtained by the core preform producing step; and   (3) a measuring step of measuring a residual He-concentration in a region corresponding to the center rod in a sample preform cut out from the optical fiber preform obtained by the cladding portion producing step, wherein   the core preform producing step includes: as a step to be performed between the rod producing step and the doping step, an annealing time determining step of determining time taken for annealing treatment of the transparent glass rod by referring to result data including a table indicating a relationship between the annealing time and the residual He-concentration, the relationship depending on a specific outer diameter of the transparent glass rod; and a first annealing treatment step of annealing the transparent glass rod in an atmosphere not containing the He gas at least for the determined annealing time, and   the measuring step updates the result data by accumulating the annealing time determined by the annealing time determining step and the measured residual He-concentration, as treatment result for each outer diameter of the transparent glass rod annealed in the first annealing treatment step.   
     
     
         2 . The method for producing the optical fiber preform according to  claim 1 , further comprising a second annealing treatment step, between the cladding portion producing step and the measuring step, of annealing the optical fiber preform obtained by the cladding portion producing step for predetermined time in the atmosphere not containing He gas. 
     
     
         3 . The method for producing the optical fiber preform according to  claim 2 , wherein the annealing time of the first annealing treatment step is longer than the annealing time of the second annealing treatment step. 
     
     
         4 . (canceled)