Method of manufacturing optical fiber preform and optical fiber preform

Abstract

The present invention relates to a method of manufacturing an optical fiber preform for obtaining an optical fiber with low transmission loss. A core preform included in the optical fiber preform comprises three or more core portions, which are each produced by a rod-in-collapse method, and in which both their alkali metal element concentration and chlorine concentration are independently controlled. In two or more manufacturing steps of the manufacturing steps for each of the three or more core portions, an alkali metal element is added. As a result, the mean alkali metal element concentration in the whole core preform is controlled to 7 atomic ppm or more and 70 atomic ppm or less.

1 . A method of manufacturing an optical fiber preform which comprises a first core portion including an alkali metal element, a second core portion surrounding the first core portion, a third core portion surrounding the second core portion, and a cladding portion surrounding the third core portion and further having a refractive index lower than each refractive index of the first to third core portions, the method comprising:
 a first doping step of doping an alkali metal element into an inner surface of a first glass pipe with a mean chlorine concentration of 10 atomic ppm or more and 600 atomic ppm or less;   a first collapse step of collapsing the first glass pipe after the first doping step by heating, thereby producing a first intermediate rod from the first glass pipe;   a first diameter-reduction step comprising removing an outer peripheral portion of the first intermediate rod to produce a first core rod constituting a part of the first core portion, the first core rod having a diameter smaller than a diameter of the first intermediate rod;   a second doping step of doping an alkali metal element into an inner surface of a second glass pipe with the mean chlorine concentration of 10 atomic ppm or more and 600 atomic ppm or less;   a second collapse step of integrating the first core rod and the second glass pipe by heating while the first core rod is inserted into the second glass pipe after the second doping step, thereby producing a second intermediate rod including a first glass region to be the first core portion and a second glass region to be the second core portion, the first glass region having an alkali metal element concentration of 100 atomic ppm or more, the second glass region having an alkali metal element concentration of 10 atomic ppm or less;   a first build-up step of adding a third glass region to be the third core portion onto an outer periphery of a second core rod formed from the whole or part of the second intermediate rod to obtain a third intermediate rod including the second core rod, the third glass region having an alkali metal element concentration of 10 atomic ppm or less and a chlorine concentration of 2,000 atomic ppm or more and 15,000 atomic ppm or less; and   a second build-up step of adding a fourth glass region to be the cladding portion onto an outer periphery of a third core rod formed from the whole or part of the third intermediate rod to obtain the optical fiber preform including the third core rod.   
     
     
         2 . The method of manufacturing an optical fiber preform according to  claim 1 , wherein the first collapse step comprises introducing oxygen gas in the inside of the first glass pipe in a reduced pressure condition. 
     
     
         3 . The method of manufacturing an optical fiber preform according to  claim 1 , wherein the second collapse step comprising introducing oxygen gas in the inside of the second glass pipe in a reduced pressure condition. 
     
     
         4 . The method of manufacturing an optical fiber preform according to  claim 1 , the method further comprising a second diameter-reduction step carried out after the second collapse step, wherein the second diameter-reduction step comprises removing an outer peripheral portion of the second intermediate rod to produce the second core rod having a diameter smaller than a diameter of the second intermediate rod. 
     
     
         5 . The method of manufacturing an optical fiber preform according to  claim 1 , the method further comprising a third diameter-reduction step carried out after the first build-up step, wherein the third diameter-reduction step comprises removing an outer peripheral portion of the third intermediate rod to produce the third core rod having a diameter smaller than a diameter of the third intermediate rod. 
     
     
         6 . The method of manufacturing an optical fiber preform according to  claim 1 , wherein a concentration of OH group contained in the third core rod is 0.01 wt·ppm or less. 
     
     
         7 . The method of manufacturing an optical fiber preform according to  claim 1 , wherein the alkali metal element is potassium. 
     
     
         8 . The method of manufacturing an optical fiber preform according to  claim 1 , wherein the third core rod includes a region containing the alkali metal element having a concentration of 100 atomic ppm or more, and in a part or the whole of the region an oxygen molecule concentration is 30 mol·ppb or more and 200 mol·ppb or less. 
     
     
         9 . An optical fiber preform, comprising:
 a core preform including:   a first core portion with a maximum alkali metal element concentration of 500 atomic ppm or more and 5,000 atomic ppm or less, the first core portion having a profile of an alkali metal element concentration along a radial direction of the first core portion, the profile having at least two local maxima points, which are separated from the center of the cross section of the first core portion,   a second core portion surrounding the first core portion, the second core portion having the alkali metal element concentration of 10 atomic ppm or less, and   a third core portion surrounding the second core portion, the third core portion having the alkali metal element concentration of 10 atomic ppm or less and the chlorine concentration of 2,000 atomic ppm or more and 15,000 atomic ppm or less, wherein the core preform has a mean alkali metal element concentration of 7 atomic ppm or more and 70 atomic ppm or less, and   a cladding portion surrounding the core preform, the cladding portion having a refractive index lower than each refractive index of the first to third core portions.   
     
     
         10 . The optical fiber preform according to  claim 9 , wherein the alkali metal element is potassium. 
     
     
         11 . The optical fiber preform according to  claim 9 , wherein the chlorine concentration of the first core portion and second core portion is 10 atomic ppm or more and 600 atomic ppm or less. 
     
     
         12 . The optical fiber preform according to  claim 9 , wherein the core preform includes a region containing the alkali metal element having a concentration of 100 atomic ppm or more, and in a part or the whole of the region an oxygen molecule concentration is 30 mol·ppb or more and 200 mol·ppb or less. 
     
     
         13 . The optical fiber preform according to  claim 9 , wherein a mean concentration of OH group in the core preform is 0.01 wt·ppm or less.