Method of manufacturing optical fiber preform and optical fiber preform

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.