Method of manufacturing optical fiber preform and optical fiber preform

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 include a first local maximum point and a second local maxima point on a circumference around the first local maximum point,
• 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.