Optical fiber preform

An optical fiber preform comprising:

• a core portion comprised of silica glass and extending along a central axis thereof, in a cross-section of the optical fiber preform orthogonal to the central axis, the core portion having
• a first dopant region containing a first dopant and including the central axis, the first dopant being selected from among Na, K, and their compounds, a concentration of the first dopant being set at 10 atomic ppm or more but 2,000 atomic ppm or less, and
• a second dopant region containing a second dopant for reducing viscosity of the silica glass and surrounding the central axis while being away from the central axis, the second dopant having, as a characteristic at a temperature of 2,000° C. to 2,300° C., a diffusion coefficient of 1×10−12 cm2/s or higher but lower than that of the first dopant, a concentration of the second dopant being set at 10 atomic ppm or more;
• and
• a cladding portion comprised of silica glass and surrounding an outer periphery of the core portion, the cladding portion containing F and having a refractive index lower than that of the core portion.