Optical fiber and light source device

An optical fiber that is composed of silica glass, the optical fiber comprising:

• a core having a refractive index n1 and a diameter 2a and containing germanium;
• a depressed section that surrounds the core, has a refractive index n2 and an outer diameter 2b, and contains fluorine;
• and
• cladding surrounding the depressed section and having a refractive index n3,
• the optical fiber having a relationship n1>n3≥n2, a relative refractive-index difference Δ1 of the core relative to the depressed section being 3.0% or more and 4.2% or less, a relative refractive-index difference Δ2 of the depressed section relative to the cladding being −0.8% or more and −0.3% or less, the diameter 2a being 4 μm or more and 6.0 μm or less, and b/a being 2.0 or more and 3.0 or less,
• wherein the optical fiber has a zero-dispersion wavelength of 1290 nm to 1350 nm, has an effective area of 14 μm2 or smaller at a wavelength of 1550 nm, and has a chromatic dispersion at the wavelength of 1550 nm of 10 ps/nm/km or more and 22 ps/nm/km or less.