Patent ID: 7711238

Claim:
An optical fiber, comprising: a core that is provided at a center, the core being made of silica glass containing at least germanium; an inner cladding made of silica glass, the inner cladding being provided concentric to the core; and an outer cladding made of silica glass, the outer cladding being provided around the inner cladding, wherein the inner cladding contains germanium, and a concentration of germanium in the core is about 200% or more of a concentration of germanium in the inner cladding, a diameter of the inner cladding is between 10 μm and 40 μm, the inner cladding containing germanium of between 0.2% by weight and 1.5% by weight, phosphorus of between 0.5% by weight and 1.5% by weight, and fluorine of between 0.1% by weight and 1.0% by weight, a diffusion coefficient of germanium diffused in the inner cladding being relatively large, a refractive index of the inner cladding being substantially the same as a refractive index of silica glass, a diffusion coefficient of germanium diffused in the outer cladding is relatively smaller than in the inner cladding, and an enlarged mode field diameter of the optical fiber during splicing with a second silica glass optical fiber by means of heating using an arc discharge for a time to a minimum splicing loss is matched with an enlarged mode field diameter of the second silica glass optical fiber, a failure tension with which a cumulative failure probability becomes 50% is over 2.0 GPa after the fusion splicing.