Patent ID: 7406236

Claim:
An erbium-doped optical fiber amplifier comprising: a single mode optical fiber; an erbium-doped optical fiber; and an optical fiber coupler comprising a pair of optical fibers that are fused together and elongated, one end of one of the pair of optical fibers being fusion-spliced with the single mode optical fiber, and the other end of the one of the pair of optical fibers being fusion-spliced with the erbium-doped optical fiber, the one of the pair of optical fibers 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 disposed concentric to the core; and an outer cladding made of silica glass disposed 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 one of the single mode optical fiber and the erbium-doped 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 one of the single mode optical fiber and the erbium-doped optical fiber, a failure tension with which a cumulative failure probability becomes 50% is over 2.0 GPa after the fusion splicing.