Patent ID: 7835609

Claim:
An optical fiber having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ 1 in a region from the center of the core to a radius of R 1 μm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ 2 in a region from the radius of R 1 μm to a radius of R 2 μm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ 3 in a region from the radius of R 2 μm to a radius of R 3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ 2 is equal to or less than 0.4%, Δ 1 , Δ 2 , and Δ 3 have relationships of Δ 1 >Δ 2 , Δ 3 >Δ 2 , and Δ 3 >Δ 1 , when Δ 1 , Δ 2 , and Δ 3 have relationships of Δ 1 −Δ 2 =X and Δ 3 −Δ 2 =Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%, Δ 2 , Δ 3 , R 1 , and R 2 satisfy relationships of (Δ 2 +Δ 3 )+1.0≦R 2 /R 1 ≦7*(Δ 2 +Δ 3 )−1.45 and Δ 2 +Δ 3 ≦1.15, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm 2 ·km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and an SBS threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.