Patent ID: 8135255

Claim:
An optical fiber comprising: a core at a center thereof; a first cladding adjacent to the core to cover a circumference of the core; and a second cladding adjacent to the first cladding to cover a circumference of the first cladding, wherein 0.35%≦(Δ 1 −Δ 2 )≦0.65%, 0.30%≦Δ 1 ≦0.55%, −0.20%≦Δ 2 ≦−0.05%, 0.22≦a/b≦0.34, and 4≦b √|Δ 2 |≦10 hold, and loss increase resulting when the optical fiber is wound on a mandrel having a diameter of 20 mm is 0.5 dB/turn or smaller at a wavelength of 1550 nm, where Δ 1 (%) is a specific refractive index difference of a maximum of a refractive index distribution of the core with reference to a refractive index of the second cladding, Δ 2 (%) is a specific refractive index difference of a minimum of a refractive index distribution of the first cladding with reference to the refractive index of the second cladding, a(μm) is a radius of a boundary between the core and the first cladding with respect to a center of the core, and b(μm) is a radius of a boundary between the first cladding and the second cladding with respect to the center of the core.