Patent ID: 8655133

Claim:
A silica-based optical fiber having a transmission loss of 0.19 dB/km or less at a wavelength of 1550 nm, an effective area A eff of 120 μm 2 or more but 150 μm 2 or less at the wavelength of 1550 nm, and a fiber cutoff wavelength λ C of 1.3 μm or more but 1.53 μm or less, the optical fiber comprising: a) a core comprised of pure silica, the core having a refractive index n 1 and a diameter 2 a; b) an inner cladding, provided at an outer periphery of the core, having a refractive index n 2 and a diameter 2 b ; and c) an outer cladding, provided at an outer periphery of the inner cladding, having a refractive index n 3 , d) wherein the refractive index n 3 is smaller than the refractive index n 1 but greater than the refractive index n 2 , e) wherein the ratio Ra(= 2 b / 2 a ) of the diameter of the inner cladding to the diameter of the core is 2.5 or more but 3.5 or less, f) wherein the core has the diameter of 12.9 μm or more but 15.0 μm or less, g) wherein the relative refractive index difference Δ*(=100×(n 1 −n 2 )/n 2 ) of the core with the inner cladding of 0.265% or more but 0.345% or less, h) wherein, letting λ FCup be the fundamental-mode cutoff wavelength when a fundamental mode begins to leak at 1625 nm, a fundamental-mode cutoff wavelength λ FC is set to a wavelength exceeding λ FCup , and i) wherein the relative refractive index difference Δ − (=100 ×(n 2 −n 3 )/n 3 ) of the inner cladding with respect to the outer cladding at which the bending loss is minimized is −0.12% or more but −0.06% or less.