Patent ID: 7583878

Claim:
An optical fiber, made of silica-based glass, comprising: a core; and a cladding, wherein the optical fiber includes a mode field diameter of 5.4 micrometers or larger at a wavelength of 1300 nanometers, the optical fiber is configured to transmit light with a wavelength of 1250 nanometers in a single mode, and the optical fiber includes a bending loss of 1 dB/turn or smaller at a wavelength of 1300 nanometers when the optical fiber is bent with a curvature radius of 1 millimeter, and wherein the optical fiber includes a relative refractive index difference of a first core of 0.8% or larger with respect to the cladding, the first core being located at a center of the optical fiber, a parameter α of 1.5 or larger, and a relative refractive index difference of a second core of −0.05% or smaller with respect to the cladding, the second core surrounding the first core.