Patent ID: 7366386

Claim:
An optical fiber made of silica glass, comprising: a core; and a cladding surrounding the core, wherein a mode field diameter of the optical fiber at a wavelength of 1300 nanometers is equal to or larger than 5.4 micrometers, a light of a wavelength of 1250 nanometers is propagated through the optical fiber in a single mode, and a bending loss of the optical fiber with a bending radius of 1 millimeter at the wavelength of 1300 nanometers is equal to or lower than 1 dB/turn, wherein the core includes a first core located substantially in a center of the optical fiber along a longitudinal direction and having a diameter of “a”; and a second core surrounding the first core and having a diameter of “b”, and a relative refractive-index difference of the first core to the cladding is equal to or larger than 0.8%, an α value is equal to or larger than 1.5, a relative refractive-index difference of the second core to the cladding is equal to or less than −0.2%, and a ratio b/a, defined by a ratio of the diameter “b” of the second core to the diameter “a” of the first core, is equal to or lower than 2.0.