Patent ID: 7894697

Claim:
An optical fiber mainly composed of silica glass, said optical fiber comprising a core region extending along a designated axis, a depressed region provided on an outer periphery of said core region, a ring region provided on an outer periphery of said depressed region, and a cladding region provided on an outer periphery of said ring region and doped with F, wherein as compared with a refractive index of pure silica glass, a relative refractive index difference of said core region is greater than 2% but less than 3%, a relative refractive index difference of said depressed region is −1% or more but −0.5% or less, a relative refractive index difference of said ring region is 0.01% or more but 0.24% or less, and a relative refractive index difference of said cladding region is −0.3% or more but −0.1% or less, wherein a ratio Ra (= 2 a / 2 b ) of an outside diameter 2 a of said core region to an outside diameter 2 b of said depressed region is 0.15 or more but 0.35 or less and a ratio Rb (= 2 b / 2 c ) of the outside diameter 2 b of said depressed region to an outside diameter 2 c of said ring region is 0.55 or more but 0.85 or less, and wherein the outside diameter 2 a of said core region is 1.5 μm or more but 3.0 μm or less, and at a wavelength of 1550 nm an FOM (=|dispersion|/loss) is 250 ps/nm/dB or more.