Patent ID: 8346041

Claim:
An optical communications system in which an optical signal is transmitted via an optical fiber that is laid in a predetermined transmission section, and the optical signal is distributed-Raman-amplified in the optical fiber, wherein a transmission loss α (dB/km) and an effective area A eff (μm 2 ) of the optical fiber satisfy, at the wavelength of 1550 nm, the following relationships: 0.13≦α≦0.15 A eff ≧120, wherein the optical fiber comprises a central core extending along a predetermined axis, and a cladding surrounding an outer periphery of the central core and having a refractive index lower than that of the central core, and wherein the central core of the optical fiber comprises pure silica glass, or silica glass obtained by doping pure silica glass with at least one of P 2 O 5 , of 1 mol % or more but 10 mol % or less, Cl of less than 2000 mol ppm, F of 2000 mol ppm or more but 10000 mol ppm or less, and A 2 O (where A is an alkali metal element) of 1 mol ppm or more but 10000 mol ppm or less.