PURPOSE: To provide an optical fiber having an excellent signal to noise ratio and high Raman efficiency in an optical transmission system which uses a Raman amplification and to provide an optical transmission system in which the optical fiber is used. CONSTITUTION: The optical transmission system is constituted by using an optical fiber which has a refractive index distribution structure containing one or more annular region between a central core and a clad, and in which Rayleigh scattering coefficient is <=1 μm¬4.dB/km, the specific refractive index difference with respect to the clad of the central core is a positive value of <=0.9%, the specific refractive index difference with respect to the clad of the annular region adjacent to the central core is >=-0.7% and <=-0.2% and the effective core cross-sectional area is <=60 μm¬2, and obtain an amplification effect by making the excited wavelength-multiplexed light incident on the optical fiber.
OPTICAL FIBER AND OPTICAL SIGNAL TRANSMISSION SYSTEM USING THIS OPTICAL FIBER