Optical fiber and optical signal transmission system using this optical fiber

An optical fiber having a good signal-to-noise ratio (SNR) and high Raman efficiency in an optical signal transmission system using Raman amplification, the optical fiber having a refractive index profile, including at least one annular region (12) between a center core (11) and a cladding layer (13), a Rayleigh scattering coefficient not more than 1 mu m .dB/km, a relative refractive index difference of the center core with respect to the cladding layer ( DELTA 1) of a positive value not more than 0.9%, a relative refractive index difference of an annular region adjoining the center core with respect to the cladding layer ( DELTA 2) from -0.7% to -0.2%, an effective area (Aeff) Aeff not more than 60 mu m , and a zero dispersion wavelength outside of the range of 1400 to 1600nm, and an optical signal transmission system using that optical fiber for part of the optical signal transmission line in a Raman amplifier or as an optical signal amplification fiber and introducing wavelength multiplexed stimulated light into the optical fiber so as to obtain an amplification effect.