Optical fiber for Raman amplification, optical fiber coil, Raman amplifier, and optical communication system

An optical fiber for Raman amplifying a signal light with a pumping light, the optical fiber comprising:

• a glass part including,
• a first core,
• a second core surrounding the first core, and
• a cladding surrounding the second core;
• and
• a coating surrounding the glass part,
• wherein a chromatic dispersion at a wavelength of 1,550 nanometers is in a range between −70 ps/nm/km and −30 ps/nm/km,
• a Raman gain efficiency with a pumping light of 1,450 nanometers is equal to or more than 5 (W×km)−1,
• a nonlinear coefficient at the wavelength of 1,550 nanometers is equal to or less than 5.0×10−9 W−1,
• a zero-dispersion wavelength is not at a wavelength of the signal light nor at a wavelength of the pumping light,
• a cut-off wavelength is equal to or less than the wavelength of the pumping light,
• the first core has a relative refractive index difference to the cladding of Δ1 and the second core has a relative refractive index difference to the cladding of Δ2 where Δ1 is 2.2 to 2.8%, and Δ2 is −0.55 to −0.23%, and
• a ratio of a diameter of the first core to a diameter of the second core is 0.33 or more and 0.48 or less.