Patent ID: 7440167

Claim:
A Raman amplifier that Raman amplifies a signal light, the Raman amplifier comprising: an input unit to which the signal light is input; an output unit from which amplified signal light is output; a first optical fiber for Raman amplifying the signal light with a pumping light provided in at least a part between the input unit and the output unit; and a pumping light source that supplies a pumping light to the first optical fiber, wherein a chromatic dispersion of the first optical fiber 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 of the first optical fiber with a pumping light of 1,450 nanometers is equal to or more than 5 (W×km) −1 , a nonlinear coefficient of the first optical fiber at the wavelength of 1,550 nanometers is equal to or less than 5.0×10 −9 W −1 , a zero-dispersion wavelength of the first optical fiber is not at a wavelength of the signal light nor at a wavelength of the pumping light, and a cut-off wavelength of the first optical fiber is equal to or less than the wavelength of the pumping light, the first optical fiber includes, 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, 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.