Optical amplifier, optical amplifying system, wavelength converter, and optical communication system

An optical amplifier comprising:

• a plurality of optical amplifying fibers;
• a relative phase shifter that is inserted between two or more of the optical amplifying fibers and that changes a relative phase of light input;
• and
• a pump light source that supplies phase-modulated pump light to the optical amplifying fibers, the pump light being used for parametrically amplifying signal light input to the optical amplifying fibers by using a non-linear optical effect of the optical amplifying fibers, wherein
• a fluctuation of a zero-dispersion wavelength of the optical amplifying fibers in a longitudinal direction is within a limit of 0.5 nm/100 m,
• the each of optical amplifying fibers has
• a core portion including:
• a center core part;
• an outer core layer which is formed in a surrounding of the center core part and of which a refractive index is lower than that of the center core part;
• and
• at least one buffer core layer which is formed between the center core part and the outer core layer and of which a refractive index is lower than that of the center core part and is higher than that of the outer core layer, and
• a cladding portion which is formed in a surrounding of the outer core layer and of which a refractive index is lower than that of the center core part and is higher than that of the outer core layer, and
• an effective core area at a wavelength of 1550 nm is 18 μm2 or smaller,
• the optical amplifying fibers are configured so that
• the effective core area at the wavelength 1550 nm is in a range from 10.27 μm2 to 18 μm2 inclusive,
• a relative refractive-index difference of the center core part to the cladding portion is in a range from 1.8% to 3.0% inclusive,
• a relative refractive-index difference of the outer core layer to the cladding portion is in a range from −1.2% to −0.2% inclusive,
• a relative refractive-index difference of the buffer core layer to the cladding portion is in a range from 0.1% to 0.6% inclusive,
• an outside diameter of the outer core layer is in a range from 9.4 μm to 21.4 μm inclusive,
• a ratio of a diameter of the center core part to the outside diameter of the outer core layer is in a range from 0.20 to 0.40 inclusive,
• a ratio of an outside diameter of the buffer core layer to the outside diameter of the outer core layer is in a range from 0.24 to 0.80 inclusive, and
• in a range where an absolute value of a wavelength dispersion at the wavelength 1550 nm is 5 ps/nm/km or smaller, a fluctuation of the wavelength dispersion at the wavelength 1550 nm caused by a 1% change in the outside diameter of the outer core layer is 0.7 ps/nm/km or smaller.