The present invention provides a Raman amplifier and the like comprising a structure
for keeping the flatness of power spectrum of Raman-amplified signal light. The
Raman amplifier comprises an optical fiber for Raman-amplifying a plurality of
signal channels of signal light having respective center optical frequencies different
from each other; a pumping light supply section for supplying N (N being an integer
of 2 or more) pumping channels of pumping light having respective center optical
frequencies different from each other to the optical fiber; and a feedback section
for detecting a part of the signal light Raman-amplified within the optical fiber
when the pumping light is supplied thereto, and controlling the pumping light supply
section such that the Raman-amplified signal light has a substantially flat power
spectrum with respect to an optical frequency direction according to the result
of detection. In particular, the feedback section divides the detected Raman-amplified
signal light into N optical frequency ranges defined so as to include respective
Raman amplification peaks as optical frequencies lower than respective center optical
frequencies of the pumping channels of pumping light by an optical frequency shift
of about 15 THz, and controls the pumping light supply section such that the Raman-amplified
signal light has a power fluctuation of 2 dB or less in each of thus divided N
optical frequency ranges.