Patent ID: 7068421

Claim:
A Raman amplifier, comprising: an optical fiber that propagates and amplifies a second signal light that is a wavelength-multiplexed signal of a first signal light and a reference light, wherein the first signal light includes a plurality of wavelengths and the reference light is out of a wavelength range of amplification; an excitation light source that outputs an excitation light for amplifying the second signal light; a beam splitter that splits a portion of the second signal light into the first signal light and the reference light; a signal light level detecting unit that detects a level of the first signal light; a reference light level detecting unit that detects a level of the reference light; and a signal level setting unit that, based on the level of the reference light, calculates a target value to control signal light level for constantly maintaining a Raman gain, and controls the output level of the excitation light in such a way that the first signal level matches with the target value.