Patent ID: 7796325

Claim:
A distributed Raman amplifier comprising: a pumping light generating section that generates a pumping light for distributed Raman amplifying a WDM signal light propagated through an optical transmission path; a pumping light supply section that supplies the pumping light output from said pumping light generating section in a direction opposite to a propagation direction of the WDM signal light; and a gain equalizer which receives the WDM signal light propagated through said optical transmission path to be subjected to the distributed Raman amplification, and has fixed transmission wavelength characteristics capable of flattening gain wavelength characteristics of the distributed Raman amplification, said distributed Raman amplifier further comprising; a reference light source which generates a reference light having a wavelength set within a gain band of the distributed Raman amplification but longer or shorter than a wavelength of the WDM signal light; a multiplexer which multiplexes the reference light output from said reference light source with the WDM signal light to supply the multiplexed light to one end of said optical transmission path; a monitor section that monitors the powers of the WDM light and the reference light, which are output from the other end of said optical transmission path; and a control section that obtains a Raman gain in the WDM signal light and a Raman gain in the reference light based on the monitor result in said monitor section, to control said pumping light generating section so that a difference between the Raman gains is maintained at constant, wherein the wavelength of the reference light is set in a wavelength region in which, when an average gain of the distributed Raman amplification is changed, a fluctuation amount of the Raman gain in the wavelength of the reference light becomes equal to or lower than ⅕ of a fluctuation amount of the Raman gain in the wavelength band of the WDM signal light.