A Raman amplifier for an optical transmission system. A light source unit generates
pumping light to amplify wavelength-multiplexed signal light having a plurality
of wavelengths including a wavelength band of the signal light. A transmission
path propagates the signal light and the pumping light and amplifies the signal
light. A pumping light supplying unit supplies the pumping light to the transmission
path. A filter narrows a spread of a spectrum of the pumping light within the wavelength
band of the signal light and outputs it to the pumping light supplying unit. The
present invention increases transmission capacity while suppressing degradation
of the transmission characteristic due to crosstalk.