In a Raman optical amplification system in which WDM optical signals comprising
two or more signal bands are transmitted in a transmission line, and in which a
plurality of pumping lightwaves are introduced into said transmission line so that
WDM optical signals in the shortest signal band are Raman-amplified by said plurality
of pumping lightwaves, the interval between the longest pumping wavelength and
the next-longest pumping wavelength differs by 0.2 to 1.4 THz in frequency. This
makes it possible to flatten the level of the WDM signals by means of canceling
the Raman gain ripple on the longer wavelength region of the distributed amplification.
