Patent ID: 7095553

Claim:
An optical amplifier, comprising: a rare earth doped fiber amplifying section to be disposed in the case where a number of operating channels exceeds a number of allowable channels A; a Raman amplifying section being capable of amplifying signal light of an entire band being composed of an amplifying band of the rare earth doped fiber amplifying section and a band, which can have a number of allowable channels A, located out of said number of allowable channels A, located out of said amplifying band; and a control section; controlling wavelength of pumping light of said Raman amplifying section such that the signal light of said entire band is amplified by said Raman amplifying section, without amplification of said amplifying band being provided by the rare earth doped fiber amplifying section, in the case where the number of operating channels is equal to that of allowable channels A or less, and controlling the wavelength of pumping light of said Raman amplifying section such that the signal light of the band located out of said amplifying band, but not the entire band, is amplified by said Raman amplifying section, and amplification of said amplifying band is provided by the rare earth doped fiber amplifier, in the case where the number of operting channels becomes greater than the number of allowable channels A.