Patent ID: 7397599

Claim:
An amplifier for amplifying an optical signal, the amplifier comprising: a doped ring amplifying optical fiber; a multimode pump coupled to said fiber perform amplification by active rare earth ions, and and at least one single-mode pump coupled to the amplifying optical fiber to perform Raman amplification in addition to said amplification by the active rare earth ions; the doped ring amplifying optical fiber comprising: a single-mode core of given diameter; and a multimode core surrounding the single-mode core and containing a doped layer, having a predetermined concentration of active rare earth ions, the fiber being configured, because of the active rare earth ions, to amplify an optical signal for injection into the amplifying fiber; the fiber being characterized in that it is of a length and has Raman efficiency such that the product of said length multiplied by said Raman efficiency is greater than or equal to 0.5 W −1 , and less than or equal to 5.0 W −1 , and in that, for said fiber presenting absorption for an injected optical signal due to the presence of active rare earth ions, said absorption being defined by an absorption coefficient expressed in dB/m and presenting a maximum value as a function of the wavelength of said signal, which value is referred to as the absorption maximum, said fiber presents accumulated absorption, corresponding to the product of said length multiplied by said absorption maximum, of the doped layer of the multimode core which is greater than or equal to 100 dB and less than or equal to 300 dB; wherein a gain is less than or equal to 60 dB.