Patent ID: 7038845

Claim:
An optical amplifier including: an amplifier medium allowing a propagation of a signal light which is subject to an amplification, said amplifier medium being doped with at least one kind of rare-earth ions, and each of said at least one kind of rare-earth ions having an energy level system which includes: a ground level; a first pair of a laser upper level and a laser lower level which is higher than said ground level and lower than said laser upper level; and a second pair of an excited state absorption upper level, and an excited state absorption lower level which is lower than said excited state absorption upper level, wherein said second pair allows absorbing an emission light generated by a transition from said laser upper level to said laser lower level, and wherein said excited state absorption lower level is higher than said ground level and lower than said laser upper level, and wherein said excited state absorption lower level is different from said laser lower level; a first excitation generator for causing a first type excitation of said at least one kind of rare-earth ions to cause a population inversion between said laser upper level and said laser lower level; and a second excitation generator for causing a second type excitation of said at least one kind of rare-earth ions from said excited state absorption lower level to a high excited level which is equal to or higher than said laser upper level.