An optical amplifier which includes an optical fiber having an erbium doped core surrounded by cladding, a pump for pumping the fiber with pump light at a pump wavelength coupled to the fiber, input means for inputting a signal to be amplified to the amplifier and output means for outputting an amplified...http://www.google.es/patents/US5491581?utm_source=gb-gplus-sharePatente US5491581 - Rare earth doped optical fibre amplifiers
