In an optical amplifier, comprising a rare earth doped optical waveguide, in which signals at signal wavelength and pump power at pump wavelength are coupled together, a differential loss inducing means is located in a predetermined position along the length of the waveguide, causing a loss for the signal greater by a predetermined loss amount than the loss for the pump wavelength, said predetermined loss amount and said predetermined loss position being chosen in a mutual relationship to cause an amplification of lower power input signals, in said signal input power range, greater than the amplification of higher power input signals in said range, by an amount causing a substantially constant output power in a dynamic range of input signals greater than 15 dB.