An optical amplifier for amplifying signals of different wavelengths throughout a spectral window modifies the amplification of each signal such that the output levels of the signals are more equal than the input levels thereof when the input levels differ by more than a predetermined amount. In one embodiment this is achieved by providing a dichroic reflector at one end of an amplifying fibre so that standing wave patterns are set up in the amplifying fibre by interference of the forward and reflected signal lights, at the different wavelengths, the signal at each wavelength preferentially decreasing its own gain with increasing signal level.