Spectral linewidth broadening in erbium-doped and Raman fibre amplifiers

A systematic experimental investigation of signal spectral broadening in erbium doped amplifiers and the first such measurement for Raman fibre amplifiers are reported. In both cases the linewidth is found to be broadened significantly in the region above the noise floor, the amount of broadening being determined by the amplifier operating conditions. These results have implications for coherent transmission systems using cascades of fibre amplifiers.