Double Rayleigh backscattering in long-haul transmissionsystems employing distributed and lumped fibre amplifiers

Double Rayleigh backscattering in long-haul transmission systems employing ‘lumped’ and distributed fibre amplifiers has been analysed. We find that, for singlemode fibre employing ‘lumped’ erbium-doped fibre amplifiers (EDFAs) with isolators on both sides of the EDFAs, the degradation is very small; however, for systems using ‘lumped’ praseodymium-doped fluoride fibre amplifiers or distributed EDFAs the power penalties can be large.