Double Rayleigh backscattering in long-haul transmission systems 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