Impact of single and double Rayleigh backscattering on available system gain in optically amplified fiber systems without isolators

Rayleigh backscattering in optically amplified fiber systems without optical isolators is discussed, and its impact on these systems is quantified in terms of a receiver power penalty. We show that amplifiers with large gain should be avoided in these system. On the other hand, using moderate gain amplifiers, large system gains can be obtained with little power penalties.