Method of operating concatenated optical amplifiers

In a transmission system having concatenated optical amplifiers (OA), gain saturation of the optical amplifiers at any stage is caused by the extraction of power from the amplifier by the ASE (amplified spontaneous emission) and amplification of the signal and of the ASE from previous stages. Three methods of operating transmission systems are classified as free-running, constant-total power and constant-signal power. Differences in performance of the three systems arise from the propagation of signal power and the buildup of the ASE. It is now disclosed that the buildup of ASE noise in a system of either lumped or distributed concatenated optical amplifiers can be limited by operating at least a plurality of the optical amplifiers to provide unsaturated gain which is greater than the loss of the system and/or each optical amplifier is operated in its saturated state.