A semiclassical model for noise propagation in depleted-pump optical amplifiers

Erbium-doped and parametric fiber-optic amplifiers can be modeled as a cascade of "amplifying" beamsplitters, each representing a short fiber section in which the pump is approximately constant. Each beamsplitter obeys simple noise combination relations and its gain is calculated from the standard rate equations. The proposed model allows for the management of pump depletion in a very simple way by numerical analysis, avoiding direct quantum mechanical calculations. Sample noise propagation diagrams are reported for technically significant values of the parameters. For the four-wave-mixing amplifier, it has been found that the 3-dB noise limit can be reached even for a strong gain compression.