Semiclassical particle-like description of optical amplifier noise

A semiclassical model of optical amplifier noise in terms of photon-particle description and simple calculations is developed. The active-medium amplifier is modelled as a stochastic photon multiplier, whose statistical properties are derived from well-known results about branching processes. The effects on amplifier output noise caused by random amplification of input photons and by spontaneous emission are treated separately, and it is thus possible to ascribe output noise terms to specific physical mechanisms. In view of this, the 3 dB noise figure limit of optical amplifiers can be entirely ascribed to optical gain randomness. Since the results obtained here are coincident with those from quantum theories, it is concluded that the randomness of optical gain in a particle-like description is the correct semiclassical counterpart of vacuum-field amplification in wave-like formulations.