Noise in optically amplified links with a new vacuum-field model

Recently, considerable interest has been attracted by the evaluation of performances of optically-amplified fiber links, in which noise and saturation of the optical amplifier (OA) as well as fiber non-linearities interact to determine the evolution of the S/N ratio along the line. In this work we also include the vacuum field noise, added by the lossy fiber, and take into account the Parametric Gain (PG) that the signal generates on it. To calculate the evolution of the noise spectra along an optically amplified line, we describe the OA by means of a recent semiclassical noise-model: the amplifier is modeled as a four port device, with an idler port opened to the vacuum field; the fiber is similarly modeled as a beamsplitter with transmittance equal to its attenuation. The saturation of the OAs is taken into account by a simple phenomenological description