A new method for calculating the probability density function of the photon number propagating through a travelling wave optical amplifier with no restriction on its working regime (linear and nonlinear) is reported. The authors show that the widely used Gaussian approximation of the probability density function does not match the real statistics if the incident optical power is small.
Guitart, J., Ruiz-Moreno, S. Strict calculation of the light statistics at the output of a travelling wave optical amplifier. "Electronics Letters", Agost 1993, vol. 29, núm. 17, p. 1589-1591.