Quantum fluctuations in multimode semiconductor lasers

We derive a quantum macroscopic model for a multimode semiconductor laser, starting from a complete microscopic description. Nonlinear gain terms describing saturation and mode-mode interaction are obtained by a suitable treatment of the spectral hole burning in the distribution of carriers. In the case in which there are two neighbouring active longitudinal modes above threshold and for quiet pumping, we calculate the spectrum of the total intensity fluctuations at zero frequency. For moderate pumping level the nonlinear gain terms do not reduce the degree of anticorrelation between modes, leaving the squeezing in total intensity unchanged with respect to the single-mode case. Only for very high pumping does the nonlinear gain suppression become relevant, and the total intensity noise is increased.