Influence of nonlinear gain and loss on the intensity noise of a multimode semiconductor laser

We developed a model for the intensity noise of a multimode semiconductor laser which includes both the nonlinear gain and the nonlinear loss. The nonlinear gain stems from spectral hole burning and population pulsation. Saturable absorption by deep trap levels results in the nonlinear loss. We find that the nonlinear gain alone does not enhance the total intensity noise above the quantum efficiency limit. The nonlinear loss, however, does increase the intensity noise when longitudinal side modes have considerable intensities, and thus explains the observed excess intensity noise. In this case the perfect anticorrelation among the longitudinal modes is degraded, which in turn converts the mode-partition noise to the total intensity noise. We remark that larger (smaller) saturable absorption in the quantum-well (transverse-junction-stripe) lasers could explain the larger (smaller) excess noise behavior observed in these lasers.