Effect of nonlinear gain on the single-frequency behavior of semiconductor lasers.

The effect of nonlinear gain on the single-frequency behavior of semiconductor lasers is analyzed using a two-mode rate-equation model. We find that the asymmetric nature of the nonlinear gain is responsible for bringing the sidemode above threshold when the power in the main mode exceeds a critical level. We obtain an analytic expression for the critical current density at which the sidemode reaches above threshold and apply it to discuss the single-frequency range of distributed feedback semiconductor lasers.