Effect of nonlinear gain on single-frequency behaviour of semiconductor lasers

The effect of nonlinear gain on the single-frequency behaviour of semiconductor lasers is analysed 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 threshold and apply it to discuss the single-frequency range of distributed-feedback semiconductor lasers.