Importance of nonlinear gain in semiconductor lasers

This paper reviews the importance of nonlinear gain and its impact on the performance of semiconductor lasers. The physical mechanisms which can lead to an intensity dependence of the optical gain in the above-threshold regime are described briefly. A specific nonlinear-gain mechanism, referred to as intraband gain saturation, is discussed in detail by considering its effect on the important laser characteristics such as the modulation bandwidth, intensity noise, and the laser linewidth. Particular attention is paid to the effects of cross saturation in nearly single-mode semiconductor lasers. Even a weak side mode can lead to saturation and rebroadening of the main-mode linewidth due to mode coupling induced by the nonlinear gain.