SATURATION OF GAIN AND CARRIER-INDUCED REFRACTIVE INDEX IN SEMICONDUCTOR LASERS.

A theory for a semiconductor active medium interacting with a laser field is described. The transitions in a semiconductor laser occur between distributions of electronic states in the conduction and valence bands. The semiconductor gain medium is modeled as an ensemble of two-level systems, each consisting of a conduction-band electronic state and a valence-band electronic state. The theory is capable of predicting from first principles the gain profile, efficiency, saturation intensity, saturated gain, and carrier-induced refractive index of semiconductor lasers. A prediction of the theory is that the gain and carrier-induced refractive index saturate differently, with the carrier-induced refractive index being a much weaker function of the laser intensity. This result has important implications for the transverse-mode structure of semiconductor lasers, particularly in the formation of filaments.