Non-linear optical gain of InGaAs/InGaAsP-quantum wells

InGaAs/InGaAsP-multi-quantum-well-layers are preferably used as the active region in long-wavelength semiconductor lasers. The authors present calculations of their optical gain in dependence of the carrier density. These calculations are compared with measurements of electrically and optically excited gain spectra of such laser structures for 1.55 mu m emission wavelength with different numbers of wells. In contrast to conventional double heterostructure lasers a distinct sublinear increase of the gain g with increasing carrier densities N is observed, which is well pronounced for low well numbers and can be described approximately with a logarithmic equation g approximately ln(N) quite in accordance with theoretical predictions.