Epitaxial structure dependence of gain, refractive index, and linewidth enhancement factor in gallium arsenide and indium gallium arsenide broad-area quantum well lasers

Semiconductor lasers have taken a prominent position in the present technological areas of optoelectronics, optical communications, spectroscopy, laser pumping, and numerous other fields. Filamentation tendencies and spatial coherence of high-power semiconductor lasers are strongly influenced by the linewidth enhancement factor or alpha parameter. The alpha parameter is also a critical factor controlling the linewidth and modulation characteristics of high frequency diode lasers. This investigation centers on the epitaxial dependence of the alpha parameter with the goal of finding ways to reduce it and thus increase the output powers of broad-area devices and improve their beam quality.; In this study, experimental and theoretical results for gain, refractive index change, and linewidth enhancement factor in four broad-area quantum well lasers are reported, analyzed, and discussed. Two of the laser structures have GaAs quantum well layers that vary in width, and the other two have InGaAs quantum well layers that vary in quantum well depth. Experimental data from the Hakki-Paoli method are used to compare gain, index change, and alpha parameter between these pairs of devices.; Also, the results of two computer models for the structures are compared to the experimental data and to each other. The first is based on the approximation of parabolic bands for both the conduction and valence bands, and the second employs the k·p method to calculate the valence bands. The findings include the following: (1) narrower and deeper quantum wells yield lower alpha values; (2) modeling results from the k·p method agree somewhat better, but not greatly, with the experimental data than the results from the parabolic band model; (3) stimulated emission below threshold has been found to be prominent in these devices; (4) carriers in the barrier energy states above the well are shown to be responsible for high alpha values. Finally, a description is presented for a method to reduce alpha to nearly zero by using an absorbing quantum well coupled with an emitting one in the same epitaxy of a semiconductor laser device.