The linewidth enhancement factor is a fundamental parameter that characterizes the limit of spectral purity and the tendency for filamentation in a semiconductor laser. For either narrow-line or high-power operation, it is generally desirable that the linewidth enhancement factor be minimized. This parameter depends primarily on the shape of the differential gain spectrum of the active region material. In mid-infrared laser materials, band structure engineering has been used routinely to minimize the effects of nonradiative recombination. This same approach can be used to tailor the differential gain spectrum of these materials to minimize the linewidth enhancement factor.
Favorable features of the electronic structure of the material include 1) band-edge dispersion in the conduction band which shifts the peak of the gain away from the band edgeCand 2) intersubband absorption resonances, which can be designed to lie above the peak gain in energy. Both of these strategies shift the peak of the differential gain closer to the peak gain, and thus reduce the linewidth enhancement factor. These electronic structure design strategies can be used to reduce the linewidth enhancement factor in type-II InAs/GaInSb systems to almost 1.0, which is significantly smaller than typical values of 2.5–5 in type-I strained quantum wells. We note that a linewidth enhancement factor close to 1 is much smaller even than a typical linewidth enhancement factor in visible or near-infrared semiconductor lasers. Furthermore in some materials the peak of the differential gain lies in a region of positive gain, indicating that a grating could be used to select a mode with negligible linewidth enhancement factor.