We show how temporally and spatially resolved lasing spectra may be interpreted to obtain quantitative information on the lattice temperature and refractive index distribution in an optically pumped GaInAsSb/AlGaAsSb quantum well 1.8 &mgr;m midwave-infrared laser. We present this technique as a method for evaluating and comparing high power laser structures in terms of optical and thermal efficiency. We also suggest how this method might provide insight into the particulars of filamentation and heat transport in both electrically and optically pumped structures. ©1997 American Institute of Physics.