Progress in Quantum Cascade Lasers

Recent progresses in quantum cascade laser research are reported. By a systematic measurement of otherwise identical devices, it is shown that the doping level is a key optimization parameter that both determine the maximum drive current, optical losses as well as the threshold current. An other important optimization is the one of the optical waveguide. By fabricating large optical waveguides, powers up to 14W at 15K and 5W at 280K at λ ≈ 5.2μm are demonstrated. Beam divergence could be reduced by almost a factor of two compared to conventional waveguides. Finally, a measurement of the linewidth enhancement factor α of a distributed feedback quantum cascade laser, based on an optical heterodyne techniqe, is presented. As expected, very low values of α = 0.02 ± 0.20 are experimentally demonstrated.