Spectral Behavior of Linewidth Enhancement Factor of a Mid-Infrared Quantum Cascade Laser

We report the experimental linewidth enhancement factor (α-factor) results on mid-infrared InGaAs/InAlAs quantum cascade lasers (QCLs) obtained using the self-mixing method. The waveforms appearing under self-mixing conditions were different from those originally predicted, which assumed that the gain was symmetric around the gain peak and that α=0. The signs of the QCL α-factors were identified to be negative and asymmetric to the gain peak by using the nonsymmetrical mirror drive, and by comparing their wave patterns to those of conventional 1.55-µm laser diodes. The nonparabolicity of the subbands was a possible cause of the α-factor's asymmetry. The subbands are no longer parallel if the lasing energy is restricted by the nonparabolicity, which produces inhomogeneous broadening and an asymmetric optical gain spectrum with a long wavelength tail. Taking the nonparabolicity into account, we determined the QCL gain spectrum and the dependence of the α-factor on the spectral position relative to the gain peak. The dependence of the α-factor on detuning was in reasonable agreement with the calculation that accounts for the asymmetry in gain.