Detuning characteristics of the linewidth enhancement factor of a midinfrared quantum cascade laser

The linewidth enhancement factor of a 5μm distributed feedback (DFB) quantum cascade laser above the threshold current detuned from the gain peak was investigated. This laser oscillates in two DFB modes located on either side of the gain peak. Using the optical feedback self-mixing method, we have observed variations in the linewidth enhancement factor, deviating from zero. The signs of the two DFB modes are both negative, which was verified by comparing their waveforms with that of conventional 1.55μm laser diodes, suggesting that the gain curve is asymmetric with respect to the gain peak.