Hot-cavity linewidth enhancement factor of a quantum cascade laser

The linewidth enhancement factor (LEF) of quantum cascade lasers (QCLs) is an important parameter, recently tightly linked to many phenomena that occur in this type of laser — from self-starting frequency combs to the emergence of solitons. The dynamic processes involved act at frequencies similar to the roundtrip frequency of the lasers (typically GHz), reflected in the high-frequency component of the LEF. Its value in QCLs is predicted to increase under laser operation with increasing light intensity, as the stronger gain saturation effectively increases the spectral gain asymmetry. Here, we investigate the hot-cavity LEF of a free-running frequency comb far above the laser threshold and at high frequencies, employing shifted wave interference Fourier transform spectroscopy (SWIFTS). Our measurements confirm an increasing LEF with laser current, which is supported by numerical simulations. From the spectral slope of the LEF we can extract further important parameters, such as the gain peak frequency and the decoherence time of the laser transition, which is tightly linked to the available gain bandwidth. • Intensity dependence of the LEF of a QCL frequency comb — experiment and simulation. • Investigation of LEF at high frequencies (GHz) and wide range of laser operation. • High-frequency LEF relevant for dynamic processes — frequency comb/soliton formation. • Findings strongly support claim that Bloch gain is a dominant mechanism in QCLs. • Spectrally resolved LEF allows to calculate decoherence time and gain peak position.