Measurement of the cavity dispersion in quantum cascade lasers using subthreshold luminescence

The measurement of the cavity dispersion is a key ingredient in the characterization of a laser frequency comb due to its fundamental role in predicting the multimode laser dynamics. In this work, we discuss the measurement of the modal refractive index dispersion in quantum cascade lasers obtained through the analysis of their sub-threshold luminescence measured using Fourier-transform spectrometers. In particular, we demonstrate the effects of apodization and zero-padding. Finally, we use selected examples to show the effects of different cold-cavity dispersion profiles on the lasing state of the laser. Our results provide a comprehensive guide for determining the cavity dispersion of Fabry-Pérot quantum cascade lasers, which is essential for designing high-performance frequency combs.