Retrieving the phase relation of a quantum cascade laser frequency comb and reconstructing its emission profile

In the direction of miniaturizing and expanding optical frequency comb (FC) operation in the infrared (IR), the most relevant results have recently been achieved with quantum cascade lasers (QCLs) [1-4]. By using broadband Fabry-Pérot QCLs [5] designed to have a low group velocity dispersion, FC generation was demonstrated in fully free-running operation (QCL-combs) [6,7]. Various techniques to characterize the emission of mid- and far-infrared QCL-combs have been recently developed [6, 8-12].