Evidence of linear chirp in mid-infrared quantum cascade lasers

Since the first demonstration of the quantum cascade laser (QCL) frequency comb in 2012, there have been open questions concerning the reproducibility of comb states, and, critically for metrological applications, the phase stability. To address these important issues, we measure the intermodal phase relation of a QCL comb operating at 8 mu m using a coherent beatnote spectroscopy. We find these intermodal phase differences to be constant within experimental uncertainty over several repeat measurements, and the comb states reproducible to an average of 21 mrad after cycling the power of the device. These phases describe a comb state exhibiting a simple, linear chirp, which in fact corresponds to the lowest state of chirp to minimize the amplitude modulation, as required for combs driven by four-wave mixing in a gain medium with a short gain recovery lifetime. All together, these findings could pave the way for pulse shaping in the QCL platform. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement