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Retrieval of phase relation and emission profile of quantum cascade laser frequency combs
preprintposted on 2023-11-30, 18:21 authored by Francesco Cappelli, Luigi Consolino, Giulio Campo, Iacopo Galli, Davide Mazzotti, Annamaria Campa, Mario Siciliani de Cumis, Pablo Cancio Pastor, Roberto Eramo, Markus Rösch, Mattias Beck, Giacomo Scalari, Jérôme Faist, Paolo De Natale, Saverio Bartalini
The major development recently undergone by quantum cascade lasers has effectively extended frequency comb emission to longer-wavelength spectral regions, i.e. the mid and far infrared. Unlike classical pulsed frequency combs, their mode-locking mechanism relies on four-wave mixing nonlinear processes, with a temporal intensity profile different from conventional short-pulses trains. Measuring the absolute phase pattern of the modes in these combs enables a thorough characterization of the onset of mode-locking in absence of short-pulses emission, as well as of the coherence properties. Here, by combining dual-comb multi-heterodyne detection with Fourier-transform analysis, we show how to simultaneously acquire and monitor over a wide range of timescales the phase pattern of a generic frequency comb. The technique is applied to characterize a mid-infrared and a terahertz quantum cascade laser frequency comb, conclusively proving the high degree of coherence and the remarkable long-term stability of these sources. Moreover, the technique allows also the reconstruction of electric field, intensity profile and instantaneous frequency of the emission.