摘要： 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-P′erot 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]. Here we demonstrate the possibility of monitoring the Fourier phases of the modes of a FC (sample FC). The proposed approach bases on dual-comb multi-heterodyne detection using a metrological FC as reference (LO-FC) and a subsequent Fourier transform analysis (see ?g. 1a), allowing a simultaneous retrieval of the modes phases (?g. 1b). The required stability is obtained by post-detection RF common frequency noise suppression. This approach is the most direct method for FCs characterization, being suitable for IR FCs. A mid-IR and a THz QCL-comb (sample FCs) have been characterized. As LO-FCs two difference-frequency-generated FCs have been used. The measurements consisted in series of seven consecutive acquisitions. A strong correlation among the modes and a remarkable phase stability over 10s of minutes have been evidenced (see ?g. 1c). For the THz QCL-comb all the emitted modes can be acquired, therefore it is possible to reconstruct the emission pro?le and the instantaneous frequency (?g. 1d and e), con?rming a substantial deviation with respect to typical FC pulsed operation. The observed standard deviations on the obtained phases are in the order of 8 ? 10° at tens-of-minutes timescale. Concerning the phase relation, the scattering of THz QCL-comb phases (Fourier phases) encompasses a range of 300°, a value which cannot be associated to short-pulses operation. The quadratic term found in the phase relation (?g. 1b) can be attributed to chirping in QCL-comb emission. The reconstructed emission pro?les are roughly the same from measurement to measurement over the 30 minutes. Noticeably, the QCL-comb emission deviates from a pulsed emission, even though its amplitude is deeply modulated during the round trip. The obtained instantaneous frequency con?rms that the THz QCL-comb operates in a hybrid amplitude-/frequency-modulated regime. The gained knowledge of the phase relation provides the basis for future implementation of programmable pulse shaping. The remarkable phase stability attained for the QCL-combs modes conclusively proves the high coherence characterizing their emission, demonstrating that these sources are potentially suitable for metrological applications.