[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Retrieving the Phase Relation of a Quantum Cascade Laser Frequency Comb and Reconstructing its Emission Profile

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.