Abstract
The road towards the realization of quantum cascade laser (QCL) frequency combs [1,2] has undoubtedly attracted ubiquitous attention from the scientific community. These devices promise to deliver an all-in-one (i.e. a single, miniature, active device) frequency comb synthesizer in a range as wide as the QCL spectral coverage itself (from about 4 microns to the THz range), with the unique possibility to tailor their spectral emission by band structure engineering. For these reasons, vigorous efforts have been spent to characterize the emission of four-wave-mixing (FWM) multi-frequency QCLs, aiming to seize their comb-like mode-locked operation [3–6].
Highlights
Miniaturized frequency comb sources across hard-to-access spectral regions, i.e. mid- and far-infrared, have long been sought
While QCLbased frequency comb synthesizers (FCs) have been already used in a variety of sensing setups, mostly related to dual-comb spectroscopy[17,18], their relevance as metrological-grade, phase-stabilized sources has not been proven, yet
The OR-comb and the QCL-comb beams are overlapped by means of a wire grid polarizer (WGP) and generate the dual-comb multi-heterodyne signal on a fast non-linear detector, i.e. a hot electron bolometer (HEB—Scontel RS0.3-3T1)
Summary
Miniaturized frequency comb sources across hard-to-access spectral regions, i.e. mid- and far-infrared, have long been sought. Providing a phase-coherent link between the optical and the microwave/radio-frequency regions[2], FCs have become groundbreaking tools for precision measurements[3,4] Despite these inherent advantages, developing miniaturized comb sources across the whole infrared (IR), with an independent and simultaneous control of the two comb degrees of freedom at a metrological level, has not been possible, so far. Here applied to a far-IR emitter and open ended to other spectral windows, enables Hzlevel narrowing of the individual comb modes, and metrologicalgrade tuning of their individual frequencies, which are simultaneously measured with an accuracy of 2 × 10−12, limited by the frequency reference used These fully-controlled, frequencyscalable comb emitters will allow an increasing number of midand far-IR applications, including quantum technologies, due to the quantum nature of the gain media[21]
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