Quantum cascade laser based hybrid dual comb spectrometer

Abstract

Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a powerful photonic tool, driving a recent revolution in major molecular fingerprint regions, i.e. mid-and far-infrared domains. Their compact and frequency-agile design, together with their high optical power and spectral purity, promise to deliver an all-in-one source for the most challenging spectroscopic applications. Here, we demonstrate a metrological-grade hybrid dual comb spectrometer, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optically-rectified THz frequency comb. A proof-of-principle application to methanol molecular transitions is presented. The multi-heterodyne molecular spectra retrieved provide state-of-the-art results in line-center determination, achieving the same precision as currently available molecular databases. The devised setup provides a solid platform for a new generation of THz spectrometers, paving the way to more refined and sophisticated systems exploiting full phase control of QCL-FCs, or Doppler-free spectroscopic schemes.

Tunable, Grating‐Gated, Graphene‐On‐Polyimide Terahertz Modulators

“…The tuning coefficients of the three beatnotes in the 515-558 mA and 558-600 mA ranges are dissimilar, indicating that, while in the first portion of the dynamic range the three active regions embedded in the QCL core are individually behaving like frequency combs, in the second current range, higher order lateral modes can interfere with the main QCL modes, preventing comb operation. [68] In all cases (Figure 5d-f) the beatnote turns again single and narrow (7.6 kHz, Figure 6b) at larger driving currents in the range 760-820 mA, when coupling the QCL with SLG modulators with p = 5, 10 µm, and over a slightly narrower range (780-820 mA) when p = 22 µm. In this new operating regime, not observed either in the bare-QCL frequency comb, [54] or while coupling the same QCL with an Au mirror, [58] we retrieve a stable, high-intensity (30 dBm), narrow (5-7 kHz) single beatnote, with an emission spectrum covering a 1.2 THz bandwidth (Figure 5c), with 98 equidistant optical modes.…”

Homogeneous quantum cascade lasers operating as terahertz frequency combs over their entire operational regime

“…We furthermore prove the RF injection locking capability of the devised laser bars in a regime conventionally characterized by electric field instabilities. The achieved results provide a concrete route for the development of miniaturized homogeneous chipscale FC spectroscopic setups for addressing metrologicalgrade applications in the far-infrared, addressing absorption line strengths comparable or even stronger than fundamental, mid-infrared vibration transitions, but with much narrower Doppler-limited LWs, ruled by inverse linear relationship with the wavelength [3]. A B Figure 5: (A) Injection locking map for an injected radio frequency (RF) power of +25 dBm.…”

Toward new frontiers for terahertz quantum cascade laser frequency combs

“…After the first attempts of a proof-of-principle acquisition of an etalon signal simulating a molecular absorption [45], or of a low resolution spectrum of ammonia gas, dual-comb THz setups have been developed for pioneering hyperspectral imaging [46], self-detection schemes [47] and time resolved spectroscopy on molecular gas mixtures [48]. At the same time, a hybrid dual-comb spectrometer based on a OR-and a QCL-FC has been recently demonstrated [49], merging the sensitivity allowed by a 4 mW [23] THz QCL, with the accuracy of the absolute frequency scale, provided by the referenced OR-FC. Such a technique leads to a state-of-the-art accuracy, of the order of 10 −8 , in the determination of molecular transition line centers.…”