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1. Broadband 1-GHz mid-infrared frequency comb

   https://doi.org/10.1038/s41377-022-00947-w  

   Hoghooghi, Nazanin; Xing, Sida; Chang, Peter; Lesko, Daniel; Lind, Alexander; Rieker, Greg; Diddams, Scott (December 2022, Light: Science & Applications)

   Abstract Mid-infrared (MIR) spectrometers are invaluable tools for molecular fingerprinting and hyper-spectral imaging. Among the available spectroscopic approaches, GHz MIR dual-comb absorption spectrometers have the potential to simultaneously combine the high-speed, high spectral resolution, and broad optical bandwidth needed to accurately study complex, transient events in chemistry, combustion, and microscopy. However, such a spectrometer has not yet been demonstrated due to the lack of GHz MIR frequency combs with broad and full spectral coverage. Here, we introduce the first broadband MIR frequency comb laser platform at 1 GHz repetition rate that achieves spectral coverage from 3 to 13 µm. This frequency comb is based on a commercially available 1.56 µm mode-locked laser, robust all-fiber Er amplifiers and intra-pulse difference frequency generation (IP-DFG) of few-cycle pulses in χ (2) nonlinear crystals. When used in a dual comb spectroscopy (DCS) configuration, this source will simultaneously enable measurements with μs time resolution, 1 GHz (0.03 cm −1 ) spectral point spacing and a full bandwidth of >5 THz (>166 cm −1 ) anywhere within the MIR atmospheric windows. This represents a unique spectroscopic resource for characterizing fast and non-repetitive events that are currently inaccessible with other sources. 

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2. 1-GHz mid-infrared frequency comb spanning 3 to 13 μm

   https://doi.org/10.48550/arXiv.2201.07134  

   Hoghooghi, N.; Xing, S.; Chang, P.; Lesko, D.; Lind, A.; Rieker, G.; Diddams, S (July 2022, ArXivorg)

   Mid-infrared (MIR) spectrometers are invaluable tools for molecular fingerprinting and hyper-spectral imaging. Among the available spectroscopic approaches, GHz MIR dual-comb absorption spectrometers have the potential to simultaneously combine the high-speed, high spectral resolution, and broad optical bandwidth needed to accurately study complex, transient events in chemistry, combustion, and microscopy. However, such a spectrometer has not yet been demonstrated due to the lack of GHz MIR frequency combs with broad and full spectral coverage. Here, we introduce the first broadband MIR frequency comb laser platform at 1 GHz repetition rate that achieves spectral coverage from 3 to 13 {\mu}m. This frequency comb is based on a commercially available 1.56 {\mu}m mode-locked laser, robust all-fiber Er amplifiers and intra-pulse difference frequency generation (IP-DFG) of few-cycle pulses in \c{hi}(2) nonlinear crystals. When used in a dual comb spectroscopy (DCS) configuration, this source will simultaneously enable measurements with {\mu}s time resolution, 1 GHz (0.03 cm-1) spectral point spacing and a full bandwidth of >5 THz (>166 cm-1) anywhere within the MIR atmospheric windows. This represents a unique spectroscopic resource for characterizing fast and non-repetitive events that are currently inaccessible with other sources. 

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3. Dual-comb mid-infrared spectromicroscopy with photothermal fluorescence detection

   https://doi.org/10.1364/OE.557991  

   Turner, Gwendylan A; Herman, Daniel I; Razumtcev, Aleksandr; Mangold, Markus; Li, Minghe; Cao, Ziyi; Rong, Jiayue; Bouman, Charles A; Buzzard, Gregery; Simpson, Garth J (January 2025, Optics Express)

   An approach is described for spectrally parallel hyperspectral mid-infrared imaging with spatial resolution dictated by fluorescence imaging. Quantum cascade laser (QCL)-based dual-comb mid-infrared spectroscopy enables the acquisition of infrared spectra at high speed (<1 millisecond) through the generation of optical beat patterns and radio-frequency detection. The high-speed nature of the spectral acquisition is shown to support spectral mapping in microscopy measurements. Direct detection of the transmitted infrared beam yields high signal-to-noise spectral information, but long infrared wavelengths impose low diffraction-limited spatial resolution. The use of fluorescence detected photothermal infrared (F-PTIR) imaging provides high spatial resolution tied directly to the integrated IR absorption. Computational imaging using a multi-agent consensus equilibrium (MACE) approach combines the high spatial resolution of F-PTIR and the high spectral information of dual-comb infrared transmission in a single optimized equilibrium hyperspectral data cube. 

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4. Ultrafast-laser-absorption-spectroscopy measurements of gas temperature in multi-phase, high-pressure combustion gases

   https://doi.org/10.2514/6.2021-0719  

   Tancin, Ryan J.; Ruesch, Morgan; Son, Steven F.; Lucht, Robert P.; Goldenstein, Christopher S. (January 2021, AIAA SciTech 2021 Forum)

   This manuscript describes the first application of ultrafast-laser-absorption spectroscopy (ULAS) to characterizing high-pressure (up to 40 bar), multi-phase combustion gases. Single-shot measurements of temperature and CO were acquired at 5 kHz in AP-HTPB propellant flames with and without aluminum. An ultrafast light source was used to produce broadband pulses of light near 4.96 𝜇m at a repetition rate of 5 kHz and a high-speed mid-infrared imaging spectrometer was used to image the pulses across an 86 nm bandwidth with a spectral resolution of 0.7 nm. Measurements of temperature and CO concentration were obtained by least-squares fitting simulated absorbance spectra of CO to measured spectra. A system of corrective optics was used to diminish the e˙ect of beam steering during high-pressure experiments, greatly increasing the pressure capabilities of the diagnostic. The diagnostic was used to characterize AP-HTPB propellant flames in an argon bath gas at pressures of 1, 10, 20, and 40 bar. An aluminized AP-HTPB propellant was also characterized at 10 and 20 bar to demonstrate that ULAS can provide high-fidelity measurements in particulate-laden flames. The results demonstrate that ULAS is capable of providing single-shot temperature and species measurements at high pressures with 1-𝜎 precisions less than 1.1% and 3% for temperature and species respectively, despite non-absorbing transmission losses in excess of 90%. 

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5. Mid-infrared quantum cascade laser-based structured illumination microscope

   https://doi.org/10.1364/OL.583804  

   Le, M; Myers, O; Squier, J; Phal, Y (January 2026, Optics Letters)

   We demonstrate, for the first time to the best of our knowledge, the implementation of Spatial Frequency Modulated Imaging (SPIFI) microscopy in the mid-infrared region using a Quantum Cascade Laser (QCL) source. Hyperspectral imaging in the mid-infrared (3–20 μm) enables chemically specific mapping and structural imaging based on molecular contrast, which is otherwise inaccessible in the visible or near-infrared region. But mid-infrared microscopes are often limited by slow raster scanning and costly detector arrays. Our approach uses spatial frequency-encoded structured illumination and a low-cost single-pixel mid-infrared detector to acquire hyperspectral images in transflection mode. The system—structured illumination single-pixel imaging system in the mid-infrared—provides a practical trade-off between speed and complexity, offering acquisition speeds comparable to selected QCL-based point-scanning implementations, while maintaining a simple, cost-effective optical design utilizing off-the-shelf components. We present a preliminary resolution measurement as well as absorption images of coronal sections of a mouse brain at both 6 and 7 μm, demonstrating chemically selective contrast and robust image formation suitable for future high-resolution and compressive implementations. 

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