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Dual-comb spectroscopy has recently attracted significant interest due to its fast acquisition times, absolute frequency accuracy, negligible lineshape, and coherent probe light. We have recently expanded our near-infrared dual-comb spectroscopy efforts to the mid-infrared, which offers significantly improved sensitivity for many trace gas species and access to other species which cannot be measured in the Near-Infrared.
Our Mid-Infrared spectrometer is based on two erbium fiber optical frequency combs that generate light spanning from about 3 to 5 microns using a two-branch difference frequency generation (DFG) design with a periodically poled lithium niobate crystal (PPLN). The data product of the spectrometer are interferograms. Once digitized, the interferograms are corrected for residual phase noise of the frequency combs and coadded in real-time on a field-programmable gate array (FPGA). Finally, the optical spectrum is calculated through a Fourier transform of the coadded interferogram.
I will present three measurement modalities we implemented with this spectrometer. In a laboratory gas cell measurement, we characterized low-pressure gas phase propane, demonstrating excellent agreement with literature spectra obtained with high-resolution FTIR. In a separate measurement, we performed in-situ monitoring of a chemical reaction using attenuated total reflection spectroscopy. Finally, open-path measurements of atmospheric trace gases (methane, CO2, water, ethane) and volatile organic compounds (acetone, isopropanol) demonstrate the spectrometer's capability to monitor atmospheric trace gases and quantify emissions from sources like oil and gas, forest fires and industry.
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