We utilize mid-infrared dual frequency comb spectroscopy for the detection of methane in ambient air. Two mid-infrared frequency comb sources based on femtosecond Er:fiber oscillators are produced through difference frequency generation with periodically poled MgO-doped lithium niobate crystals and stabilized at slightly different repetition rates at about 250 MHz. We performed dual frequency comb spectroscopy in the spectral range between 2900 cm −1 and 3150 cm −1 with 0.07 cm −1 resolution using a multipass cell of ~580 m path length, and achieved the sensitivity about 7.6 × 10 −7 cm −1 with 80 ms data acquisition time. We determined the methane concentration as ~1.5 ppmv in the ambient air of the laboratory, and the detection limit as ~60 ppbv for the current setup.
We employed a midinfrared frequency comb source for methane detection in ambient air. The transmitted spectra over a bandwidth of about 500 nm were recorded with an optical spectrum analyzer under various experimental conditions of different path lengths. The normalized absorption spectra were compared and fitted with simulations, yielding quantitative values of concentrations of methane and water vapor in the ambient air. The 3σ detection limit was ∼6.6×10 cm in ambient air for a broad spectral range, achieved with a path length of ∼590 m. This approach provides a broad spectral range, a large dynamic range, high sensitivity, and accurate calibration. The performed analysis of the residuals shows that an excellent agreement between the measured and calculated spectral profiles was obtained.
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