We present a system for molecular spectroscopy using a broadband mid-infrared laser with near-infrared detection. Difference frequency generation of a Yb:fiber femtosecond laser produced a mid-infrared (MIR) source tunable from 2100–3700 cm-1 (2.7–4.7 μm) with average power up to 40 mW. The MIR spectrum was upconverted to near-infrared wavelengths for broadband detection using a two-dimensional dispersion imaging technique. Absorption measurements were performed over bandwidths of 240 cm-1 (7.2 THz) with 0.048 cm-1 (1.4 GHz) resolution, and absolute frequency scale uncertainty was better than 0.005 cm-1 (150 MHz). The minimum detectable absorption coefficient per spectral element was determined to be 4.4×10-7 cm-1 from measurements in low pressure CH4, leading to a projected detection limit of 2 parts-per-billion of methane in pure nitrogen. In a natural atmospheric sample, the methane detection limit was found to be 30 parts-per-billion. The spectral range, resolution, and frequency accuracy of this system show promise for determination of trace concentrations in gas mixtures containing both narrow and broad overlapping spectral features, and we demonstrate this in measurements of air and solvent samples.
Johnson T, Diddams SA. 2012. Mid-infrared upconversion spectroscopy based on a Yb:fiber femtosecond laser. Applied Physics B: Lasers and Optics 107(1): 31-39.