Real-time trace gas sensing of fluorocarbons using a swept-wavelength external cavity quantum cascade laser

Abstract

We present results demonstrating real-time sensing of four different fluorocarbons at low part-per billion (ppb) concentrations using an external cavity quantum cascade laser (ECQCL) designed for infrared vibrational spectroscopy of molecules with broad absorption features. The ECQCL was repeatedly swept at 20 Hz over its full tuning range of 1145–1265 cm⁻¹ providing a scan rate of 3535 cm⁻¹ s⁻¹, and a detailed characterization of the ECQCL scan stability and repeatability is presented. The ECQCL was combined with a 100 meter path length multi-pass cell for direct absorption spectroscopy. A portable sensor system is described, which was deployed on a mobile automotive platform to provide spatially-resolved detection of fluorocarbons in outdoor experiments. Noise-equivalent detection limits of 800–1000 parts-per-trillion (ppt) are demonstrated for 1 s integration times.