Hollow Waveguide-Enhanced Mid-Infrared Sensor for Real-Time Exhaled Methane Detection

A hollow waveguide (HWG)-enhanced tunable laser absorption spectroscopy sensor was demonstrated for the detection of exhaled methane in the mid-infrared range. A 3392-nm interband cascade laser was used to simultaneously target two neighboring strong absorption peaks and wavelength modulation spectroscopy (WMS) with the second harmonic signal normalized by the first-harmonic signal (WMS-$2f/1f$) was employed for data processing. With a time resolution of 0.1 s and a detection limit of 48 ppb, the evolution of exhaled methane concentration in a single breath was observed. In addition, the HWG-based optical path is mirror-less, flexible, and light, which makes the proposed sensor suited for clinical instrumentation, point-of-care breath monitoring, and disease diagnostics.