Near-Infrared Off-Axis Integrated Cavity Output Spectroscopic Gas Sensor for Real-Time, In Situ Atmospheric Methane Monitoring

A near-infrared methane (CH ₄ ) sensor system was demonstrated by using off-axis integrated cavity output spectroscopy (OA-ICOS), which was an effective gas-phase analytical technique and was adopted for fast, real-time and in-situ ambient atmospheric CH ₄ monitoring. A 60 cm-long cavity with an effective absorption path length of 2150 m was fabricated with a fast response time of ~ 0.8 s. An Allan deviation analysis yielded a minimum detection limit of 13 parts-per-billion in volume (ppbv) using wavelength modulation spectroscopy (WMS) technique, which corresponds to a noise equivalent absorption sensitivity (NEAS) of 9.7 × 10 ^-10 cm ^-1 ·Hz ^-1/2 . By applying multiple denoising scheme of wavelet transform (WT), Kalman filtering (KF) and empirical mode decomposition (EMD) method, the precision levels of CH ₄ measurements were improved by a factor of 4, 5 and 6, respectively. For field application, dual-gas sensing was implemented for simultaneous measurement of CH ₄ and water vapor (H ₂ O) in a single sensor system. Also, a three-day continuous monitoring of atmospheric CH ₄ concentration levels was carried out to verify the validity and reliability of the demonstrated ppbv-level CH ₄ sensor system.