Ultra-compact multi-pass cell-based TDLAS sensor for simultaneous dual-position atmospheric methane detection

This paper presents the development and evaluation of an ultra-compact multi-pass cell (MPC) for multi-point methane (CH₄) detection in environmental monitoring. The MPC, with a 2.45 m optical path length and a volume of ∼7 mL, was fabricated using two spherical mirrors to form a non-intersecting seven-circle spot pattern, maximizing the optical path length while minimizing volume. A near-infrared distributed feedback (DFB) laser at 1.65 μm was used as the light source, achieving detection sensitivity at the ppb level. The sensor system was tested in a dual-point setup over seven days, monitoring methane concentrations in a sewage environment. Results show that the ground-floor sensor recorded CH₄ concentrations ranging from 1.70 to 36.80 ppm, with an average of 2.90 ± 1.03 ppm, while the rooftop sensor showed concentrations between 1.60 and 4.40 ppm, averaging 2.40 ± 0.33 ppm. The linearity of the sensor was confirmed with R² values exceeding 0.999 for methane concentrations from 0.5 ppm to 20 ppm. The minimum detectable limit (MDL) was 288 ppb, and the rise and fall times were measured at 29s and 30s, respectively. The ultra-compact MPC design is cost-effective, easy to manufacture, and suitable for real-time methane detection in distributed networks, offering significant potential for environmental monitoring.