Ultrasensitive Optical Gas Sensor Based on a Rectangular-like Herriott Cell for Real-Time Atmospheric Methane Detection and Emission Monitoring

Tunable diode laser absorption spectroscopy (TDLAS) provides high sensitivity, superior spectral resolution, and a fast response for quantitative gas analysis across applications such as greenhouse gas monitoring, industrial process control, combustion diagnostics, and respiratory medicine. However, the traditional Herriott multipass cell (MPC)─a key optical component in TDLAS─faces challenges due to its large physical size, low mirror utilization efficiency, and strict alignment requirements, limiting portability and field deployment. In this work, we designed and fabricated a compact rectangular-like Herriott cell (RLHC) with a 12.7 m optical path length and physical dimensions of 9.00 × 6.60 × 3.45 cm³ (closed-cell volume ∼58.9 mL), representing the smallest MPC reported for a comparable path length. By transforming a circular (24 mm diameter) beam-spot distribution into an elliptical one (24 mm long axis and 6 mm short axis) and folding the optical axis six times using two high-reflectivity plane mirrors, the RLHC achieves a fill factor of 21.9 cm^-2. Integration of a fiber-coupled collimator and an InGaAs photodetector eliminates the need for active optical alignment, resulting in a self-contained sensing module. Using a 1.65 μm distributed feedback laser, the RLHC-based methane sensor achieves a minimum detection limit (MDL) of 38.93 ppbv and a noise-equivalent absorption coefficient of 1.36 × 10^-5 Hz^-1/2 (approximately an order of magnitude lower than conventional TDLAS systems). Continuous three-day measurements near sewage systems and in ambient air demonstrate strong robustness and long-term stability, underscoring the potential of RLHC-based TDLAS sensors for distributed environmental monitoring, hand-held operation, and large-scale field applications.