An Ultrahighly Sensitive CH₄-TDLAS Sensor Based on an 80-m Optical Path Length Multipass Cell with a Dense Circular Spot Pattern

In this paper, we present an ultrahighly sensitive methane (CH₄) sensor based on tunable diode laser absorption spectroscopy (TDLAS), utilizing an 80-m optical path length (OPL) multipass cell (MPC) with 15 independent circles pattern. The designed MPC achieved an actual OPL of 81.58 m within a compact volume of 361 cm³, resulting in an exceptional ratio of optical path length to volume (RLV) of 22.6 cm^-2. Wavelength modulation spectroscopy (WMS) was employed to minimize background noise and enhance detection performance. The CH₄-TDLAS sensor demonstrated excellent linear response to CH₄ concentrations. The minimum detection limit (MDL) of the sensor was determined to be 13.46 ppb. Further analysis using Allan deviation revealed that the MDL improved to 1.36 ppb at an average time of 400 s, highlighting the sensor's exceptional long-term stability and ultrahigh sensitivity. Finally, to further validate the sensor's performance, an experiment simulating laboratory CH₄ leakage was conducted. The results demonstrated that the CH₄-TDLAS sensor exhibits ultrahigh sensitivity, effectively detecting and tracking changes in CH₄ concentration in real time.