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.