Highly sensitive CH_4,C₂H₂and CO simultaneous measurement LITES sensor based on multi-pass cell with overlapped spots pattern and QTFs with low resonant frequency

Abstract

This paper presents a simultaneous measurement light-induced thermoelectric spectroscopy (LITES) sensor with high sensitivity for detecting methane (CH 4 ), carbon monoxide (CO) and acetylene (C 2 H 2 ). It employs a multi-pass cell (MPC) with an overlapped spots pattern and low resonant frequency circle-head quartz tuning forks (QTFs) for the first time. The fiber-coupled MPC with an optical length (OPL) of 40 m was combined with a thin-film filter (TFF) to improve the laser absorption and enable light spots multiplexing on mirrors. Three self-designed circle-head QTFs with low resonant frequencies of less than 10 kHz and a quality factor of ∼ 11500 were adopted to improve the detection ability. The LITES sensor detected multi-gas signals based on their unique absorption spectrum, enabling real-simultaneous measurement of CH 4 , CO, and C 2 H 2 . After optimization, the minimum detection limits (MDLs) of 0.5 ppm, 126.9 ppm and 0.4 ppm for these gases were obtained, respectively. With integration times of 300 s for CH 4 and 200 s each for CO and C 2 H 2 , the MDLs could be further reduced to 0.09 ppm, 57.1 ppm, and 0.07 ppm, correspondingly. The paper concludes with a discussion of potential strategies for further improving the performance of such LITES sensors.