Highly Sensitive CH4/C2H2 Dual-Gas Light-Induced Thermoelastic Spectroscopy Sensor Based on a Dual-Path Multiring Multipass Cell and a Circle-Head Quartz Tuning Fork

In this paper, a highly sensitive methane (CH4) and acetylene (C2H2) dual-gas light-induced thermoelastic spectroscopy (LITES) sensor based on a novel dual-path multiring multipass cell (DPMR-MPC) and a circle-head quartz tuning fork (QTF) was demonstrated for the first time. A theoretical model was established on the basis of double concave spherical mirrors to realize a 14-interlaced-ring spot pattern in DPMR-MPC with an optical path length (OPL) of 26.7 m per path. The total optical path length to volume ratio (OPL/V) reached 22 cm-2. A circle-head QTF with a low resonant frequency of ∼9.5 kHz was adopted to enhance the LITES sensor's detection ability. A Raman fiber amplifier (RFA) and an erbium-doped fiber amplifier (EDFA) were employed to amplify the two-laser excitation source. When the integration time of the LITES sensor was increased to 100 s, the minimum detection limits (MDLs) for simultaneous detection of CH4 and C2H2 could be improved to 50.9 and 6.64 ppb, respectively. Compared with the current spectroscopic techniques used for detecting multiple gases, the dual-gas LITES sensor in this study offers an obviously high detection sensitivity.