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.