Highly Sensitive T-Shaped Quartz Tuning Fork Based CH₄-Light-Induced Thermoelastic Spectroscopy Sensor with Hydrogen and Helium Enhanced Technique

In this paper, a highly sensitive methane (CH₄) sensor based on light-induced thermoelastic spectroscopy (LITES) and a T-shaped quartz tuning fork (QTF) with hydrogen (H₂) and helium (He) enhancement techniques are reported for the first time. The low resonant frequency self-designed T-shaped QTF was exploited for improving the energy accumulation time. H₂ and He were utilized as surrounding gases for the T-shaped QTF to minimize energy loss, thereby enhancing the sensitivity of the LITES sensor. Additionally, a fiber-coupled multi-pass cell (FC-MPC) with a 40 m optical length was utilized to improve the optical absorption of CH₄. The frequency response of the T-shaped QTF with different concentrations of H₂ and He was investigated, and the Q factor in the H₂ and He environment increased significantly. Compared to operating QTF in a nitrogen (N₂) environment, the signal amplitude was enhanced by 2.9 times and 1.9 times in pure H₂ and He environments, respectively. This enhancement corresponded to a minimum detection limit (MDL) of 80.3 ppb and 113.6 ppb. Under different CH₄ concentrations, the T-shaped QTF-based H₂-enhanced CH₄-LITES sensor showed an excellent linear response. Furthermore, through Allan deviation analysis, the MDL of the T-shaped QTF-based H₂-enhanced CH₄-LITES can reach 38 ppb with an 800 s integration time.