Parts-per-Trillion-Level Methane Detection Based on Light-Induced Thermoelastic Spectroscopy with Extended Optical Path Absorption

This paper describes a highly sensitive methane (CH₄) sensor utilizing light-induced thermoelastic spectroscopy (LITES), which incorporates a multipass cell (MPC) with a dense spot configuration and a self-designed round-tip quartz tuning fork (QTF). The designed MPC can achieve a long optical path length of 77.36 m and a large ratio of optical path length to volume (RLV) of 20.55 cm^-2. A novel round-tip QTF with a low frequency of 9.7 kHz is used as the photothermal detector of the sensor. A continuous wavelength distributed feedback (CW-DFB) diode laser capable of a 10 mW maximum output is employed to stimulate the CH₄ absorption line at 1650.96 nm. Aiming to boost the CH₄-LITES sensor's detection ability, the laser power is amplified to 200 mW via the integration of a Raman fiber amplifier (RFA) into the optical path. Detailed testing is conducted to assess the sensor's performance. The results reveal that the CH₄-LITES sensor has excellent linearity and an optical power response. The novel round-tip QTF brings about a 3.61-fold enhancement in the minimum detection limit (MDL) of the sensor when compared with the commercial QTF. Eventually, the Allan deviation method enables the LITES sensor to reach a detection limit of 600 ppt for CH₄.