Ultra-Sensitive CH ₄ -LITES Sensor Enabled by Low-Frequency Slingshot-Shaped Quartz Tuning Fork and Optical Enhancement

This paper reports for the first time an ultrasensitive methane (CH₄)-LITES sensor based on a low-frequency slingshot-shaped quartz tuning fork (QTF) and optical enhancement. The slingshot-shaped QTF serves as the core innovative component, featuring low resonant frequency (∼7.9 kHz), wide prong gap (>1000 μm), and slingshot-shaped curved transition structure, which significantly extends acoustic energy accumulation time, suppresses optical scattering noise, and optimizes stress distribution, thereby comprehensively enhancing sensing performance. Finite element simulation results demonstrate that, compared to a standard commercial QTF, the maximum temperature gradient and total surface charge of the slingshot-shaped QTF increased by factors of 3.53 and 2.68, respectively. Experimental validation shows that the signal-to-noise ratio (SNR) of the LITES system based on this slingshot-shaped QTF improved by 2.26 times compared to the system using a standard QTF. To further optimize the detection performance for CH₄, a Raman fiber amplifier (RFA) was employed to enhance the power of the diode laser, while a multipass cell (MPC) with a dense spot pattern and an optical path length of approximately 80 m, along with a self-designed amplifier, were used to promote gas absorption and enhance signal gain, respectively. These three components work synergistically to effectively improve the sensor's detection capability by increasing excitation intensity, promoting gas absorption, and enhancing signal gain. In this configuration, the sensor's minimum detection limit (MDL) for CH₄ was calculated to be 8.42 ppb, and the noise-equivalent normalized absorption coefficient (NNEA) was calculated as 1.38 × 10^-9 cm^-1·W·Hz^-1/2. Allan deviation analysis indicated that at an averaging time of 350 s, the sensor's MDL was optimized to 0.72 ppb. This study provides a novel QTF structure with significant performance advantages and an optical enhancement strategy for highly sensitive CH₄ gas detection, holding important application prospects in fields such as environmental monitoring and industrial safety.