Overtone acoustic feedback enhanced light-induced thermoelastic spectroscopy sensing

In this paper, a novel light-induced thermoelastic spectroscopy (LITES) sensor based on overtone acoustic feedback enhancement (OAFE) is proposed for the first time to improve the sensing performance. A self-designed low-frequency quartz tuning fork (QTF) is employed as the sensing element. Using dual-frequency modulation, the QTF’s first overtone signal is amplified to drive an electroacoustic transducer (EAT), generating a fundamental acoustic signal. This creates an acoustic feedback loop that significantly enhances the QTF’s fundamental vibration. Methane (CH₄), a highly flammable and explosive gas, is selected as the target gas in this study. Experimental results show that compared to the traditional LITES sensor, the signal-to-noise ratio (SNR) of the OAFE-based LITES sensor is improved by a factor of 6.19. The reported OAFE-based LITES sensor exhibits an excellent linear response characteristic to CH₄ concentration, with a minimum detection limit (MDL) of 1.08 ppm. Additionally, through Allan deviation analysis, when the integration time reaches 70 s, the MDL of the OAFE-based LITES sensor can be optimized to 0.29 ppm.