Fast Step Heterodyne Light-Induced Thermoelastic Spectroscopy Gas Sensing Based on a Quartz Tuning Fork with High-Frequency of 100 kHz
This study presents the first demonstration of a fast step heterodyne light-induced thermoelastic spectroscopy (SH-LITES) sensor utilizing a high-frequency quartz tuning fork (QTF) with a resonant frequency of approximately 100 kHz. The theoretical basis of heterodyne LITES (H-LITES) signal generation is analyzed, and an acetylene (C2H2) H-LITES sensor is constructed to evaluate performance. Comparative experiments between the high-frequency QTF and a standard commercial QTF (resonant frequency ~32.768 kHz) reveal that the high-frequency QTF achieves a tenfold faster response time, with a measurement cycle of 33 ms—90% shorter than commercial counterparts. The proposed SH-LITES technique further reduces the scanning time to 15 ms, representing the shortest LITES measurement time reported to date. To validate dynamic gas detection capabilities, an H2O-LITES system integrating both QTF types is employed for real-time monitoring of H2O concentration during various respiration patterns. Results demonstrate that SH-LITES more accurately captures rapid H2O concentration fluctuations during respiration, outperforming the commercial QTF-based H-LITES sensor in fast-response scenarios. These findings establish a new benchmark for high-speed trace gas sensing with potential applications in combustion diagnostics, healthcare monitoring, and environmental surveillance.