Fast step heterodyne light-induced thermoelastic spectroscopy gas sensing based on a quartz tuning fork with high-frequency of 100 kHz

In this paper, a fast step heterodyne light-induced thermoelastic spectroscopy (SH-LITES) sensor using a high-frequency quartz tuning fork (QTF) with resonant frequency of ~100 kHz is reported for the first time. The theoretical principle of heterodyne LITES (H-LITES) signal generation is analyzed firstly, and an acetylene (C₂H₂) H-LITES sensor is established to verify its performance. Experimental comparisons between the high-frequency QTF and a standard commercial QTF with resonant frequency of ~32.768 kHz reveal that the high-frequency QTF exhibits a tenfold faster response time. Specifically, the H-LITES sensor with this QTF achieves a 33 ms measurement cycle, 90% shorter than commercial counterparts. Furthermore, The SH-LITES technique is proposed to further shorten the scanning time to 15 ms, which achieves the shortest LITES measurement time known to date. To demonstrate its advantages in dynamic gas detection, an H₂O-LITES system integrating both QTF types is constructed for real-time monitoring of H₂O concentration during different respiration patterns. Comparative measurements show that the SH-LITES more accurately captures dynamic H₂O concentration fluctuations during respiration, outperforming the commercial QTF-based H-LITES sensor in rapid response scenarios.