A gas sensing method based on quartz-enhanced photothermal spectroscopy (QEPTS) demodulated by quartz tuning fork (QTF) sensing acoustic wave is reported for the first time. Different from traditional QEPTS, the method proposed in this paper utilizes the second QTF to sense acoustic wave produced by the first QTF owing to the vibration resulted from photo-thermo-elastic effect. This indirect demodulation by acoustic wave sensing can avoid QTF being irradiated by laser beam and therefore get less noise and realize better detection sensitivity. Four different sensing configurations are designed and verified. Acetylene (C 2 H 2 ) with a volume concentration of 1.95 % is selected as the target gas. A model of sound field produced by the first QTF vibrating is established by finite element method to explain the variation trend of signal and noise in the second QTF. The measured results indicate that this technique had an enhanced signal-to-noise ratio (SNR) of 1.36 times when compared to the traditional QEPTS. Further improvement methods for such technique is proposed.

In this paper, we report on the performance of quartz tuning fork (QTF) based laser spectroscopy sensing employing multiple QTFs. To avoid that resonance frequency mismatching of the QTFs degrades the sensor performance, two types of resonance frequency matching method are here proposed. A system based on the coupling of two sensing modules, one based on quartz-enhanced photoacoustic spectroscopy (QEPAS) and one on light-induced thermoelastic spectroscopy (LITES) technique, was realized to validate the proposed methods. Each module employed a different QTF (QTF1 and QTF2, respectively). Operating temperature or pressure of QTF2 were regulated to match the resonance frequency of QTF1, which operated at 25.0 °C and atmospheric pressure. Without regulation, the difference between QTF1 and QTF2 resonance frequencies was 2.42 Hz and the superposition coefficient η was only 54.7%. When the temperature regulation was carried out, at a QTF2 operating temperature of 67.5 °C, an optimal η value of 95.0% was obtained. For the pressure regulation approach, if operating QTF2 at pressure of 500 Torr, η reached a value of 97.2%. The obtained results show that the proposed two methods are effective in resonance frequency matching of QTFs for gas sensing systems.

An H-shaped acoustic micro-resonator (AmR)-based quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor is demonstrated for the first time. The H-shaped AmR has the advantages of easy optical alignment, high utilization of laser energy, and reduction in optical noise. The parameter of the H-shaped AmR is designed based on the standing wave enhancement characteristic. The performance of the H-shaped AmR-based QEPAS sensor system and bare quartz tuning fork (QTF)-based sensor system are measured under the same conditions by choosing water vapor (H2O) as the target gas. Compared with the QEAPS sensor based on a bare QTF, the detection sensitivity of the optimal H-shaped AmR-based QEPAS sensor exhibits a 17.2 times enhancement.

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