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• A long-path QEPTS gas sensor using a high power Q-switched fiber laser was proposed. • Long optical path and high power are beneficial to the QEPTS sensor. • A minimum detection limit (MDL) of 6.1 ppbv at the 48 s integration time was achieved. A long-path quartz tuning fork enhanced photothermal spectroscopy (QEPTS) gas sensor using a Q-switched fiber laser is proposed. A theoretical model is used to describe the process of the long-path QEPTS sensor and the factors that affect its performance. We can conclude that long optical path and high power are beneficial to the QEPTS sensor from the theoretical analysis. And we use our long-path QEPTS sensor for C 2 H 2 detection at the wavelength of 1531.59 nm. With the scanning time of 4 s and the peak pulse power of 127.5 mW, an Allan deviation analysis shows that our sensor has a minimum detection limit (MDL) of 6.1 ppbv at the 48 s integration time and a good linear response (R-square = 0.99796). [ABSTRACT FROM AUTHOR]

• A QTF-self-difference QEPTS sensor for C 2 H 2 detection was proposed. • The SNR could be increased 5.1 times by using the QTF-self-difference technique. • The system achieved a MDL of 723 ppbv, corresponding to a NNEA of 7.85 × 10−10 cm−1 W/Hz1/2. A novel self-difference technique based on quartz tuning fork (QTF) is proposed in this manuscript. C 2 H 2 is selected as the target analyte to verify the effectiveness of this technique. The measurement technique consists of two laser beams, one laser beam passes through the gas cell as the probe laser beam and the other beam is used as the reference laser beam. The vibration direction of the QTF caused by the two laser beams is opposite by adjusting the position where the two laser beams illuminate QTF. The two quartz tuning fork enhanced photothermal spectroscopy (QEPTS) signals are subtracted by utilizing the mechanical vibration characteristics of the QTF. As a result, the SNR increases 5.1 times in QTF-self-difference based system compared to a traditional differential gas detection system. Finally, the system achieved a minimum detection limit (MDL) of 723 ppbv, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 7.85 × 10−10 cm−1 W/Hz 1/2. [ABSTRACT FROM AUTHOR]

• Scanning cycle is optimized in detail for photoacoustic energy accumulation. • Optimum modulation index is verified to be ∼5 by experiments instead of 2.2. • Frequency and efficiency of QTF are evaluated in a long-term (∼8 months) experiment. • Photoacoustic spectrophones are discussed and compared in their optimum geometric dimensions. Efforts were made in this study to evaluate a series of pivotal techniques in standard quartz tuning fork (QTF) based quartz enhanced photoacoustic spectroscopy (QEPAS) systems for engineering applications. In current QEPAS systems, a scanning ramp combined with high frequency sinusoidal signal is used to drive the light source for photoacoustic excitation, the scanning cycle was optimized for the first time in experiments guaranteeing that photoacoustic energy accumulation finished to achieve the largest photoacoustic signal, and the optimum modulation index was verified by experiments to be ∼5 instead of the simulated result 2.2. As an important step for engineering applications, a long-term (∼8 months) experiment was also carried out to evaluate the stability of QTF's property. In addition, several photoacoustic spectrophone configurations were compared in the optimum geometry parameters, dual-tubes on beam was verified in experiments to be the most efficient one compared with half on-beam and off-beam structures in standard QTF based systems, which is consist with the summary of published papers. [ABSTRACT FROM AUTHOR]

• A novel single quartz tuning fork (QTF) compact integrated dual-spectroscopy gas detection structure based on quartz-enhanced photoacoustic spectroscopy (QEPAS) and quartz-enhanced photothermal spectroscopy (QEPTS) was designed for improving detection intensity and reducing sensor size. • In the structure, an off-beam acoustic micro-resonator (AMR) was used to enhance the photoacoustic signal. After passing the AMR, the laser beam was turned back by the right-angle prism and incident on the side of the QTF fork again to generate the photothermal signal. • The silver layer on the QTF surface was chemically etched to further enhance the photothermal signal. For the traditional off-beam photoacoustic excitation, the photothermal signal is excited by the way of laser retracting. The superposition of the dual-spectroscopy detection technology on a single QTF is realized. • Under the same conditions, photoacoustic signal, photothermal signal and dual-spectroscopy signal were measured respectively. The measured dual-spectroscopy signal was approximately the superposition value of photoacoustic signal and photothermal signal, which showed the effective signal lifting effect. • With C 2 H 2 as the target gas, the experimental results showed that the signal value of the dual-spectroscopy structure was 1.94 times that of QEPAS structure, 2.18 times that of QEPTS structure. And the minimum detection limit at atmospheric pressure was 6.64 ppm. The integrated dual-spectroscopy structure was compact, and was suitable for small space measurement, mobile measurement, and other fields. If the acetylene (C 2 H 2) content in the oxygen production device exceeds the standard, it is highly susceptible to explosion. It is necessary to detect C 2 H 2 gas and reach the detection limit of ppm. A novel single quartz tuning fork (QTF) compact integrated dual-spectroscopy gas detection structure based on quartz-enhanced photoacoustic spectroscopy (QEPAS) and quartz-enhanced photothermal spectroscopy (QEPTS) was designed for improving detection intensity and reducing sensor size. The detection structure (40 mm × 90 mm × 50 mm) includes gas buffer, gas detection structure, and preamplifier circuit. In the structure, an off-beam acoustic micro-resonator (AMR) was used to enhance the photoacoustic signal. After passing the AMR, the laser beam was turned back by the right-angle prism and incident on the side of the QTF fork again to generate the photothermal signal. The silver layer on the QTF surface was chemically etched to further enhance the photothermal signal. For the traditional off-beam photoacoustic excitation, the photothermal signal is excited by the way of laser retracting. The superposition of the dual-spectroscopy detection technology on a single QTF is realized. And the structure is finely integrated which is convenient to carry and measure. Under the same conditions, photoacoustic signal, photothermal signal and dual-spectroscopy signal were measured respectively. The measured dual-spectroscopy signal was approximately the superposition value of photoacoustic signal and photothermal signal, which showed the effective signal lifting effect. With C 2 H 2 as the target gas, the experimental results showed that the signal value of the dual-spectroscopy structure was 1.94 times that of QEPAS structure, 2.18 times that of QEPTS structure. And the minimum detection limit at atmospheric pressure was 6.64 ppm. The integrated dual-spectroscopy structure was compact, and was suitable for small space measurement, mobile measurement, and other fields. [ABSTRACT FROM AUTHOR]

• A comprehensive dual-spectroscopy detection technique based on tunable diode laser absorption spectroscopy (TDLAS) and quartz enhanced photoacoustic spectroscopy (QEPAS) is demonstrated using a single quartz tuning fork (QTF) for signal detection. • A QTF was used as a photodetector and an acoustic wave transducer to detect the TDLAS and QEPAS signal simultaneously for 2f signal enhancement. • A gas absorption cell with 3 m optical path length was utilized to generate and enhance the TDALS signal. We tested the feasibility of the proposed dual-spectroscopy detection technique by detecting acetylene (C 2 H 2) at 1532.83 nm. • The signal of the dual-spectroscopy technique was approximately 1.13 and 8.71 times higher than that of the QEPAS and TDLAS techniques based on QTF. The proposed dual-spectroscopy detection technique also showed superior gas sensing capability for trace gas detection and could achieve a minimum detection limit of 1.05 ppm. A comprehensive dual-spectroscopy detection technique based on tunable diode laser absorption spectroscopy (TDLAS) and quartz enhanced photoacoustic spectroscopy (QEPAS) is demonstrated using a single quartz tuning fork (QTF) for signal detection. The QTF was utilized as an acoustic wave transducer for QEPAS signal detection. The QTF also served as a photoelectric detector for TDLAS signal detection based on the thermoelastic effect. The dual-spectroscopy detection structure was designed for TDLAS and QEPAS detection. The on-beam acoustic micro resonator (AMR) structure was placed on the upper end of the QTF for QEPAS signal enhancement, and the laser beam transmitted through the AMR was aligned to the QTF prong by an optical collimator for TDLAS signal detection. An absorption gas cell with an optical path length of 3 m was utilized in the TDLAS setup. The gas cell enhanced the absorption signal of the gas by virtue of its relatively long optical path. We tested the feasibility of the proposed dual-spectroscopy detection technique by detecting acetylene (C 2 H 2) at 1532.83 nm. The experimental show that the signal of the dual-spectroscopy detection technique is approximately 1.13 times that of the QEPAS signal. The proposed dual-spectroscopy detection technique also showed superior gas sensing capability for trace gas detection and could achieve a minimum detection limit of 1.05 ppm. The signal strength of the proposed dual-spectroscopy detection technique can be further enhanced by using an absorption gas cell with a longer optical path. [ABSTRACT FROM AUTHOR]