In recent years, harmful gases in the atmosphere have gradually become an important issue of concern for people. Gas sensing technology can perform highly sensitive monitoring of trace gas concentrations, obtaining information on the composition, knowing the concentration changes of gases, and understanding the distribution changes of gases. Quartz-enhanced photoacoustic spectroscopy (QEPAS) technology based on quartz tuning fork detection has the advantages of simple structure, low cost, and strong anti-noise ability, which is a hot spot in the field of gas sensing. In common QEPAS technology, commercial quartz tuning forks are generally used with a resonance frequency of 32.768 kHz, but the performance of QEPAS systems is limited due to high resonance frequency, short energy accumulation time, and small interfinger spacing.

In this paper, the finite element analysis method is used to simulate the stress and charge distribution of quartz tuning forks. A T-shaped quartz tuning fork is designed, and the resonance frequency of the T-shaped quartz tuning fork is 8930.93 Hz, with a Q value of 11164 and a cross interfinger spacing of 1.73 mm. In the experimental verification phase, by using water vapor in the atmosphere as the measurement object, the QEPAS water vapor detection system is built. Under the same conditions, we test two types of quartz tuning forks. One is a commercial quartz tuning fork, and the other is a T-shaped quartz tuning fork. A comparison of experimental results between these two quartz tuning forks is performed to verify their detection performance.

The T-shaped quartz tuning fork has a length of 9.4 mm, a width of 1.2 mm, and a thickness of 0.25 mm (Table 1). By using the optical excitation method, firstly, the performance of commercial quartz tuning forks and T-shaped quartz tuning forks is tested separately, so as to obtain the resonance frequency curves of two types of quartz tuning forks. The resonance frequency f₀ of a commercial quartz tuning fork is 32767.76 Hz, and the quality factor is 9128. f₀ of the T-shaped quartz tuning fork is 8930.93 Hz, and the quality factor is 11164. The resonance frequency of the T-shaped quartz tuning fork is reduced by 73%, and the quality factor is improved by 22% compared with the widely used commercial quartz tuning fork (Fig. 4). The signal level of the QEPAS system is related to the laser incidence position. The optimal laser incidence position for commercial quartz tuning forks is 0.7 mm from the top, and the optimal laser incidence position for T-shaped quartz tuning forks is 1.6 mm from the top (Fig. 6). The amplitude of the 2f signal using a commercial quartz tuning fork is 16.44 μV, and the noise level and the signal-to-noise ratio are 58.86 nV and 279.31, respectively. The amplitude of the 2f signal measured using a T-shaped quartz tuning fork is 25.37 μV, and the noise level and the signal-to-noise ratio are 56.54 nV and 448.71, respectively. Compared with commercial quartz tuning forks, the signal amplitude detected by T-shaped quartz tuning forks has increased by 54.32%, and the signal-to-noise ratio has increased by 60.65% (Fig. 7).

The commercial quartz tuning forks widely used in QEPAS technology currently have certain limitations. For example, the high resonance frequency makes the system unable to detect gases with low molecular relaxation rates, short energy accumulation time leads to weak collection ability of acoustic signals, and small interdigital spacing is not conducive to coupling transmission of laser beams and reducing system noise. We use finite element analysis to simulate a T-shaped quartz tuning fork with low resonance frequency, high Q value, and large interdigital gap. After actual measurement, the resonance frequency of this T-shaped quartz tuning fork is 8930.93 Hz with a Q value of 11164 and interdigital spacing of 1.73 mm. Compared with the widely used commercial quartz tuning fork, the resonance frequency of the T-shaped quartz tuning fork is reduced by 73%, and the quality factor is increased by 22%. Finally, this quartz tuning fork is applied to the near-infrared QEPAS water vapor detection system to further verify its sensing performance. Compared with commercial quartz tuning forks, the signal-to-noise ratio of the water vapor QEPAS system based on the T-shaped quartz tuning fork has increased by 60.65%, proving the superiority of the sensing performance of this quartz tuning fork. However, the equivalent resistance value of this quartz tuning fork is still too high, which has an impact on the overall detection performance. Further optimization will be carried out to reduce the equivalent resistance value and further improve the sensing performance of the system.