Quartz-enhanced photoacoustic spectroscopy (QEPAS) has become an important optical sensing technique for trace gas detection due to its ability to combine high sensitivity with a compact and robust system architecture [1–3]. In QEPAS, modulated optical radiation is absorbed by gas molecules, generating periodic thermal expansion and acoustic waves, which are subsequently detected by a quartz tuning fork (QTF) operating as a piezoelectric resonator [4]. Compared with conventional microphone-based photoacoustic spectroscopy, the use of a high-Q mechanical resonator enables operation at elevated frequencies, effectively suppressing low-frequency environmental noise and improving detection stability [5].

A distinctive feature of QEPAS is that the acoustic transducer simultaneously defines the detection bandwidth, noise rejection capability, and excitation efficiency of the system. The resonance frequency and quality factor of the QTF directly influence the conversion of absorbed optical energy into an electrical signal [6–9]. For this reason, the selection and optimization of the tuning fork are central to QEPAS sensor performance. Since the early development of the technique, the majority of QEPAS implementations have relied on a standard commercial quartz tuning fork with a nominal resonance frequency of 32.768 kHz, originally designed for timing applications [10–13]. Owing to its high quality factor, mechanical reliability, and widespread availability, this tuning fork has served as a convenient and well-established reference in many laboratory and applied studies.

In recent years, substantial efforts have been directed toward the design of custom quartz tuning forks with tailored geometries, reduced resonance frequencies, or enlarged prong spacing. L. Dong et al. developed a low-frequency custom QTF with enhanced sensitivity for water vapor detection [14]. M. Duquesnoy et al. demonstrated a QEPAS sensor using a radial acoustic resonator based on a custom QTF to enhance detection sensitivity [15]. Y. Ma et al. demonstrated an in-plane single-quartz-enhanced dual spectroscopy sensor for H₂O detection, achieving a minimum detection limit of ∼14.6 ppm [16]. A compact QEPAS–TDLAS sensor using a commercial custom quartz tuning fork enables fast and sensitive CO₂ detection for fruit respiration analysis [17].

It should be noted that commercial quartz tuning forks are not limited to a single standardized geometry. In addition to the widely used 32.7 kHz tuning fork, a variety of low- and intermediate-frequency commercial devices with different prong dimensions and spacings are readily available. Despite this diversity, commercial QTFs are often treated as functionally equivalent acoustic transducers in QEPAS experiments. In practice, differences in tuning fork geometry inevitably affect mechanical resonance characteristics, acoustic field distribution between the prongs.

Although the performance advantages of custom-designed quartz tuning forks have been widely demonstrated in QEPAS systems, their fabrication complexity and limited availability can pose challenges for large-scale deployment [18]. In contrast, commercially available quartz tuning forks benefit from mature manufacturing processes, low cost, and excellent reproducibility, making them particularly attractive for product-oriented and field-deployable QEPAS sensors. Beyond the widely used 32.7 kHz tuning fork, a range of commercial devices with different resonance frequencies and geometrical dimensions are readily accessible.

In this work, four commercially available quartz tuning forks with distinct geometrical parameters and resonance frequencies are investigated within a unified QEPAS framework. Their resonance properties are first experimentally characterized, followed by a systematic optimization of the laser excitation position for each tuning fork. The QEPAS performance is then evaluated by comparing second-harmonic signals obtained from a 1.8% water vapor sample under individually optimized conditions. By focusing on commercially available devices, this study aims to provide practical insights into tuning fork selection and system-level optimization for real-world QEPAS applications.

QTFs used in QEPAS are mechanical resonators whose vibration characteristics are governed by their geometrical dimensions. In order to investigate how the geometry of commercially available tuning forks influences their QEPAS behavior, four quartz tuning forks with distinct structural parameters and nominal resonance frequencies were selected and studied in this work. These tuning forks are denoted as QTF1–QTF4.

A schematic illustration of the tuning fork geometry is shown in Figure 1, where the key geometrical parameters include the prong width w , the prong spacing g , the prong thickness t , and the prong length l . These parameters determine the effective vibrating mass, stiffness, and acoustic interaction region between the prongs, and are therefore expected to influence both the mechanical resonance properties and the photoacoustic excitation efficiency.

Table 1 summarizes the geometrical dimensions and nominal resonance frequencies of the four commercial tuning forks investigated. QTF1 and QTF2 share identical prong widths of ∼230 μm, spacings of ∼130 μm, and thicknesses of ∼130 µm. Differing mainly in prong length, which decreases from 3,100 µm for QTF1 to 2,600 µm for QTF2. This structural difference leads to a clear shift in resonance frequency while maintaining similar cross-sectional geometry.

QTF3 exhibits a noticeable increase in prong width, thickness, and spacing compared with QTF1 and QTF2. The larger cross-sectional dimensions of QTF3 result in an increased effective mass and a wider gap between the prongs, which can modify the spatial distribution of the acoustic pressure field generated by laser absorption.

QTF4 corresponds to the widely used standard commercial tuning fork with a nominal resonance frequency of 32.7 kHz. Among the four devices, QTF4 has the largest prong width and spacing, as well as the longest prong length. Owing to its widespread use in QEPAS, QTF4 serves as a reference device for evaluating the performance of the other custom tuning forks with lower resonance frequencies and different geometrical characteristics.

Although all four tuning forks are commercially available devices fabricated from quartz, their geometrical parameters span a broad range. These differences provide an opportunity to systematically examine how variations in tuning fork geometry affect resonance behavior, laser excitation conditions, and ultimately QEPAS signal generation.

The mechanical resonance properties of the four commercial quartz tuning forks were experimentally characterized prior to QEPAS measurements. Each tuning fork was electrically driven, and its frequency response was obtained by sweeping the excitation frequency across the fundamental flexural resonance. The resulting resonance curves were recorded under ambient conditions and analyzed to extract the resonance frequency and quality factor.

Figure 2 presents the measured resonance amplitude as a function of excitation frequency for QTF1–QTF4. Clear resonance peaks are observed for all tuning forks, indicating stable and well-defined fundamental vibration modes. The resonance frequency f 0 and quality factor Q were determined by fitting the experimental data with a Lorentzian function. The extracted parameters are summarized in Table 2.

QTF1 exhibits the lowest resonance frequency of 19,989.8 Hz, accompanied by a quality factor of 4,138. Increasing the resonance frequency to 25,589.8 Hz, QTF2 shows a moderate increase in quality factor to 4,701. For QTF3, operating at 27,988.4 Hz, a significantly higher quality factor of 8,215 is obtained. QTF4, corresponding to the standard commercial tuning fork, presents the highest resonance frequency of 32,756.3 Hz and the largest quality factor of 10,533 among the four devices.

The experimental setup used for the QEPAS measurements is schematically shown in Figure 3. A distributed feedback (DFB) diode laser emitting at 1,392 nm was employed as the excitation source. The laser temperature was set to 17.5 °C.This wavelength corresponds to a strong near-infrared absorption line of water vapor falling at 1389.89 nm and was selected to ensure efficient photoacoustic excitation.

The laser was operated in wavelength modulation mode. Its injection current was driven by a composite modulation signal consisting of a slow periodic waveform for wavelength scanning and a high-frequency sinusoidal signal for photoacoustic excitation. The sinusoidal modulation frequency was set to half of the resonance frequency of the quartz tuning fork under test, enabling second-harmonic (2f) detection at the tuning fork resonance frequency. The slow scanning signal allowed the laser wavelength to sweep across the selected H₂O absorption line, generating a characteristic 2f spectral profile.

The laser output was delivered through a single-mode optical fiber and focused into the gap between the prongs of the quartz tuning fork using a fiber-coupled focusing lens. The laser beam was carefully aligned to propagate through the prong spacing without directly illuminating the quartz surfaces, thereby minimizing photothermal background signals. The vertical position of the laser focus relative to the tuning fork prongs was adjustable, which enabled systematic optimization of the excitation position for tuning forks with different geometrical parameters.

The quartz tuning fork was mounted inside a gas cell filled with the target gas mixture containing water vapor. Absorption of the modulated laser radiation by H₂O molecules generated periodic pressure fluctuations, which excited flexural vibrations of the tuning fork prongs. The resulting piezoelectric current was converted into a voltage signal using a low-noise preamplifier and subsequently demodulated by a lock-in amplifier referenced to the tuning fork resonance frequency.

All experimental instruments, including the laser driver, function generator (AFG3102, Tektronix), and lock-in amplifier (SR830, SRS), were controlled via a computer interface. For all measurements reported in this work, the same QEPAS system configuration and electronic settings were maintained. Only the modulation frequency was adjusted to match the resonance frequency of each tuning fork.

The spatial overlap between the laser-induced photoacoustic source and the vibration mode of the quartz tuning fork has a strong influence on the excitation efficiency in QEPAS. For tuning forks with different geometrical dimensions, the distribution of the acoustic pressure field and the mechanical vibration amplitude along the prongs are not identical. As a result, the optimal laser excitation position is expected to depend on the specific tuning fork geometry.

To experimentally investigate this effect, the position of the laser focus was varied along the longitudinal direction of the tuning fork prongs while keeping all other experimental parameters unchanged. As illustrated in Figure 4a, the distance d is defined as the vertical displacement of the laser focus with respect to the top of the tuning fork prongs.

Figures 4b–e show the normalized second-harmonic (2f) QEPAS signal amplitude as a function of the laser excitation position for QTF1–QTF4, respectively. For all tuning forks, the QEPAS signal exhibits a clear dependence on the laser focus position (QTF1∼400 µm, QTF2∼300 µm, QTF3∼500 µm and QTF4∼600 µm), with a well-defined maximum appearing at an intermediate value of d .

Although the overall trends are similar, the location of the optimal excitation position differs noticeably among the four tuning forks. For the low-frequency tuning forks QTF1 and QTF2, the maximum signal occurs relatively close to the prong tips. In contrast, QTF3 and QTF4 exhibit optimal excitation positions located further down along the prong length.

It is worth emphasizing that the optimal excitation position does not scale linearly with the resonance frequency or with a fixed fraction of the prong length. Instead, it reflects a combined effect of tuning fork geometry, mechanical resonance characteristics, and acoustic coupling conditions. These results demonstrate that a single, universal laser excitation position cannot be assumed when employing different commercial quartz tuning forks in QEPAS systems.

After optimizing the laser excitation position for each quartz tuning fork, QEPAS measurements were carried out to evaluate and compare the sensing performance of the four commercial devices under individually optimized conditions. A gas mixture containing 1.8% water vapor was selected as the target analyte to ensure strong absorption and efficient photoacoustic signal generation. For each tuning fork, the laser focus was fixed at its respective optimal position, while all other optimal settings.

Figure 5 shows the measured 2f QEPAS signals obtained with QTF1–QTF4 as the laser wavelength was scanned across the selected H₂O absorption line. The baseline signal recorded away from the absorption feature was used to evaluate the system 1σ noise level.

The extracted signal amplitudes, noise levels, and corresponding signal-to-noise ratios (SNRs) are summarized in Table 3. For the low-frequency tuning forks QTF1 and QTF2, the measured SNR are 79.6 and 107.3, respectively. QTF3 shows a pronounced enhancement in SNR, reaching 206.4, while the standard tuning fork QTF4 delivers the highest SNR of 430.4 under identical measurement conditions.

In conclusion, this work presents a systematic experimental comparison of four commercially available quartz tuning forks for QEPAS-based water vapor detection. Although all devices are standard commercial products, significant differences are observed in their resonance characteristics, optimal excitation conditions, and achievable QEPAS signal levels. These findings highlight that commercial quartz tuning forks should not be regarded as interchangeable components in QEPAS systems. Instead, careful selection of the tuning fork and optimization of excitation parameters are essential for achieving reliable and high-performance sensing. The results of this study provide practical guidance for the design and optimization of QEPAS sensors employing commercially available quartz tuning forks in real-world applications.