Search Results (104)

This study designed and validated a dual-component beat-frequency quartz-enhanced photoacoustic spectroscopy (BF-QEPAS) gas detection system utilizing time-division multiplexing (TDM). By applying TDM to drive distributed feedback lasers, the system achieved the simultaneous detection of acetylene and methane. Its key innovation lies in exploiting the transient response of the quartz tuning fork (QTF) to acquire gas concentrations while concurrently capturing the QTF resonant frequency and quality factor in real-time. Owing to the short beat period and rapid system response, this approach significantly reduces time-delay constraints in time-division measurements, eliminating the need for periodic calibration inherent in conventional methods and preventing detection interruptions. The experimental results demonstrate minimum detection limits of 5.69 ppm for methane and 0.60 ppm for acetylene. Both gases exhibited excellent linear responses over the concentration range of 200 ppm to 4000 ppm, with the R² value for methane being 0.996 and for acetylene being 0.997. The system presents a viable solution for the real-time, calibration-free monitoring of dissolved gases in transformer oil. Full article

To enable the highly sensitive detection of acetylene (C₂H₂) dissolved in transformer oil, a high-power quartz-enhanced photoacoustic spectroscopy (QEPAS) sensing system is proposed. A standard 32.7 kHz quartz tuning fork (QTF) was employed as an acoustic transducer, coupled with an optimized acoustic resonator to enhance the acoustic signal. The laser power was boosted to 150 mW using a C-band erbium-doped fiber amplifier (EDFA), achieving a detection limit of 469 ppb for C₂H₂ with an integration time of 1 s. The headspace degassing method was utilized to extract dissolved gases from the transformer oil, and the equilibrium process for the release of dissolved C₂H₂ was successfully monitored using the developed high-power QEPAS system. This approach provides reliable technical support for the real-time monitoring of the operational safety of power transformers. Full article

This work demonstrates an ultra-compact optical gas-sensing system, consisting of a pigtailed laser diode emitting at 1392.5 nm for water vapor (H₂O) detection, a silicon nitride (Si₃N₄) optical waveguide to guide the laser light, and a custom-designed, low-frequency, and T-shaped Quartz Tuning Fork (QTF) as the sensitive element. The system employs both Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) and Light-Induced Thermoelastic Spectroscopy (LITES) techniques for trace gas sensing. A 3.8 mm-wide, S-shaped waveguide path was designed to prevent scattered laser light from directly illuminating the QTF. Both QEPAS and LITES demonstrated comparably low signal-to-noise ratios (SNRs), ranging from 1.6 to 3.2 for a 1.6% indoor H₂O concentration, primarily owing to the reduced optical power (~300 μW) delivered to the QTF excitation point. These results demonstrate the feasibility of integrating photonic devices and piezoelectric components into portable gas-sensing systems for challenging environments. Full article

Quartz crystal out-of-plane vibration units are critical components of QMEMS devices. However, the fabrication of their 3D sidewall electrode structures presents significant challenges, particularly within ultrafine etched grooves. These challenges seriously limit further miniaturization, which is critical for portable and wearable electronic applications. In this paper, we propose a novel 3D self-masking fabrication strategy that enables the precise formation of sidewall electrodes by using the etched beam structure as a self-aligned pattern transfer medium. Based solely on photolithography and wet etching processes, this approach overcomes the limitations of the conventional shadow mask technique by improving alignment accuracy, process efficiency, and fabrication yields. In addition, a predictive mathematical model was developed to guide process optimization, enabling adaptive and reliable fabrication. Sidewall electrodes were successfully achieved in etched grooves as narrow as 45 μm, closely matching the theoretical predictions. To validate the approach, an ultra-miniaturized out-of-plane vibration unit with a beam spacing of just 150 μm—the narrowest reported to date—was fabricated, representing an 80% reduction compared to previously documented structures. The unit exhibited a repeatability error below 1.13%, confirming the precision and reliability of the proposed fabrication strategy. Full article

In this paper, an all-fiber light-induced thermoelastic spectroscopy (LITES) sensor based on hollow-core anti-resonant fiber (HC-ARF) and self-designed low-frequency quartz tuning fork (QTF) is reported for the first time. By utilizing HC-ARF as both the transmission medium and gas chamber, the laser tail fiber was spatially coupled with the HC-ARF, and the end of the HC-ARF was directly guided onto the QTF surface, resulting in an all-fiber structure. This design eliminated the need for lens combinations, thereby enhancing system stability and reducing cost and size. Additionally, a self-designed rectangular-tip QTF with a low resonant frequency of 8.69 kHz was employed to improve the sensor’s detection performance. Acetylene (C₂H₂), with an absorption line at 6534.37 cm⁻¹ (1.53 μm), was chosen as the target gas. Experimental results clearly demonstrated that the detection performance of the rectangular-tip QTF system was 2.9-fold higher than that of a standard commercial QTF system. Moreover, it exhibited an outstanding linear response to varying C₂H₂ concentrations, indicating its high sensitivity and reliability in detecting C₂H₂. The Allan deviation analysis was used to assess the system’s stability, and the results indicated that the system exhibits excellent long-term stability. Full article

A carbon dioxide (CO₂) sensor based on light-induced thermoelastic spectroscopy (LITES) using a 2 μm diode laser and a self-designed low-frequency trapezoidal-head QTF is reported for the first time in this invited paper. The self-designed trapezoidal-head QTF with a low resonant frequency of 9464.18 Hz and a high quality factor (Q) of 12,133.56 can significantly increase the accumulation time and signal level of the CO₂-LITES sensor. A continuous-wave (CW) distributed-feedback (DFB) diode laser is used as the light source, and the strongest absorption line of CO₂ located at 2004.01 nm is chosen. A comparison between the standard commercial QTF with the resonant frequency of 32.768 kHz and the self-designed trapezoidal-head QTF is performed. The experimental results show that the CO₂-LITES sensor with the self-designed trapezoidal-head QTF has an excellent linear response to CO₂ concentration, and its minimum detection limit (MDL) can reach 46.08 ppm (parts per million). When the average time is increased to 100 s based on the Allan variance analysis, the MDL of the sensor can be improved to 3.59 ppm. Compared with the 16.85 ppm of the CO₂-LITES sensor with the commercial QTF, the performance is improved by 4.7 times, demonstrating the superiority of the self-designed trapezoidal-head QTF. Full article

This study compares the performance of a quartz tuning fork (QTF) with a highly sensitive ultrasound microphone in the context of acoustic measurements, applying the substitution calibration method. QTF sensors are increasingly used for high-precision tasks due to their sensitivity and stability, while microphones are still the standard in general acoustic measurements. The aim of this study is to evaluate both technologies across several key performance metrics, including linearity of response, sensitivity, noise characteristics, and acoustic detection limit. Which sensor is better suited to which acoustic and physical condition? The results show that QTFs perform exceptionally well in applications requiring high precision, especially in high-frequency and narrow-band measurements. The signal-to-noise-ratio (SNR) of the QTF at its resonance frequency is 14 dB higher than that of the microphone, whereas the detection limit and linearity are comparable. The findings suggest that QTF sensors are particularly advantageous for specialized applications like photoacoustic spectroscopy (PAS). Full article

Quartz-enhanced photoacoustic spectroscopy (QEPAS) has shown great promise for monitoring greenhouse gases and pollutants with a high measurement accuracy and limit of detection. A QEPAS sensor, which can achieve high photoacoustic signal gain without requiring the laser beam to pass through the two prongs of a quartz tuning fork (QTF), is reported. A custom QTF with a resonant frequency of 7.2 kHz and a quality factor of 8406 was employed as a sound detection element, and the parameters of the acoustic micro-resonator (AmR) in the off-beam QEPAS spectrophone were optimized. A signal-to-noise ratio (SNR) gain of 16 was achieved based on the optimal AmR dimensions compared to the bare custom QTF. Water vapor (H₂O) was detected utilizing the QEPAS sensor equipped with the off-beam spectrophone, achieving a minimum detection limit (MDL) of 4 ppm with a normalized noise equivalent absorption coefficient (NNEA) of 5.7 × 10⁻⁸ cm⁻¹·W·Hz^−1/2 at an integration time of 300 ms. Full article

A highly sensitive light-induced thermoelastic spectroscopy (LITES) sensor employing a charge amplifier (CA) is reported for the first time in this invited paper. CA has the merits of high input impedance and strong anti-interference ability. The usually used transimpedance amplifier (TA) and voltage amplifier (VA) were also studied under the same conditions for comparison. A standard commercial quartz tuning fork (QTF) with a resonant frequency of approximately 32.76 kHz was used as the photothermal signal transducer. Methane (CH₄) was used as the target gas in these sensors for performance verification. Compared to the TA-LITES sensor and VA-LITES sensor, the reported CA-LITES sensor shows improvements of 1.83 times and 5.28 times in the minimum detection limit (MDL), respectively. When compared to the LITES sensor without an amplifier (WA-LITES), the MDL has a 19.96-fold improvement. After further optimizing the gain of the CA, the MDL of the CA-LITES sensor was calculated as 2.42 ppm, which further improved the performance of the MDL by 30.3 times compared to the WA-LITES. Additionally, long-term stability is analyzed using Allan deviation analysis. When the average time of the sensor system is increased to 50 s, the MDL of the CA-LITES sensor system can be improved to 0.58 ppm. Full article

In this paper, a highly sensitive methane (CH₄) sensor based on light-induced thermoelastic spectroscopy (LITES) and a T-shaped quartz tuning fork (QTF) with hydrogen (H₂) and helium (He) enhancement techniques are reported for the first time. The low resonant frequency self-designed T-shaped QTF was exploited for improving the energy accumulation time. H₂ and He were utilized as surrounding gases for the T-shaped QTF to minimize energy loss, thereby enhancing the sensitivity of the LITES sensor. Additionally, a fiber-coupled multi-pass cell (FC-MPC) with a 40 m optical length was utilized to improve the optical absorption of CH₄. The frequency response of the T-shaped QTF with different concentrations of H₂ and He was investigated, and the Q factor in the H₂ and He environment increased significantly. Compared to operating QTF in a nitrogen (N₂) environment, the signal amplitude was enhanced by 2.9 times and 1.9 times in pure H₂ and He environments, respectively. This enhancement corresponded to a minimum detection limit (MDL) of 80.3 ppb and 113.6 ppb. Under different CH₄ concentrations, the T-shaped QTF-based H₂-enhanced CH₄-LITES sensor showed an excellent linear response. Furthermore, through Allan deviation analysis, the MDL of the T-shaped QTF-based H₂-enhanced CH₄-LITES can reach 38 ppb with an 800 s integration time. Full article

The quartz tuning fork (QTF) is a promising instrument for biosensor applications due to its advanced properties such as high sensitivity to physical quantities, cost-effectiveness, frequency stability, and high-quality factor. Nevertheless, the fork’s small size and difficulty in modifying the prongs’ surfaces limit its wide use in experimental research. Our study presents the development of a QTF immunosensor composed of three active layers: biocompatible natural melanin nanoparticles (MNPs), glutaraldehyde (GLU), and anti-IgG layers, for the detection of immunoglobulin G (IgG). Frequency shifts of QTFs after MNP functionalization, GLU activation, and anti-IgG immobilization were measured with an Asensis QTF F-master device. Using QTF immunosensors that had been modified under optimum conditions, the performance of QTF immunosensors for IgG detection was evaluated. Accordingly, a finite element method (FEM)-based model was produced using the COMSOL Multiphysics software program (COMSOL License No. 2102058) to simulate the effect of deposited layers on the QTF resonance frequency. The experimental results, which demonstrated shifts in frequency with each layer during QTF surface functionalization, corroborated the simulation model predictions. A modelling error of 0.05% was observed for the MNP-functionalized QTF biosensor compared to experimental findings. This study validated a simulation model that demonstrates the advantages of a simulation-based approach to optimize QTF biosensors, thereby reducing the need for extensive laboratory work. Full article

Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) is a technique in which the sound wave is detected by a quartz tuning fork (QTF). It enables particularly high specificity with respect to the excitation frequency and is well known for an extraordinarily sensitive analysis of gaseous samples. We have developed the first photoacoustic (PA) cell for QEPAS on solid samples. Periodic heating of the sample is excited by modulated light from an interband cascade laser (ICL) in the infrared region. The cell represents a half-open cylinder that exhibits an acoustical resonance frequency equal to that of the QTF and, therefore, additionally amplifies the PA signal. The antinode of the sound pressure of the first longitudinal overtone can be accessed by the sound detector. A 3D finite element (FE) simulation confirms the optimal dimensions of the new cylindrical cell with the given QTF resonance frequency. An experimental verification is performed with an ultrasound micro-electromechanical system (MEMS) microphone. The presented frequency-dependent QEPAS measurement exhibits a low noise signal with a high-quality factor. The QEPAS-based investigation of three different solid synthetics resulted in a linearly dependent signal with respect to the absorption. Full article

Microfabrication technology with quartz crystals is gaining importance as the miniaturization of quartz MEMS devices is essential to ensure the development of portable and wearable electronics. However, until now, there have been no reports of dimension compensation for quartz device fabrication. Therefore, this paper studied the wet etching process of Z-cut quartz crystal substrates for making deep trench patterns using Au/Cr metal hard masks and proposed the first quartz fabrication dimension compensation strategy. The size effect of various sizes of hard mask patterns on the undercut developed in wet etching was experimentally investigated. Quartz wafers masked with initial vias ranging from 3 μm to 80 μm in width were etched in a buffered oxide etch solution (BOE, HF:NH₄F = 3:2) at 80 °C for prolonged etching (>95 min). It was found that a larger hard mask width resulted in a smaller undercut, and a 30 μm difference in hard mask width would result in a 17.2% increase in undercut. In particular, the undercuts were mainly formed in the first 5 min of etching with a relatively high etching rate of 0.7 μm/min (max). Then, the etching rate decreased rapidly to 27%. Furthermore, based on the etching width compensation and etching position compensation, new solutions were proposed for quartz crystal device fabrication. And these two kinds of compensation solutions were used in the fabrication of an ultra-small quartz crystal tuning fork with a resonant frequency of 32.768 kHz. With these approaches, the actual etched size of critical parts of the device only deviated from the designed size by 0.7%. And the pattern position symmetry of the secondary lithography etching process was improved by 96.3% compared to the uncompensated one. It demonstrated significant potential for improving the fabrication accuracy of quartz crystal devices. Full article

Transformer oil, crucial for transformer and power system safety, demands effective monitoring. Aiming to address the problems of expensive and bulky equipment, poor real-time performance, and single parameter detection of traditional measurement methods, this study proposes a quartz tuning fork-based simultaneous measurement system for online monitoring of the density, viscosity, and dielectric constant of transformer oil. Based on the Butterworth–Van Dyke quartz tuning fork equivalent circuit model, a working mechanism of transformer oil density, viscosity, and dielectric constant was analyzed, and a measurement model for oil samples was obtained. A miniaturized simultaneous measurement system was designed based on a dedicated chip for vector current-voltage impedance analysis for data acquisition and a Savitzky–Golay filter for data filtering. A transformer oil test platform was built to verify the simultaneous measurement system. The results showed that the system has good repeatability, and the measurement errors of density, viscosity, and dielectric constant are lower than 2.00%, 5.50%, and 3.20%, respectively. The online and offline results showed that the system meets the requirements of the condition maintenance system for online monitoring accuracy and real-time detection. Full article

This paper reports a self-temperature compensation barometer based on a quartz resonant pressure sensor. A novel sensor chip that contains a double-ended tuning fork (DETF) resonator and a single-ended tuning fork (SETF) resonator is designed and fabricated. The two resonators are designed on the same diaphragm. The DETF resonator works as a pressure sensor. To reduce the influence of the temperature drift, the SETF resonator works as a temperature compensation sensor, which senses the instantaneous temperature of the DETF resonator. The temperature compensation method based on polynomial fitting is studied. The experimental results show that the accuracy is 0.019% F.S. in a pressure range of 200~1200 hPa over a temperature range of −20 °C~+60 °C. The absolute errors of the barometer are within ±23 Pa. To verify its actual performance, a drone flight test was conducted. The test results are consistent with the actual flight trajectory. Full article