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Quartz tuning forks (QTFs) are piezo-transducers that have been implemented for numerous applications, such as chemical gas sensing, atomic force microscopy, rheology, and industrial process control. The most important parameter for QTFs’ sensing application is the resonance quality factor (Q-factor). An experimental investigation and theoretical analysis of the influence of QTFs’ geometries on the Q-factor of the flexural fundamental and first overtone resonance modes are reported. The resonance frequencies and related Q-factors for five different QTFs have been measured. The QTF response was recorded at different air pressures to investigate the influence of the surrounding medium on the Q-factor. A data analysis demonstrated that air viscous damping is the dominant energy dissipation mechanism for both flexural modes. Thermoelastic and support losses are additional contributions that depend on the QTF geometry. A study of the QTF damping mechanism dependence upon the prong geometry is also provided. [ABSTRACT FROM AUTHOR]

This review aims to discuss the latest advancements in quartz-enhanced photoacoustic spectroscopy (QEPAS) based trace-gas sensing. Starting from the QEPAS basic physical principles, the most used QEPAS configurations will be described. This is followed by a detailed theoretical analysis and experimental study regarding the influence of quartz tuning forks (QTFs) geometry on their optoacoustic transducer performance. Furthermore, an overview of the latest developments in QEPAS trace-gas sensor technology employing custom QTFs will be reported. Results obtained by exploiting novel micro-resonator configurations, capable of increasing the QEPAS signal-to-noise ratio by more than two orders of magnitude and the utilization of QTF overtone flexural modes for QEPAS based sensing will be presented. A comparison of the QEPAS performance of different spectrophone configurations is reported based upon signal-to-noise ratio. Finally, a novel QEPAS approach allowing simultaneous dual-gas detection will be described. [ABSTRACT FROM AUTHOR]

A review on the design of quartz tuning forks and their applications in quartz-enhanced photoacoustic spectroscopy (QEPAS)-based trace gas sensors is reported. Standard commercial quartz tuning forks are used as sound wave transducers in the conventional QEPAS technique. With the development of QEPAS technology, the advent of custom quartz tuning forks has further improved the sensitivity level of QEPAS sensors. The manufacturing process of quartz tuning forks will be explained in detail in this review and the application of quartz tuning forks in QEPAS trace gas sensors in recent years will be summarized. [ABSTRACT FROM AUTHOR]

We present a gas sensing system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) employing a monolithic distributed-feedback quantum cascade laser (QCL) array operated in a pulsed mode as a light source. The array consists of 32 quantum cascade lasers emitting in a spectral range from 1190 cm−1 to 1340 cm−1. The optoacoustic detection module was composed of a custom quartz tuning fork with a prong spacing of 1 mm, coupled with two micro-resonator tubes to enhance the signal-to-noise ratio. The QEPAS sensor was validated by detecting the absorption of the P- and R-branches of nitrous oxide. The measurements were performed by switching the array QCLs in sequence while tuning their operating temperature to retrieve the fine structure of the two N2O branches. A sensor calibration was performed, demonstrating a linear responsivity for N2O:N2 concentrations from 1000 down to 200 parts-per-million. With a 10 s lock-in integration time, a detection sensitivity of less than 60 parts-per-billion was achieved permitting the monitoring of nitrous oxide at global atmospheric levels. [ABSTRACT FROM AUTHOR]

We report here on the use of the Allan deviation plot to analyze the long-term stability of a quartz-enhanced photoacoustic (QEPAS) gas sensor. The Allan plot provides information about the optimum averaging time for the QEPAS signal and allows the prediction of its ultimate detection limit. The Allan deviation can also be used to determine the main sources of noise coming from the individual components of the sensor. Quartz tuning fork thermal noise dominates for integration times up to 275 s, whereas at longer averaging times, the main contribution to the sensor noise originates from laser power instabilities. [ABSTRACT FROM AUTHOR]

An innovative quartz enhanced photoacoustic (QEPAS) gas sensing system operating in the THz spectral range and employing a custom quartz tuning fork (QTF) is described. The QTF dimensions are 3.3 cm × 0.4 cm × 0.8 cm, with the two prongs spaced by ∼800 μm. To test our sensor we used a quantum cascade laser as the light source and selected a methanol rotational absorption line at 131.054 cm−1 (∼3.93 THz), with line-strength S = 4.28 × 10−21 cm mol−1. The sensor was operated at 10 Torr pressure on the first flexion QTF resonance frequency of 4245 Hz. The corresponding Q-factor was 74 760. Stepwise concentration measurements were performed to verify the linearity of the QEPAS signal as a function of the methanol concentration. The achieved sensitivity of the system is 7 parts per million in 4 seconds, corresponding to a QEPAS normalized noise-equivalent absorption of 2 × 10−10 W cm−1 Hz−1/2, comparable with the best result of mid-IR QEPAS systems. [ABSTRACT FROM AUTHOR]

Infrared laser technology over the last decades has led to an increasing demand for optical detectors with high sensitivity and a wide operative spectral range suitable for spectroscopic applications. In this work, we report on the performance of a custom quartz tuning fork used as a sensitive and broadband infrared photodetector for absorption spectroscopy. The photodetection process is based on light impacting on the tuning fork and creating a local temperature increase that generates a strain field. This light-induced, thermoelastic conversion produces an electrical signal proportional to the absorbed light intensity due to quartz piezoelectricity. A finite-element-method analysis was used to relate the energy release with the induced thermal distribution. To efficiently exploit the photo-induced thermoelastic effects in the low-absorbance spectral region of quartz also, chromium/gold layers, acting as opaque surface, have been deposited on the quartz surface. To demonstrate the flat response as photodetectors, a custom tuning fork, having a fundamental resonance frequency of 9.78 kHz and quality factor of 11 500 at atmospheric pressure, was employed as photodetector in a tunable diode laser absorption spectroscopy setup and tested with five different lasers with emission wavelength in the 1.65–10.34 μm range. A spectrally flat responsivity of ∼2.2 kV/W was demonstrated, corresponding to a noise-equivalent power of 1.5 nW/√Hz, without employing any thermoelectrical cooling systems. Finally, a heterodyne detection scheme was implemented in the tunable diode laser absorption spectroscopy setup to retrieve the resonance properties of the quartz tuning fork together with the gas concentration in a single, fast measurement. [ABSTRACT FROM AUTHOR]

A theoretical model describing the acoustic coupling between two resonator tubes in spectrophones exploiting custom-made quartz tuning forks (QTFs) is proposed. The model is based on an open-end correction to predict the optimal tube length. A calculation of the sound field distribution from one tube exit allowed for the estimation of the optimal radius as a function of the QTF prong spacing and the sound wavelength. The theoretical predictions have been confirmed using experimental studies employing a custom QTF with a fundamental flexural mode resonance frequency of 15.8 kHz and a quality factor of 15,000 at atmospheric pressure. The spacing between the two prongs was 1.5 mm. Spectrophones mounting this QTF were implemented for the quartz-enhanced photoacoustic detection of water vapor in air in the mid-infrared spectral range. [ABSTRACT FROM AUTHOR]

An optical gas sensing technique based on in-plane quartz-enhanced photoacoustic spectroscopy (IP-QEPAS) is reported. In IP-QEPAS, the laser beam is aligned in the plane of the quartz tuning fork (QTF) to increase the interaction area between the acoustic wavefront and the QTF. A custom T-shaped QTF with a prong length of 9.4 mm and a resonance frequency of 9.38 kHz was designed and employed in the IP-QEPAS sensor. For comparison, the traditional QEPAS sensor in which the laser beam is perpendicular to the QTF plane (PP-QEPAS) is investigated with the same operating conditions. Theoretical calculations of strain and displacement of the QTF prong were performed to support the advantage of using the IP-QEPAS technique. By selecting water vapor as the gas target, the IP-QEPAS sensor results in a signal more than 40 times higher than that measured with the PP-QEPAS configuration, confirming the potential of this approach. [ABSTRACT FROM AUTHOR]

A highly sensitive trace gas sensor based on light-induced thermoelastic spectroscopy (LITES) and a custom quartz tuning fork (QTF) is reported. The QTF has a T-shaped prong geometry and grooves carved on the prongs' surface, allowing a reduction of both the resonance frequency and the electrical resistance but retaining a high resonance quality factor. The base of the QTF prongs is the area maximizing the light-induced thermoelastic effect. The front surface of this area was left uncoated to allow laser transmission through the quartz, while on the back side of the QTF, a gold film was coated to back-reflect the laser beam and further enhance the light absorption inside the crystal. Acetylene (C2H2) was chosen as the target gas to test and validate the LITES sensor. We demonstrated that the sensor response scales linearly with the laser power incident on the prong base, and the optimum signal to noise ratio was obtained at an optical power of 4 mW. A minimum detection limit of ∼325 ppb was achieved at an integration time of 1 s, corresponding to a normalized noise equivalent absorption coefficient of 9.16 × 10−10 cm−1W/√Hz, nearly one order of magnitude better with respect to the value obtained with a standard 32.768 kHz QTF-based LITES sensor under the same experimental conditions. [ABSTRACT FROM AUTHOR]

A ppb-level mid-IR quartz-enhanced photoacoustic sensor for the highly sensitive detection of dimethyl methyl phosphonate (DMMP) —the sarin simulant—is developed using a T-shaped tuning fork. The sensor employs a narrow-linewidth mid-IR quantum cascade laser (QCL) to target a strong, interference-free DMMP absorption band. A custom quartz tuning fork with 800-μm prong spacing is employed to avoid significant photothermal noise, thus removing the requirement for an optical spatial filter. The performance of the photoacoustic sensor was optimized in terms of exciting power, modulation frequency, and gas pressure. A minimum detection limit of 6 ppb was achieved at an integration time of 300 ms, which can be further improved to 0.45 ppb at the optimal integration time of 20 s • A ppb-level photoacoustic sensor for the sarin simulant detection was developed. • A large-gap-spacing quartz tuning fork was used to decrease noise. • A minimum detection limit of 0.45 ppb was achieved at an optimal integration time of 20 s. • The high selectivity of the sensor was verified in a real environment. [ABSTRACT FROM AUTHOR]

A single-tube on-beam quartz enhanced photoacoustic spectroscopy (SO-QEPAS) spectrophone, which employs a custom-made quartz tuning fork (QTF) having a prong spacing of 700 lm and operating at the 1st overtone flexural mode, is reported. The design of QTF prong geometry allows the bare QTF to possess twice higher Q-factor values for the 1st overtone resonance mode falling at ∼17.7 kHz than in the fundamental resonance mode at ∼2.8 kHz, resulting in an 8 times higher QEPAS signal amplitude when operating in the 1st overtone resonance mode. Both the vertical position and length of the single-tube acoustic micro-resonator (AmR) were optimized to attain optimal spectrophone performance. Benefiting from the high overtone resonance frequency and the quasi 1st harmonic acoustic standing waves generated in the SO-QEPAS configuration, the AmR length is reduced to 14.5 mm. This allows the realization of compact spectrophone and facilitates the laser beam alignment through the QTF þ AmR system. The signal enhancement in the overtone resonance mode and the high acoustic coupling efficiency between the AmR and QTF in the SOQEPAS configuration yields an overall sensitivity enhancement factor of ∼with respect to the bare custom QTF operating in the fundamental resonance mode. [ABSTRACT FROM AUTHOR]

A novel trace gas sensor exploiting fiber-ring laser intracavity quartz-enhanced photoacoustic spectroscopy (FLI-QEPAS) is reported. The gas sensor system couples an erbium-doped fiber-ring laser with a custom-designed quartz tuning fork (QTF) operating at 7.2 kHz. The QTF is placed in the fiber cavity to fully utilize intracavity optical power for photoacoustic excitation. We obtained a short intracavity absorption length of 1.5 cm, significantly extending the linear dynamic range of the gas sensor to an order of 10 5 . As a proof of principle, the fiber laser was tuned to the C 2 H 2 absorption line at 1531.6 nm. Operating at the optimal gas pressure and modulation depth selected for wavelength modulation spectroscopy, a minimum detectable C 2 H 2 concentration of 29 ppbv at 300-s integration time has been achieved. [ABSTRACT FROM AUTHOR]

We demonstrated a 19-inches 6U-rack sensing system based on quartz-enhanced photoacoustic spectroscopy for simultaneous detection of methane, nitrous oxide, and ammonia in atmosphere. The system embeds two detection modules, each one equipped with a quantum cascade laser as light source and a spectrophone composed of a T-shaped quartz tuning fork coupled with acoustic resonator tubes. The sensing system results compact, lightweight, and portable, with a computer interface for an easy end-user operation. The calibration was performed in laboratory environment with certified concentrations. Because of the NH 3 large and permanent dipole moment, ammonia QEPAS signal was used to model and characterize adsorption/desorption processes, for an accurate and reliable sensor calibration. At 0.1 s signal integration time, detection limits of 28 ppb, 9 ppb, and 6 ppb were obtained for CH 4 , N 2 O, and NH 3 , respectively. Since these detection limits were well below the natural abundance of the three analytes in atmosphere, the sensor was employed to monitor CH 4 , N 2 O, and NH 3 concentrations in laboratory air. • Development of a compact 19-inches 6U-rack QEPAS sensor box for simultaneous detection of CH 4 , N 2 O, and NH 3 in atmosphere. • Minimum detection limits of 28.0 ppb, 9 ppb and 6 ppb at 0.1 s integration time for CH 4 , N 2 O and NH 3 detection, respectively. • Sensor tested by analysing ambient air. Mean concentrations of ∼ 1.81 ppm of CH 4 , 329 ppb of N 2 O and 83 ppb of NH 3 measured. [ABSTRACT FROM AUTHOR]

• A QEPAS-based sensor with V-T relaxation self-calibration was developed. • A dual-gas detection was realized based on two-mode combined vibration of a QTF. • A self-calibration method for the CH 4 V-T relaxation was provided. • The ppb-level self-calibration CH 4 sensor has a simple setup and small sensor size. • Continuous measurements near a landfill in Shanxi province, China, were performed. A quartz-enhanced photoacoustic spectroscopy methane (CH 4) sensor with vibrational to translational (V-T) relaxation self-calibration was realized and tested for atmospheric CH 4 detection near a landfill. To normalize the influence of H 2 O vapor on the CH 4 energy relaxation rate, CH 4 and H 2 O concentrations were detected simultaneously by means of a frequency division multiplexing technique, in which a custom quartz tuning fork was operated in the fundamental and first overtone combined vibration mode. A continuous wave, thermoelectrically cooled distributed feedback interband cascade laser emitting at 3.3 μm and a near-infrared DFB laser emitting at 1.37 μm were used as the excitation source for CH 4 and H 2 O detection, respectively. A theoretical model of V-T relaxation and self-calibration method were developed to allow this CH 4 sensor to have a simple setup and a small sensor size. Continuous field measurements were carried out near the largest sanitary landfill in Shanxi province, China, to demonstrate the stability and ruggedness of the realized CH 4 sensor. [ABSTRACT FROM AUTHOR]