kHz Quartz Tuning Fork Research Articles

High-sensitivity methane detection based on QEPAS and H-QEPAS technologies combined with a self-designed 8.7 kHz quartz tuning fork

Methane (CH4) is a greenhouse gas as well as being flammable and explosive. In this manuscript, quartz-enhanced photoacoustic spectroscopy (QEPAS) and heterodyne QEPAS (H-QEPAS) exploring a self-designed quartz tuning fork (QTF) with resonance frequency (f0) of ∼8.7 kHz was utilized to achieve sensitive CH4 detection. Compared with the standard commercial 32.768 kHz QTF, this self-designed QTF with a low f0 and large prong gap has the merits of long energy accumulation time and low optical noise. The strongest line located at 6057.08 cm−1 in the 2v3 overtone band of CH4 was chosen as the target absorption line. A diode laser with a high output power of > 30 mW was utilized as the excitation source. Acoustic micro-resonators (AmRs) were added to the sensor architecture to amplify the intensity of acoustic waves. Compared to the bare QTF, after the addition of AmRs, a signal enhancement of 149-fold and 165-fold were obtained for QEPAS and H-QEPAS systems, respectively. The corresponding minimum detection limits (MDLs) were 711 ppb and 1.06 ppm for QEPAS and H-QEPAS sensors. Furthermore, based on Allan variance analysis the MDLs can be improved to 19 ppb and 27 ppb correspondingly. Compared to the QEPAS sensor, the H-QEPAS sensor shows significantly shorter measurement timeframes, allowing for measuring the gas concentration quickly while simultaneously obtaining f0 of QTF.

Dual-band light-induced thermoelastic spectroscopy utilizing an antiresonant hollow-core fiber-based gas absorption cell

In this paper, dual-band gas detection using a combination of the light-induced thermoelastic spectroscopy (LITES) and an antiresonant hollow-core fiber-based (ARHCF) gas absorption cell is demonstrated. The broad wavelength operation capability of a standard 32 kHz quartz tuning fork and the self-developed fiber-based gas absorption cell was exploited to demonstrate quasi-simultaneous detection of N2O and CO2, at 4570 nm (2188.2 cm−1) and 2006 nm (4985.9 cm−1), respectively. The signal analysis was based on the wavelength modulation spectroscopy technique, allowing to achieve a noise equivalent absorption coefficient (NEA) of 8.6 × 10–7 cm−1 and 1.7 × 10–6 cm−1 for N2O and CO2, respectively. The results indicate that the combination of ARHCFs with the LITES method is well suited for the design of broadband gas detectors and show remarkable potential in the fabrication of miniaturized, versatile and relatively inexpensive gas sensors operating over a wide spectral range, thus allowing multigas detection.

Continuous real-time monitoring of carbon dioxide emitted from human skin by quartz-enhanced photoacoustic spectroscopy

In this study, a skin gas detection system based on quartz enhanced photoacoustic spectroscopy (QEPAS) with a constant temperature collection chamber and an automatic frequency adjustment function was used to collect and monitor carbon dioxide (CO2) emissions from human skin. The detection element of the system is an on-beam structure assembled by a 30.72 kHz quartz tuning fork (QTF). A laser with a wavelength of 4991.26 cm−1 is emitted (with a wavelength adjustment range of 10 cm−1) to excite the QTF. When the integration time is 365 s, the system can achieve a minimum detection limit (MDL) of 2.6 ppmv. The sensitivity of the system is 636.9 ppmv/V. The gas detection system is used to monitor the concentration of CO2 emissions from different parts of the skin and the same part covered by different cosmetics. The CO2 emission rate is defined as the ratio of the skin gas monitoring time of 25 min to the CO2 concentration variable in the gas chamber (volume of 8 mL). The results were collected from three healthy volunteers. Among the six different parts, the cheeks emitted the fastest rate (the average rate was 365.5 ppmv/min) of CO2, and the thighs emitted the slowest rate (the average rate was 56.4 ppmv/min) of CO2. Comparing the experimental results of the six sites at different times, the order of the CO2 emission rate is identical for all six sites. In the experiments with the three cosmetic products (experimental site: forearm), comparing the CO2 emission rate from clean skin with the CO2 emission rate from cosmetic-covered skin shows that sunscreen is the most breathable, followed by barrier cream, and foundation is the least breathable.

Methane and ethane detection from natural gas level down to trace concentrations using a compact mid-IR LITES sensor based on univariate calibration

A gas sensor based on light-induced thermo-elastic spectroscopy (LITES) capable to detect methane (C1) and ethane (C2) in a wide concentration range, from percent down to part-per-billion (ppb), is here reported. A novel approach has been implemented, exploiting a compact sensor design that accommodates both a custom 9.8 kHz quartz tuning fork (QTF) used as photodetector and the gas sample in the same housing. The resulting optical pathlength was only 2.5 cm. An interband cascade laser (ICL) with emission wavelength of 3.345 µm was used to target absorption features of C1 and C2. The effects of high concentration analytes on sensor response were firstly investigated. C1 concentration varied from 1% to 10%, while C2 concentration varied from 0.1% to 1%. These ranges were selected to retrace the typical natural gas composition in a 1:10 nitrogen dilution. The LITES sensor was calibrated for both the gas species independently and returned nonlinear but monotonic responses for the two analytes. These univariate calibrations were used to retrieve the composition of C1-C2 binary mixtures with accuracy higher than 98%, without the need for further data analysis. Minimum detection limits of ∼650 ppb and ∼90 ppb were achieved at 10 s of integration time for C1 and C2, respectively, demonstrating the capability of the developed LITES sensor to operate with concentration ranges spanning over 6 orders of magnitude.

High-power near-infrared QEPAS sensor for ppb-level acetylene detection using a 28 kHz quartz tuning fork and 10 W EDFA.

A high-power near-infrared (NIR) quartz enhanced photoacoustic spectroscopy (QEPAS) sensor for part per billion (ppb) level acetylene (C2H2) detection was reported. A 1536 nm distributed feedback (DFB) diode laser was used as the excitation light source. Cooperated with the laser, a C-band 10 W erbium-doped fiber amplifier (EDFA) was employed to boost the optical excitation power to improve QEPAS detection sensitivity. A pilot line manufactured quartz tuning fork (QTF) with a resonance frequency of 28 kHz was used as the photoacoustic transducer. In the case of high excitation power, gas flow effect and temperature effect were found and studied. Benefitting from the low QTF resonance frequency, high excitation power, and vibrational-translational (V-T) relaxation promoter, a detection limit of ∼7 ppb was achieved for C2H2 detection, corresponding to a normalized noise equivalent absorption coefficient of 4.4×10-8cm-1 · W · Hz-1/2.

Integrated near-infrared QEPAS sensor based on a 28 kHz quartz tuning fork for online monitoring of CO2 in the greenhouse

In this paper, a highly sensitive and integrated near-infrared CO2 sensor was developed based on quartz-enhanced photoacoustic spectroscopy (QEPAS). Unlike traditional QEPAS, a novel pilot line manufactured quartz tuning fork (QTF) with a resonance frequency f0 of 28 kHz was employed as an acoustic wave transducer. A near-infrared DFB laser diode emitting at 2004 nm was employed as the excitation light source for CO2 detection. An integrated near-infrared QEPAS module was designed and manufactured. The QTF, acoustic micro resonator (AmR), gas cell, and laser fiber are integrated, resulting in a super compact acoustic detection module (ADM). Compared to a traditional 32 kHz QTF, the QEPAS signal amplitude increased by > 2 times by the integrated QEPAS module based on a 28 kHz QTF. At atmospheric pressure, a 5.4 ppm detection limit at a CO2 absorption line of 4991.25 cm−1 was achieved with an integration time of 1 s. The long-term performance and stability of the CO2 sensor system were investigated using Allan variance analysis. Finally, the minimum detection limit (MDL) was improved to 0.7 ppm when the integration time was 125 s. A portable CO2 sensor system based on QEPAS was developed for 24 h continuous monitoring of CO2 in the greenhouse located in Guangzhou city. The CO2 concentration variations were clearly observed during day and night. Photosynthesis and respiration plants can be further researched by the portable CO2 sensor system.

Wavelength scanning Q-switched fiber-ring laser intra-cavity QEPAS using a standard 32.76 kHz quartz tuning fork for acetylene detection

A novel fiber-ring laser intra-cavity quartz-enhanced photoacoustic spectroscopy (FLI-QEPAS) gas sensor utilizing wavelength scanning Q-switched technique is reported for acetylene detection. The system uses erbium-doped fiber as the gain medium for laser generation and a custom-made fiber Bragg grating attached to a piezoelectric transducer for wavelength scanning to cover the C2H2 absorption line at 1531.58 nm. A standard quartz tuning fork (QTF) operating at 32.76 kHz is placed inside the fiber-ring cavity to fully utilize intra-cavity optical power, realizing an ultra-short absorption path of 10 mm. Acousto-optic Q-switched working mode is employed to enable the fiber-ring laser working in high frequency enough for standard QTF excitation, up to ≃3 W intra-cavity peak power is achieved with 650 mW pump power at 980 nm. In acetylene detection experiments, the system realizes a noise equivalent concentration of 26 ppbv at 50 ms integration time, and Allan deviation shows the minimum detection limit can be improved to 2.8 ppbv at 51 s integration time. A linear dynamic range up to 10,000 ppmv is validated and the stability performance is also analyzed.

Properties of the 100 kHz Quartz Tuning Forks in Strong Magnetic Fields and Very Low Temperatures

We present the properties of the standard, commercial 100 kHz quartz tuning forks at very low temperatures and high magnetic fields up to 8 Tesla. We show that the resonance frequency of the tuning forks depends weakly on the strength of magnetic field. This makes the quartz tuning forks a promising low-temperature thermometer having the B/T ratio up to 1000. We discuss the physical origin of the observed experimental results.

Ppb-Level Quartz-Enhanced Photoacoustic Detection of Carbon Monoxide Exploiting a Surface Grooved Tuning Fork.

A compact and sensitive carbon monoxide (CO) sensor was demonstrated by using quartz enhanced photoacoustic spectroscopy (QEPAS) exploiting a novel 15.2 kHz quartz tuning fork (QTF) with grooved surfaces. The custom QTF was designed to provide a quality factor as high as 15 000 at atmospheric pressure, which offers a high detection sensitivity. A large QTF prong spacing of 800 μm was selected, allowing one to avoid the use of any spatial filters when employing a quantum cascade laser as the excitation source. Four rectangular grooves were carved on two prong surfaces of the QTF to decrease the electrical resistance and hence enhance the signal amplitude. With water vapor as the catalyst for vibrational energy transfer, the sensor system using the novel surface grooved QTF achieved a CO minimum detection limit of 7 ppb for a 300 ms averaging time, which corresponds to a normalized noise equivalent absorption coefficient of 8.74 × 10-9 cm-1W /√Hz. Continuous measurements covering a seven-day period for atmospheric CO were implemented to verify the reliability and validity of the developed CO sensor system.

Ultra-high sensitive acetylene detection using quartz-enhanced photoacoustic spectroscopy with a fiber amplified diode laser and a 30.72 kHz quartz tuning fork

An ultra-high sensitive acetylene (C2H2) Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a high power laser and a quartz tuning fork with a resonance frequency f0 of 30.72 kHz was demonstrated. An erbium-doped fiber amplifier (EDFA) amplified distributed feedback diode laser with a center wavelength of 1.53 μm was used as the exciting source. A 33.2 ppb minimum detection limit (MDL) at 6534.37 cm−1 was achieved, and the calculated normalized noise equivalent absorption coefficient was 3.54 × 10−8 cm−1 W/√Hz when the laser output power was 1500 mW. The ppb-level detection sensitivity of C2H2 validated the reported QEPAS method.

High Q value Quartz Tuning Fork in Vacuum as a Potential Thermometer in Millikelvin Temperature Range

The results of a newly developed pulse-demodulation (P-D) technique introduced to determine the resonant characteristics of a high Q value quartz tuning forks in vacuum and millikelvin temperature range are presented. Applying P-D technique to a standard 32 kHz quartz tuning fork with extremely low excitation energy of the order of a few femtojoules, we were able to measure the resonance frequency of the fork’s decay signal with resolution better than 10 $$\upmu $$ Hz. Using this highly sensitive measurement technique, we found a continuous and reproducible temperature dependence of the tuning fork’s resonance frequency in the millikelvin temperature range. The observed dependence suggests a potential application for the quartz tuning forks to be used as thermometers in the millikelvin temperature range. We also discuss the physical origin of the observed phenomenon.

Off-axis quartz-enhanced photoacoustic spectroscopy using a pulsed nanosecond mid-infrared optical parametric oscillator.

A trace-gas sensor, based on quartz-enhanced photoacoustic spectroscopy (QEPAS), consisting of two acoustically coupled micro-resonators (mR) with an off-axis 20 kHz quartz tuning fork (QTF) is demonstrated. The complete acoustically coupled mR system is optimized based on finite-element simulations and is experimentally verified. The QEPAS sensor is pumped resonantly by a nanosecond pulsed single-mode mid-infrared optical parametric oscillator. The sensor is used for spectroscopic measurements on methane in the 3.1-3.5 μm wavelength region with a resolution bandwidth of 1 cm^-1 and a detection limit of 0.8 ppm. An Allan deviation analysis shows that the detection limit at the optimum integration time for the QEPAS sensor is 32 ppbv at 190 s, and that the background noise is due solely to the thermal noise of the QTF.

Theoretical and experimental study of the dynamic response of absorber-based, micro-scale, oscillatory probes for contact sensing applications.

This paper presents two models for predicting the frequency response of micro-scale oscillatory probes. These probes are manufactured by attaching a thin fiber to the free end of one tine of a quartz tuning fork oscillator. In these studies, the attached fibers were either 75 μm diameter tungsten or 7 μm diameter carbon with lengths ranging from around 1 to 15 mm. The oscillators used in these studies were commercial 32.7 kHz quartz tuning forks. The first theoretical model considers lateral vibration of two beams serially connected and provides a characteristic equation from which the roots (eigenvalues) are extracted to determine the natural frequencies of the probe. A second, lumped model approximation is used to derive an approximate frequency response function for prediction of tine displacements as a function of a modal force excitation corresponding to the first mode of the tine in the absence of a fiber. These models are used to evaluate the effect of changes in both length and diameter of the attached fibers. Theoretical values of the natural frequencies of different modes show an asymptotic relationship with the length and a linear relationship with the diameter of the attached fiber. Similar results are observed from experiment, one with a tungsten probe having an initial fiber length of 14.11 mm incrementally etched down to 0.83 mm, and another tungsten probe of length 8.16 mm incrementally etched in diameter, in both cases using chronocoulometry to determine incremental volumetric material removal. The lumped model is used to provide a frequency response again reveals poles and zeros that are consistent with experimental measurements. Finite element analysis shows mode shapes similar to experimental microscope observations of the resonating carbon probes. This model provides a means of interpreting measured responses in terms of the relative motion of the tine and attached fibers. Of particular relevance is that, when a "zero" is observed in the response of the tine, one mode of the fiber is matched to the tine frequency and is acting as an absorber. This represents an optimal condition for contact sensing and for transferring energy to the fiber for fluid mixing, touch sensing, and surface modification applications.

Compact all-fiber quartz-enhanced photoacoustic spectroscopy sensor with a 30.72 kHz quartz tuning fork and spatially resolved trace gas detection

An ultra compact all-fiber quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor using quartz tuning fork (QTF) with a low resonance frequency of 30.72 kHz was demonstrated. Such a sensor architecture has the advantages of easier optical alignment, lower insertion loss, lower cost, and more compact compared with a conventional QEPAS sensor using discrete optical components for laser delivery and coupling to the QTF. A fiber beam splitter and three QTFs were employed to perform multi-point detection and demonstrated the potential of spatially resolved measurements.

The coefficient of the voltage induced frequency shift measurement on a quartz tuning fork.

We have measured the coefficient of the voltage induced frequency shift (VIFS) of a 32.768 KHz quartz tuning fork. Three vibration modes were studied: one prong oscillating, two prongs oscillating in the same direction, and two prongs oscillating in opposite directions. They all showed a parabolic dependence of the eigen-frequency shift on the bias voltage applied across the fork, due to the voltage-induced internal stress, which varies as the fork oscillates. The average coefficient of the VIFS effect is as low as several hundred nano-Hz per millivolt, implying that fast-response voltage-controlled oscillators and phase-locked loops with nano-Hz resolution can be built.

Quartz Tuning Forks as Cryogenic Vacuum Gauges

We report measurements of the mechanical $$Q$$ of a 32.7 kHz quartz tuning fork as a function of pressure for helium and argon at T $$=$$ 300 K and for helium in the temperature range 7.0–0.7 K. In the low pressure ballistic regime, the damping due to the surrounding gas is inversely proportional to $$P$$ , while for higher pressures, a hydrodynamic treatment accounts for most of the variation of $$Q$$ with $$P$$ . We have combined the ballistic and hydrodynamic models together with calculations of the thermal transpiration correction to correlate the tuning fork $$Q$$ at low temperature with the pressure measured with a room temperature pressure gauge. The fork was found to be useful as an in situ pressure gauge for pressures above $$\sim $$ 0.1 mTorr. A dissipation peak and frequency drop associated with the superfluid transition in the adsorbed helium film is also observed for $$T<1.4$$ K.

Mechanical response of 4He films adsorbed on single-crystalline graphite

We carried out quartz crystal microbalance (QCM) experiments of a 32 kHz quartz tuning fork for 4He films adsorbed on exfoliated single-crystalline graphite and measured the temperature dependence of the resonance frequency and Q value for various areal densities. To study the sliding direction dependence, two quartz tuning forks oscillating in the different crystal directions were prepared. It was found that the resonance frequency does not decrease greatly in the monolayer region and the decoupling from the oscillating substrate is enhanced compared with Grafoil. We observed, however, a weak decrease in frequency around the commensurate phase of monolayer and this decrease shows the sliding direction dependence.

Mutual interactions between objects oscillating in isotopically pure superfluid 4He in the T → 0 limit

We report the results of experiments to explore interactions between physically separated oscillating objects in isotopically pure superfluid 4He at T ∼ 10 mK. The investigations focused mainly on 32 kHz quartz tuning forks, but also consider a nearby 1 kHz oscillating grid. The low-drive linewidth (LDL) and resonant frequency fd of a detector fork were monitored while the maximum velocity of a transmitter fork, separated from the detector by a few mm, was varied over a wide range. Clear evidence was found for mutual interactions between the two forks, and for the influence of the grid on the forks. Monitoring the detector's LDL and fd provides evidence for a generator critical velocity in the range 0.3&amp;lt;υc1&amp;lt;1.0 cm/s for onset of the detector responses, in addition to a second critical velocity υc2∼13 cm/s probably corresponding to the production of quantum turbulence at the generator. The results are discussed, but are not yet fully understood.

Mechanical Response of 4He Films Adsorbed on Graphite with a Quartz Tuning Fork

We have carried out quartz-crystal microbalance (QCM) experiments of 32 kHz quartz tuning fork for 4He films adsorbed on Grafoil, and have measured the temperature dependence of the resonance frequency and Q value for various areal densities. It was found that the frequency does not decrease exactly in proportion to the areal density. This means that the film still undergoes decoupling partly, although it is strongly suppressed from that of 5 MHz QCM measurements. Above the three-atom thick film, the decoupling due to the superfluidity of the overlayer is observed. In addition, we found that the competition between the superfluidity and the slippage takes place for a large oscillation amplitude. From the comparison with 5 MHz QCM measurements, it is concluded that the acceleration of substrate plays an important role in the slippage.

Atomic force microscopy imaging using a tip-on-chip: Opening the door to integrated near field nanotools

We describe in detail how atomic force microscopy (AFM) images can be routinely achieved with macroscopic silicon-based chips integrating mesoscopic tips, paving the way for the development of new near field devices combining AFM imaging with any kind of functionality integrated on a chip. The chips have been glued at the end of the free prong of 100 kHz quartz tuning forks mounted in Qplus configuration. Numerical simulations by modal analysis have been carried out to clarify the nature of the vibration modes observed in the experimental spectra. It is shown that two low frequency modes can be used to drive the system and scan the surface with a great stability in amplitude modulation as well as in frequency modulation AFM under ultrahigh vacuum. The AFM capabilities are demonstrated through a series of examples including phase and dissipation contrast imaging, force spectroscopy measurements, and investigations of soft samples in weak interaction with the substrate. The lateral resolution with the tips grown by focused ion beam deposition already matches the one achieved in standard amplitude modulation mode AFM experiments.