Search

The increase in fatal accidents and chronic illnesses caused by hydrogen sulfide (H

Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a highly sensitive optical technique, suitable for real-time and in situ trace gas detection. In QEPAS, Quartz tuning forks (QTF) are employed as piezoelectric transducers of sound waves induced by gas non-radiative energy relaxation as consequence of infrared modulated light absorption. The generated electric signal depends on the gas concentration. An accurate and reliable QEPAS measurement requires the QTF characterization, in terms of resonance frequency and quality factor. Furthermore, tens of seconds are required to complete the tuning range scan of the laser employed to detect the selected gas. Beat frequency QEPAS (BF-QEPAS) is an alternative approach to standard QEPAS. In BF-QEPAS, a fast scan of the laser tuning range is employed to generate an acoustic pulse. Gas concentration, QTF resonance frequency, and quality factor can be measured acquiring and analyzing the transient response of the QTF to the acoustic pulse. In this work, a custom T-shaped QTF was employed to detect nitrogen monoxide (NO), targeting its absorption feature at 1900.07 cm−1 with an interband cascade laser. A minimum detection limit as low as 166 ppb of NO, and a highly accurate measurement of the QTF resonance frequency and quality factor were demonstrated using BF-QEPAS.

In the past decade, the rapid development of infrared laser technology has led to an increasing demand for photodetectors with high sensitivity and a wide operative spectral range suitable for spectroscopic applications [1-2]. In this work, we report on the performance of a custom quartz tuning fork (QTF), having a fundamental resonance frequency of 9.78 kHz and quality factor of 11500 at atmospheric pressure, which was used as a sensitive and broadband infrared photodetector for laser absorption spectroscopy [3]. Fourier infrared spectrometer was used to characterize the infrared absorption capacity of quartz material at the wavelength of 1-20 μm. Wide spectral response capability of the used QTF detector was investigated based on tunable diode absorption spectroscopy using lasers operating at five different wavelengths (1.6-10.35 μm). A spectrally flat responsivity of ~2.2 kV/W was demonstrated, corresponding to a noise-equivalent power of 1.5 nW/Hz1/2, without employing any thermoelectrical cooling systems. In order to compensate for the drift of inherent characteristics (resonance frequency and quality factors) of the QTF detector, a heterodyne detection scheme was implemented to retrieve the resonance properties of the QTF detector together with the gas concentration in a single, fast measurement [4]. Experimental details including theoretical simulation and application demonstration will be discussed and presented.AcknowledgmentsThe authors acknowledge financial support from National Key R&D Program of China (No. 2019YFE0118200), THORLABS GmbH, within PolySense, a joint-research laboratory, and the National Natural Science Foundation of China (Nos. 62075119 and 61805132).References[1] L. Dong, F. K. Tittel, C. Li, N. P. Sanchez, H. Wu, C. Zheng, Y. Yu, A. Sampaolo, and R. J. Griffin, Opt. Express 24 (2016) A528-A535.[2] S. Dello Russo, A. Zifarelli, P. Patimisco, A. Sampaolo, T. Wei, H. Wu, L. Dong, and V. Spagnolo, Opt. Express 28 (2020) 19074-19084.[3] T. Wei, A. Zifarelli, S. Dello Russo, H. Wu, G. Menduni, P. Patimisco, A. Sampaolo, V. Spagnolo, L. Dong, Appl. Phys. Rev. 8 (2021) 041409.[4] H. Wu, L. Dong, H. Zheng, Y. Yu, W. Ma, L. Zhang, W. Yin, L. Xiao, S. Jia, and F. K. Tittel, Nat. Commun. 8 (2017) 15331.

We report on the realization, calibration, and test outdoor of a 19-inches rack 3-units sized Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) trace gas sensor designed for real-time carbon monoxide monitoring in ambient air. Since CO acts as a slow energy relaxer when excited in the mid-infrared spectral region, its QEPAS signal is affected by the presence of relaxation promoters, such as water vapor, or quenchers like molecular oxygen. We analyzed in detail all the CO relaxation processes with typical collisional partners in an ambient air matrix and used this information to evaluate oxygen and humidity-related effects, allowing the real CO concentration to be retrieved. The sensor was tested outdoor in a trafficked urban area for several hours providing results comparable with the daily averages reported by the local air inspection agency, with spikes in CO concentration correlated to the passages of heavy-duty vehicles.

The development of a dual-gas quartz-enhanced photoacoustic (QEPAS) sensor capable of simultaneous detection of water vapor and alternatively methane or nitrous oxide is reported. A diode laser and a quantum cascade laser (QCL) excited independently and simultaneously both the fundamental and the first overtone flexural mode of the quartz tuning fork (QTF), respectively. The diode laser targeted a water absorption line located at 7181.16 cm

Hydrocarbon detection in the gas phase represents a powerful tool to guide oil exploration and production operations for the oil & gas industry. This application requires highly sensitive, selective and robust spectroscopic techniques. In this work, a quartz-enhanced photoacoustic sensor system designed to detect methane (C1), ethane (C2) and propane (C3), employing a single interband cascade laser emitting in the spectral range 3.342–3.349 μm, is reported. Detection levels in the part-per billion concentration range for C1 and C2 and a few parts per million for C3 were achieved. Measurements at both low and atmospheric pressures were carried out for mixtures simulating typical downhole hydrocarbon concentrations.

We report on the detection of the isotopologues 12CH4 and 13CH4 by employing a quartz-enhanced photoacoustic spectroscopy (QEPAS)-based sensor. By properly selecting the exciting light source and the working conditions, two absorption lines, having a negligible cross-section ratio temperature coefficient of -6.7‰/°C and a cross section ratio of ~ 0.06 for a natural abundance of each isotope, can be targeted. The QEPAS signal of the two isotopologues was acquired for mixtures in nitrogen of methane in natural abundance in a wide range of concentrations (0.02%-20%) showing a non-linear trend with high concentrations and a constant ratio comparable with the cross-section.

Quartz enhanced photo acoustic spectroscopy (QEPAS) has gained a growing interest in recent years for gas sensing technology because of the high sensitivity provided by sharp resonant tuning forks (QTFs) exploited as piezoelectric sound wave detectors; and the modularity, compactness, and robustness of the sensors. The currently used experimental configurations of the QEPAS sensors still rely on free space optics, which are potentially subject to misalignment and require a relatively high space occupation. Here we propose the modelling and the design of a QEPAS sensor with the laser source and optical components for beam delivery bonded together and semi-integrated with the tuning fork. In particular, we propose a configuration in which an optical resonator placed between the prongs of the QTF is coupled with the laser source through a waveguide (OMRSI-QEPAS). For this setup design, COMSOL simulations provided pressure values of the acoustic wavefront comparable with the standard on-beam configuration employing acoustic resonator tubes. Therefore, the design here proposed aims to package the opto-acoustic core of a QEPAS sensor in a single module of few cubic centimeters to definitively address the misalignment issues and pave the way to a further level of miniaturization, integration and deployment for application like mobile and on-drone sensing.

Graphical abstract, We demonstrated that quartz-enhanced photoacoustic spectroscopy (QEPAS) is an efficient tool to measure the vibrational relaxation rate of gas species, employing quartz tuning forks (QTFs) as sound detectors. Based on the dependence of the QTF resonance frequency on the resonator geometry, a wide range of acoustic frequencies with narrow detection bandwidth was probed. By measuring the QEPAS signal of the target analyte as well as the resonance properties of different QTFs as a function of the gas pressure, the relaxation time can be retrieved. This approach has been tested in the near infrared range by measuring the CH4 (nν4) vibrational relaxation rate in a mixture of 1% CH4, 0.15 % H2O in N2, and the H2O (ν1) relaxation rate in a mixture of 0.5 % H2O in N2. Relaxation times of 3.2 ms Torr and 0.25 ms Torr were estimated for CH4 and H2O, respectively, in excellent agreement with values reported in literature.

Mid-infrared spectral “fingerprints” of many hazardous and harmful gaseous substances are characterized by broad absorption bands, resulting from the merging of several absorption lines corresponding to ro-vibrational transitions. Broadband absorption spectroscopy is fundamental to detect and discriminate these substances. Here we report on a quartz-enhanced photoacoustic spectroscopy sensor for broadband absorbers detection, employing a monolithic array of 32, individually addressable, distributed-feedback quantum cascade lasers as the excitation source. This light source combines a fast tuning speed with a large tuning range, from 1190 cm−1 to 1340 cm−1 together with a high stability. The QEPAS sensor was tested for nitrous oxide (N2O) and methane (CH4) detection, exhibiting absorption features in the laser source emission spectral range. The two broad P- and R- absorption branches of N2O have been accurately reconstructed, as well as several absorption features of CH4, for gas concentrations ranging from 200 to 1000 part-per-million in nitrogen.

Graphical abstract, Highlights • Quartz-enhanced photoacoustic-based broadband detection is demonstrated. • Methane detection sensitivity of 200 ppb in 10 s integration time is achieved. • Multi-gas detection of mixtures of methane and nitrous oxide is demonstrated. • Analytes concentration is retrieved despite absorption spectra significant overlap., Here we report on the broadband detection of nitrous oxide (N2O) and methane (CH4) mixtures in dry nitrogen by using a quartz-enhanced photoacoustic (QEPAS) sensor exploiting an array of 32 distributed-feedback quantum cascade lasers, within a spectral emission range of 1190−1340 cm−1 as the excitation source. Methane detection down to a minimum detection limit of 200 ppb at 10 s lock-in integration time was achieved. The sensor demonstrated a linear response in the range of 200−1000 ppm. Three different mixtures of N2O and CH4 in nitrogen at atmospheric pressure have been analyzed. The capability of the developed QEPAS sensor to selectively determine the N2O and CH4 concentrations was demonstrated, in spite of significant overlap in their respective absorption spectra in the investigated spectral range.

Gas mixtures analysis is a challenging task because of the demand for sensitive and highly selective detection techniques. Partial least squares regression (PLSR) is a statistical method developed as generalization of standard multilinear regression (MLR), widely employed in multivariate analysis for relating two data matrices even with noisy and strongly correlated experimental data. In this work, PLSR is proposed as a novel approach for the analysis of gas mixtures spectra acquired with quartz-enhanced photoacoustic spectroscopy (QEPAS). Results obtained analyzing CO/N2O and CH4/C2H2/N2O gas mixtures are presented. A comparison with standard MLR approach highlights a prediction errors reduction up to 5 times.

In this work, we report on the measurement of methane (CH4) effective non-radiative relaxation rate in a mixture containing 1% of CH4 and 0.15% of water vapor in nitrogen, by using a set of custom quartz tuning forks (QTFs). The dependence of quartz-enhanced photoacoustic spectroscopy (QEPAS) peak signal and QTF quality factor as a function of operating pressure allowed the estimation of the radiation-to-sound conversion efficiency and, consequently, the calculation of the effective relaxation rate of the investigated gas mixture. We measured an effective relaxation rate of 3.2 ms·Torr, in good agreement with values reported in literature.

In oil and gas exploration and environmental monitoring fields is extremely important to rely on versatile and rugged trace gas sensors that can be mounted on remote-controlled unmanned vehicles to monitor dangerous areas inaccessible to a human operator. Thereby, a compact fiber-coupled dual-gas QEPAS sensor employing a custom tuning fork was designed and tested. Two distributed feedback pigtailed diode lasers emitting at 1654 nm and 1684 nm, for methane and ethane detection respectively, were also employed. The gas sensor was calibrated for ethane and methane trace detection and a minimum detection level of 18 parts per million and 570 parts per billion at 1 s integration time were respectively achieved. Methane detection in atmosphere was also performed.

Quartz-enhanced photoacoustic spectroscopy represents a valuable candidate for natural gas analysis in the oil & gas field. The core element of this spectroscopy technique is a quartz tuning forks, employed as high quality-factor optoacoustic transducers, capable of operating in a wide range of temperatures and pressures. The robustness and compactness of these sensors, together with the possibility to avoid the use of optical detectors, open the way to the development of a new generation of small-sized gas spectrometers to be potentially employed downhole for source rock characterization and estimation of oil & gas properties. We report here on the realization of a shoe-box sized quartz-enhanced photoacoustic spectroscopy system capable of: i) selective detection of methane and ethane in the parts-per-billion range, and propane in the parts-per-million range, by employing a single interband cascade laser emitting at 3.345 µm; ii) selective detection of 12CH4 and 13CH4 isotopes at the parts-per-billion sensitivity level by employing a quantum cascade laser operating around 7.73 µm.

We report on a statistical tool based on partial least-squares regression (PLSR) able to retrieve single-component concentrations in a multiple-gas mixture characterized by spectrally overlapping absorption features. Absorption spectra of mixtures of CO–N2O and mixtures of C2H2–CH4–N2O, both diluted in N2, were detected in the mid-IR range by exploiting quartz-enhanced photoacoustic spectroscopy (QEPAS) and using two quantum cascade lasers as light sources. Single-gas reference spectra of each target molecule were acquired and used as PLSR-based algorithm training data set. The concentration range explored in the analysis varies from a few parts-per-million (ppm) to thousands of ppm. Within this concentration range, the influence of the gas matrix on nonradiative relaxation processes can be neglected. Exploiting the ability of PLSR to deal with correlated data, these spectra were used to generate new simulated spectra, i.e., linear combinations of the reference ones. A Gaussian noise distribution was added to the created data set, simulating the real QEPAS signal fluctuations around the peak value. Compared with standard multilinear regression, PLSR predicted gas concentrations with a calibration error up to 5 times better, even with absorption features with spectral overlap greater than 97%.

It is well known that different optical techniques can be used to measure angular velocities. Interferometric Optical Gyroscopes (IFOGs) and Ring Laser Gyroscopes (RLG) exploit the Sagnac effect and are the preferred solutions for tactile grade applications. However, in the Sagnac effect the measured output is directly proportional to the gyroscope sizes. So, the miniaturization and the integration of optical gyroscopes is still a challenge in optoelectronics research. In this paper different approaches based on parity-time and anti-parity-time symmetry are explained and compared as highly performance solutions for the integration of optical gyroscopes. The sensitivity of the rotation-induced frequency-splitting (broadening) can be several orders of magnitude higher than the classical Sagnac splitting achieved with a single ring gyroscope occupying the same surface. We will show that anti-parity-time-symmetric solutions represent a better choice for angular velocity sensing.

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.

A theoretical analysis and experimental investigation of the influence of gas pressure on resonance properties, namely, the quality factor and resonance frequency, of a T-shaped quartz tuning fork (QTF) is reported here. Two configurations are considered: a bare QTF, and a QTF coupled with a pair of resonator tubes (spectrophone). In both configurations, the effect of air on resonance frequency due to the additional inertia on prong motion and the influence of air damping on the quality factor, were analysed. By comparing the bare QTF and the spectrophone results, the effect of pressure on the acoustic coupling between the QTF and the tubes was theoretically modelled and then validated. The results show that acoustic coupling is strongly influenced by air pressure, leading to a shift of resonance frequency and a decrease in the quality factor up to 24%.

We report on a quartz-enhanced photoacoustic (QEPAS) sensor employing a monolithic distributed-feedback quantum cascade laser array operating 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, which covers two absorption branches of nitrous oxide (N2O) and several absorption features of (CH4). The versatility of the QEPAS technique combined with the rapid wavelength tuning provided by the ultra-compact, low-power consuming laser source allowed the detection of N2O and CH4 with detection sensitivities below a part-per-million at atmospheric pressure.

We report on the performance of new quartz tuning fork (QTF) designs optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). We investigated the impact on resonance properties of prong geometries differing from the standard rectangular one. We proposed a QTF with T-shaped prongs and a QTF with prongs having rectangular grooves carved on the surface. QTFs were implemented in a QEPAS sensor and performances were compared in terms of signalto-noise ratio (SNR). Then, QTFs were acoustically coupled with single- and dual-tube micro-resonator systems. A record x60 SNR enhancement factor with respect to the bare QTF was achieved with QTF having T-shaped prongs.

We report on a comparison between two quartz tuning forks (QTFs) employed for quartz-enhanced photoacoustic spectroscopy (QEPAS) having quadrupole and octupole electrode pattern configurations. With respect to the quadrupole, the implementation of the octupole pattern suppresses the fundamental mode and reduces by a factor of ~ 4.4 the electrical resistance for the first overtone mode with negligible variations of the related Q-factors. Both QTFs operating at the first overtone mode were implemented in a QEPAS sensor and the results showed that the octupole configuration provides a ~2.3 signal enhancement factor.

Simultaneous detection of different trace gases is an important topic in many applications, such as breath analysis, environmental monitoring and oil exploration. Quartz-enhanced photoacoustic spectroscopy offers a good solution of this problem, due to the possibility to operate the quartz resonators at the fundamental and overtone resonance frequencies by simultaneously exciting two different antinodes with two different laser sources. In this work we employed a custom spectrophone coupled with a diode laser and a quantum cascade laser for simultaneous detection of water vapor and methane/nitrous oxide with minimum detection limits in the ppb range.

Hydrocarbon detection in the gas phase can be a powerful tool to guide downstream operations for the oil and gas industry. This application requires highly sensitive, selective and robust spectroscopic techniques. In this work we present: i) a quartz-enhanced photoacoustic (QEPAS) sensor that can individually detect methane and ethane in the part per billion range and propane in the ppm range by employing a single interband cascade laser emitting at 3345 nm; ii) a QEPAS sensor detecting 12CH4 and 13CH4 isotopes at the part-per billion sensitivity level, by employing a quantum cascade laser emitting at 7730 nm.

Hydrocarbon detection in the gas phase can be a powerful tool to guide downstream operations for the oil & gas industry. This application requires highly sensitive, selective and robust spectroscopic techniques. In this work, a quartz-enhanced photoacoustic sensor system designed to detect methane and ethane concentration levels in the part-per billion concentration range by employing a single interband cascade laser emitting at 3345 nm is reported. Measurements at both low and high pressure were carried out and mixtures simulating typical downhole compositions were analyzed.