Quartz-enhanced Photoacoustic Spectroscopy Research Articles

QEPAS sensor using a radial resonator

We report on the use of a radial acoustic resonator to increase the acoustic signal of a quartz enhanced photoacoustic spectroscopy (QEPAS) sensor. This approach is an attractive alternative to usual configurations based on microtube longitudinal resonators since it enables to substantially relax laser beam alignment and positioning and thus paves the way towards more cost-effective industrial production. This new QEPAS configuration is first investigated and designed by finite element simulation. It is then experimentally implemented and characterised by detecting acetylene around 1.5 µm. The combination of the radial acoustic resonator with a custom quartz tuning fork (QTF) leads to a QEPAS sensor with a normalised noise equivalent absorption (NNEA) of $$3.9 \times 10^{-9}$$ W cm $$^{-1}$$ Hz $$^{-1/2}$$ which is close to the performance of state-of-the-art QEPAS sensors based on microtube resonators.

An all-optical off-beam quartz-enhanced photoacoustic spectroscopy employing double-pass acoustic microresonators

An all-optical off-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor employing double-pass acoustic microresonators was proposed and experimentally investigated. An optical fiber Fabry–Perot interferometric sensor with quadrature point self-stabilization was employed for the detection of quartz prong vibration. A commercial quartz tuning fork with a resonant frequency of 30.72 kHz was employed as an acoustic transducer. Water vapor as the catalyst was added for improving the vibrational energy transfer. The minimum detection limit of ∼2.41 ppmv was obtained with a 1 s averaging time, which corresponded to a normalized noise equivalent absorption coefficient of 7.76×10−8 cm−1 W Hz−1/2 at the methane absorption line of 6046.95 cm −1 with an optimized modulation depth of 0.172 cm −1. The detection sensitivity was enhanced by a factor of ∼1.48 in comparison to the off-beam QEPAS employing a single-pass microresonator. An Allan deviation analysis indicated that the ultimate detection limit for the QEPAS-based sensor was ∼0.288 ppmv at the optimum averaging time of ∼125 s.

A comprehensive dual-spectroscopy detection technique based on TDLAS and QEPAS using a quartz tuning fork

A comprehensive dual-spectroscopy detection technique based on tunable diode laser absorption spectroscopy (TDLAS) and quartz enhanced photoacoustic spectroscopy (QEPAS) is demonstrated using a single quartz tuning fork (QTF) for signal detection. The QTF was utilized as an acoustic wave transducer for QEPAS signal detection. The QTF also served as a photoelectric detector for TDLAS signal detection based on the thermoelastic effect. The dual-spectroscopy detection structure was designed for TDLAS and QEPAS detection. The on-beam acoustic micro resonator (AMR) structure was placed on the upper end of the QTF for QEPAS signal enhancement, and the laser beam transmitted through the AMR was aligned to the QTF prong by an optical collimator for TDLAS signal detection. An absorption gas cell with an optical path length of 3 m was utilized in the TDLAS setup. The gas cell enhanced the absorption signal of the gas by virtue of its relatively long optical path. We tested the feasibility of the proposed dual-spectroscopy detection technique by detecting acetylene (C2H2) at 1532.83 nm. The experimental show that the signal of the dual-spectroscopy detection technique is approximately 1.13 times that of the QEPAS signal. The proposed dual-spectroscopy detection technique also showed superior gas sensing capability for trace gas detection and could achieve a minimum detection limit of 1.05 ppm. The signal strength of the proposed dual-spectroscopy detection technique can be further enhanced by using an absorption gas cell with a longer optical path.

Radial-cavity quartz-enhanced photoacoustic spectroscopy.

Radial-cavity quartz-enhanced photoacoustic spectroscopy (RC-QEPAS) was proposed for trace gas analysis. A radial cavity with (0,0,1) resonance mode was coupled with the quartz tuning fork (QTF) to greatly enhance the QEPAS signal and facilitate the optical alignment. The coupled resonance enhancement effects of the radial cavity and QTF were analyzed theoretically and researched experimentally. With an optimized radial cavity, the detection sensitivity of QEPAS was enhanced by >1 order of magnitude. The RC-QEPAS makes the acoustic detection module more compact and optical alignment comparable with a bare QFT, benefiting the usage of light sources with poor beam quality.

Passive Electrical Damping of a Quartz Tuning Fork as a Path to Fast Resonance Tracking in QEPAS.

In Quartz-Enhanced PhotoAcoustic Spectroscopy (QEPAS) gas sensors, the acoustic wave is detected by the piezoelectric Quartz Tuning Fork (QTF). Due to its high-quality factor, the QTF can detect very low-pressure variations, but its resonance can also be affected by the environmental variations (temperature, humidity, …), which causes an unwanted signal drift. Recently, we presented the RT-QEPAS technique that consistently corrects the signal drift by continuously measuring the QTF resonance. In this article, we present an improvement of RT-QEPAS to fasten the QTF characterization time by adding a passive electronic circuit, which causes the damping of the QTF resonance. The damping circuit is optimized analytically and through SPICE simulation. The results are supported by experimental observations, showing a 70 times improvement of the relaxation times compared to the lone QTF, which opens the way to a fast and drift-free QEPAS sensor.

Temperature-compensated calibration for quartz-enhanced photoacoustic spectroscopy by thermoelastic effects

A light-induced thermoelastic calibration method (TCM) was proposed for the temperature-compensated frequency calibration of quartz enhanced photoacoustic spectroscopy (QEPAS). The temperature effects on the resonance frequency of QEPAS was reported in traditional electrical calibration method (ECM) and TCM. With the temperature varying from 25.6 ℃ to 70.4 ℃, the resonance frequency calibrated by ECM shows a frequency shift of 36.6 ppm. The proposed TCM by using of light-induced thermoelastic wave to drive the resonance of quartz crystal tuning fork (QTF), improved the accuracy of resonance calibration by 2 orders, resulting in a frequency shift of ~ 0.3 ppm. Detailed research demonstrates the TCM is able to work with light intensity of only ~ 50 μW. It is indicated that TCM is a powerful tool for the temperature-compensation of resonance frequency of quartz tuning fork.

Multiple-sound-source-excitation quartz-enhanced photoacoustic spectroscopy based on a single-line spot pattern multi-pass cell

Multiple-sound-source-excitation quartz-enhanced photoacoustic spectroscopy (MSSE-QEPAS) based on a single-line spot pattern multi-pass cell (MPC) is reported for trace gas detection. The single-line spot pattern MPC is designed to make a laser beam pass through a quartz tuning fork (QTF) 60 times, thus producing 60 sound sources between the two QTF prongs. These sound sources excite the QTF operating at fundamental resonance mode in phase, resulting in a signal gain factor of ∼20. A theoretical mode based on convolution method is proposed to explain the working mechanism of MSSE-QEPAS.

QEPAS sensor in a butterfly package and its application.

Molecular gases are highly relevant in healthcare, production control, safety, and environmental monitoring. They often appear in small concentrations. The measurement of trace gases has increasingly become a key technique in those domains. Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a suitable method that can provide the required low detection limits in such applications at comparatively low cost and small size. For mobile implementation, the size of an entire sensor unit matters. In this paper, we present a QEPAS sensor that fits into a standard butterfly package, its characterization, and its application on CH4 and CO2.

Improved IP-QEPAS sensor based on cylindrical cavity enhancement

In this paper, a cylindrical cavity based in-plane quartz-enhanced photoacoustic spectroscopy (IP-QEPAS) was demonstrated for the first time. The cylindrical cavity was designed by theoretical mode and used as a resonator to improve the strength of acoustics wave. The relationship between radius of cylindrical cavity and acoustic pressure was analyzed theoretically. After adding cylindrical cavities, due to the coupling effect, both resonance frequency and Q-factor of quartz tuning fork (QTF) were decreased. The second harmonic (2f) signal amplitude was measured at an optimal laser wavelength modulation depth of 0.446 cm−1. A 1.57 times improvement of 2f signal amplitude was realized when a cavity with radius of 6.00 mm was added. The experimental results agreed with the theoretical analysis. The cylindrical cavity based IP-QEPAS sensor performance can be further improved when a QTF with large prongs length is used.

Advances in fiber‐based quartz enhanced photoacoustic spectroscopy for trace gas sensing

AbstractFiber‐based quartz enhanced photoacoustic spectroscopy (QEPAS) is a method for trace gas sensing with excellent properties such as high sensitivity and strong applicability. It has received extensive attention and become particularly attractive in recent years. Therefore, this invited paper analyzed and summarized the principle of its sensing as well as its technical characteristics. The latest research results, combed with the application mechanism and application fields of different types of QEPAS system, were introduced. Finally, its development trend was forecasted.

Multi-pass quartz-enhanced photoacoustic spectroscopy-based trace gas sensing

A multi-pass quartz-enhanced photoacoustic spectroscopy (MP-QEPAS)-based trace gas sensor is reported. In MP-QEPAS, a multi-pass laser beam pattern through the prong spacing of a quartz tuning fork (QTF) is obtained by means of two right-angle prisms. A large QTF with the prong length of 17 mm and prong spacing of 0.8 mm was employed to increase the passage of multi-pass time and ease the alignment of the beam reflection pattern through the QTF. This multi-pass configuration allows the laser beam to pass through the QTF prong spacing six times. Water vapor (H2O) was chosen as target gas to investigate the performance of the MP-QEPAS sensor. Compared to a conventional QEPAS measurement, the MP-QEPAS technique provided an enhancement of signal level of ∼3.2 times.

Quartz-enhanced photoacoustic-photothermal spectroscopy for trace gas sensing.

A trace gas detection technique of quartz-enhanced photoacoustic-photothermal spectroscopy (QEPA-PTS) is demonstrated. Different from quartz-enhanced photoacoustic spectroscopy (QEPAS) or quartz-enhanced photothermal spectroscopy (QEPTS), which detected only one single kind of signal, QEPA-PTS was realized by adding the photoacoustic and photothermal signals generated from two quartz tuning forks (QTFs), respectively. Water vapor (H2O) with a volume concentration of 1.01% was selected as the analyte gas to investigate the QEPA-PTS sensor performance. Compared to QEPAS and QEPTS, an enhanced signal level was achieved for this QEPA-PTS system. Further improvement of such a technique was proposed.

Parts-per-billion detection of carbon monoxide: A comparison between quartz-enhanced photoacoustic and photothermal spectroscopy

We report on a comparison between two optical detection techniques, one based on a Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) detection module, where a quartz tuning fork is acoustically coupled with a pair of millimeter-sized resonator tubes; and the other one based on a Photothermal Spectroscopy (PTS) module where a Fabry-Perot interferometer acts as transducer to probe refractive index variations. When resonant optical absorption of modulated light occurs in a gas sample, QEPAS directly detects acoustic waves while PTS probes refractive index variations caused by local heating. Compact QEPAS and PTS detection modules were realized and integrated in a gas sensor system for detection of carbon monoxide (CO), targeting the fundamental band at 4.6 μm by using a distributed-feedback quantum cascade laser. Performance was compared and ultimate detection limits up to ∼ 6 part-per-billion (ppb) and ∼15 ppb were reached for QEPAS and the PTS module, respectively, using 100 s integration time and 40 mW of laser power.

Calibration of Quartz-Enhanced Photoacoustic Sensors for Real-Life Adaptation.

We report on the use of quartz-enhanced photoacoustic spectroscopy for continuous carbon-dioxide measurements in humid air over a period of six days. The presence of water molecules alters the relaxation rate of the target molecules and thus the amplitude of the photoacoustic signal. Prior to the measurements, the photoacoustic sensor system was pre-calibrated using CO2 mole fractions in the range of 0–10−3 (0–1000 ppm) and at different relative humidities between 0% and 45%, while assuming a model hypothesis that allowed the photoacoustic signal to be perturbed linearly by H2O content. This calibration technique was compared against an alternative learning-based method, where sensor data from the first two days of the six-day period were used for self-calibration. A commercial non-dispersive infrared sensor was used as a CO2 reference sensor and provided the benchmark for the two calibration procedures. In our case, the self-calibrated method proved to be both more accurate and precise.

Research progress of quartz-enhanced photoacoustic spectroscopy based gas sensing

Laser spectroscopy based techniques have the advantages of high sensitivities, high selectivities, non-invasiveness and in situ, real-time observations. They are widely used in numerous fields, such as environmental monitoring, life science, medical diagnostics, manned space flight, and planetary exploration. Owing to the merits of low cost, compact volume and strong environment adaptability, quartz-enhanced photoacoustic spectroscopy (QEPAS) based sensing is an important laser spectroscopy-based method of detecting the trace gas, which was invented in 2002. Detection sensitivity is a key parameter for gas sensors because it determines their real applications. In this paper, focusing on the detection sensitivity, the common methods for QEPAS are summarized. High power laser including amplified diode laser by erbium doped fiber amplifier (EDFA), and quantum cascade laser are used to improve the excitation intensity of acoustic wave. The absorption line of gas molecules located at the fundamental bands of mid-infrared region is adopted to increase the laser absorption strength. Micro-resonator is employed to enhance the generated acoustic pressure by forming a standing wave cavity. Quartz tuning forks (QTFs) with low resonant frequency are used to increase the accumulation time of acoustic energy in itself. Multi-pass strategy is utilized to amplify the action length between laser beam and target gas in the prongs of QTF. The advantages and disadvantages of the above methods are discussed respectively. For the issues in real applications, the all-fiber strucure in near-infared region and mid-infrared region and miniaturization using three-dimensional(3D) printing technique for QEPAS sensor are summarized. A QEPAS technique based multi-gas sensor is used to quantify the concentration of carbon monoxide (CO), carbon dioxide (CO<sub>2</sub>), hydrogen cyanide (HCN), and hydrogen chloride (HCl) for post-fire cleanup aboard spacecraft, which is taken for example for the real application.Finally, the methods of further improving the sensitivity of QEPAS sensor are proposed.

H2S quartz-enhanced photoacoustic spectroscopy sensor employing a liquid-nitrogen-cooled THz quantum cascade laser operating in pulsed mode

In this work, we report on a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for hydrogen sulfide (H2S) detection, exploiting a liquid-nitrogen-cooled THz quantum cascade laser (QCL) operating in pulsed mode. The spectrophone was designed to accommodate a THz QCL beam and consisted of a custom quartz tuning fork with a large prong spacing, coupled with acoustic resonator tubes. The targeted rotational transition falls at 2.87 THz (95.626 cm−1), with a line-strength of 5.53 ∙ 10-20 cm/mol. A THz QCL peak power of 150 mW was measured at a heat sink temperature of 81 K, pulse width of 1 μs and repetition rate of 15.8 kHz. A QEPAS record sensitivity for H2S detection in the THz range of 360 part-per-billion in volume was achieved at a gas pressure of 60 Torr and 10 s integration time.

Quartz-enhanced photoacoustic spectroscopy exploiting low-frequency tuning forks as a tool to measure the vibrational relaxation rate in gas species

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.

Fiber-Coupled Quartz-Enhanced Photoacoustic Spectroscopy System for Methane and Ethane Monitoring in the Near-Infrared Spectral Range.

We report on a fiber-coupled, quartz-enhanced photoacoustic spectroscopy (QEPAS) near-IR sensor for sequential detection of methane (CH4 or C1) and ethane (C2H6 or C2) in air. With the aim of developing a lightweight, compact, low-power-consumption sensor suitable for unmanned aerial vehicles (UAVs)-empowered environmental monitoring, an all-fiber configuration was designed and realized. Two laser diodes emitting at 1653.7 nm and 1684 nm for CH4 and C2H6 detection, respectively, were fiber-combined and fiber-coupled to the collimator port of the acoustic detection module. No cross talk between methane and ethane QEPAS signal was observed, and the related peak signals were well resolved. The QEPAS sensor was calibrated using gas samples generated from certified concentrations of 1% CH4 in N2 and 1% C2H6 in N2. At a lock-in integration time of 100 ms, minimum detection limits of 0.76 ppm and 34 ppm for methane and ethane were achieved, respectively. The relaxation rate of CH4 in standard air has been investigated considering the effects of H2O, N2 and O2 molecules. No influence on the CH4 QEPAS signal is expected when the water vapor concentration level present in air varies in the range 0.6–3%.

Research on Detecting CO2 With Off-Beam Quartz-Enhanced Photoacoustic Spectroscopy at 2.004 μm

Research on Detecting CO2 With Off-Beam Quartz-Enhanced Photoacoustic Spectroscopy at 2.004 μm

Quartz-Enhanced Photoacoustic Spectroscopy for Four-Component Gas Detection Based on Two Off-Beam Acoustic Microresonators

A compact and portable quartz-enhanced photoacoustic spectroscopy gas sensor was developed for four-component gas detection using two off-beam acoustic microresonators (AMRs). The two AMRs were placed in parallel on opposing sides of a quartz tuning fork (QTF) for photoacoustic signal enhancement. Four distributed feedback (DFB) lasers were connected to the four ends of the two off-beam AMRs using a fiber collimator for photoacoustic signal excitation. Four-component gas sensing was achieved via time-division multiplexing of the DFB laser driver currents. The four-component gas sensing scheme was used to detect acetylene (C2H2) at 1532.83 nm, methane (CH4) at 1653.722 nm, water vapor (H2O) at 1368.597 nm and carbon dioxide (CO2) at 1577.787 nm for feasibility testing. Minimum detection limits of 3.6 ppmv for C2H2, 34.71 ppmv for CH4, 1.09 ppmv for H2O, and 341.18 ppmv for CO2 were obtained, and the linear responses reached 0.9982, 0.9969, 0.99843 and 0.99591 for C2H2, CH4, H2O and CO2, respectively, at 1.5 s intervals.