Quartz-enhanced Photoacoustic Spectroscopy Research Articles

Ultra-highly sensitive dual gases detection based on photoacoustic spectroscopy by exploiting a long-wave, high-power, wide-tunable, single-longitudinal-mode solid-state laser.

Photoacoustic spectroscopy (PAS) as a highly sensitive and selective trace gas detection technique has extremely broad application in many fields. However, the laser sources currently used in PAS limit the sensing performance. Compared to diode laser and quantum cascade laser, the solid-state laser has the merits of high optical power, excellent beam quality, and wide tuning range. Here we present a long-wave, high-power, wide-tunable, single-longitudinal-mode solid-state laser used as light source in a PAS sensor for trace gas detection. The self-built solid-state laser had an emission wavelength of ~2 μm with Tm:YAP crystal as the gain material, with an excellent wavelength and optical power stability as well as a high beam quality. The wide wavelength tuning range of 9.44 nm covers the absorption spectra of water and ammonia, with a maximum optical power of ~130 mW, allowing dual gas detection with a single laser source. The solid-state laser was used as light source in three different photoacoustic detection techniques: standard PAS with microphone, and external- and intra-cavity quartz-enhanced photoacoustic spectroscopy (QEPAS), proving that solid-state laser is an attractive excitation source in photoacoustic spectroscopy.

Quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy

The extension of dual-comb spectroscopy (DCS) to all wavelengths of light along with its ability to provide ultra-large dynamic range and ultra-high spectral resolution, renders it extremely useful for a diverse array of applications in physics, chemistry, atmospheric science, space science, as well as medical applications. In this work, we report on an innovative technique of quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy (QEMR-PAS), in which the beat frequency response from a dual comb is frequency down-converted into the audio frequency domain. In this way, gas molecules act as an optical-acoustic converter through the photoacoustic effect, generating heterodyne sound waves. Unlike conventional DCS, where the light wave is detected by a wavelength-dependent photoreceiver, QEMR-PAS employs a quartz tuning fork (QTF) as a high-Q sound transducer and works in conjunction with a phase-sensitive detector to extract the resonant sound component from the multiple heterodyne acoustic tones, resulting in a straightforward and low-cost hardware configuration. This novel QEMR-PAS technique enables wavelength-independent DCS detection for gas sensing, providing an unprecedented dynamic range of 63 dB, a remarkable spectral resolution of 43 MHz (or ~0.3 pm), and a prominent noise equivalent absorption of 5.99 × 10-6 cm-1·Hz-1/2.

Influence of the Gain–Bandwidth of the Front-End Amplifier on the Performance of a QEPAS Sensor

The quartz tuning fork used as an acoustic sensor in quartz-enhanced photo-acoustic spectroscopy gas detection systems is usually read out by means of a transimpedance preamplifier based on a low-noise operational amplifier closed in a feedback loop. The gain–bandwidth product of the operational amplifier used in the circuit is a key parameter which must be properly chosen to guarantee that the circuit works as expected. Here, we demonstrate that if the value of this parameter is not sufficiently large, the response of the preamplifier exhibits a peak at a frequency which does not coincide with the series resonant frequency of the quartz tuning fork. If this peak frequency is selected for modulating the laser bias current and is also used as the reference frequency of the lock-in amplifier, a penalty results in terms of signal-to-noise ratio at the output of the QEPAS sensor. This worsens the performance of the gas sensing system in terms of ultimate detection limits. We show that this happens when the front-end preamplifier of the quartz tuning fork is based on some amplifier models that are typically used for such application, both when the integration time of the lock-in amplifier filter is long, to boost noise rejection, and when it is short, in order to comply with a relevant measurement rate.

Quartz-enhanced photoacoustic spectroscopy sensing using trapezoidal- and round-head quartz tuning forks.

In this Letter, two novel, to the best of our knowledge, quartz tuning forks (QTFs) with trapezoidal-head and round-head were designed and adopted for quartz-enhanced photoacoustic spectroscopy (QEPAS) sensing. Based on finite element analysis, a theoretical simulation model was established to optimize the design of QTF. For performance comparison, a reported T-head QTF and a commercial QTF were also investigated. The designed QTFs have decreased resonant frequency (f0) and increased gap between the two prongs of QTF. The experimentally determined f0 of the T-head QTF, trapezoidal-head QTF, and round-head QTF were 8690.69 Hz, 9471.67 Hz, and 9499.28 Hz, respectively. The corresponding quality (Q) factors were measured as 11,142, 11,411, and 11,874. Compared to the commercial QTF, the resonance frequencies of these QTFs have reduced by 73.45%, 71.07%, and 70.99% while maintaining a comparable Q factor to the commercially mature QTF. Methane (CH4) was chosen as the analyte to verify the QTFs' performance. Compared with the commercial QTF, the signal-to-noise ratio (SNR) of the CH4-QEPAS system based on the T-head QTF, trapezoidal-head QTF, and round-head QTF has been improved by 1.75 times, 2.96 times, and 3.26 times, respectively. The performance of the CH4-QEPAS sensor based on the QTF with the best performance of the round-head QTF was investigated in detail. The results indicated that the CH4-QEPAS sensor based on the round-head QTF exhibited an excellent linear concentration response. Furthermore, a minimum detection limit (MDL) of 0.87 ppm can be achieved when the system's average time was 1200 s.

Evanescent wave quartz-enhanced photoacoustic spectroscopy employing a side-polished fiber for methane sensing

We present an all-fiber-based laser gas analyzer (LGA) employing quartz-enhanced photoacoustic spectroscopy (QEPAS) and a side-polished fiber (SPF). The LGA comprises a custom quartz tuning fork (QTF) with 0.8 mm prong spacing, two acoustic micro-resonators (mR) located on either side of the prong spacing, and a single-mode fiber containing a 17 mm polished section passing through both mRs and QTF. The SPF polished face is positioned to enable the evanescent wave (EW) to create a photoacoustic wave and excite the fundamental flexural mode of the QTF. Sensor performance was demonstrated using methane in nitrogen gas mixtures, with CH4 mixing ratios ranging from 75 ppmv to 1% (by volume), measured with an accumulation time of 300 ms, and a minimum detection limit of 34 ppmv subsequently determined. The EW-QEPAS sensor is ideal for miniaturization, as it does not contain any free-space optics and is suitable for gas sensing in harsh environments and where mobility is required.

Highly efficient and selective integrated directional couplers for multigas sensing applications

The design and fabrication of a compact, low-loss, broadband directional coupler (DC) based duplexer operating in the near-infrared (NIR) region are demonstrated. The duplexer exhibits high selectivity and coupling efficiency (CE), for target wavelengths of 1530 nm and 1653.7 nm, making it applicable in systems for the multi-gas detection of ammonia and methane. The measured CE for the duplexer is 73% and 76% at 1530 nm and 1653.7 nm respectively. These results demonstrate the effectiveness of the duplexer as a broadband and scalable power source for highly sensitive sensing techniques, like quartz-enhanced photoacoustic spectroscopy (QEPAS). Its compact size and low-loss characteristics make it highly portable and well-suited for drone-based multi-gas detection applications.

Dual-frequency modulated heterodyne quartz-enhanced photoacoustic spectroscopy.

A novel dual-frequency modulated heterodyne quartz-enhanced photoacoustic spectroscopy (DFH-QEPAS) was demonstrated for what we believe to be the first time in this study. In traditional H-QEPAS, the frequency of modulated sinusoidal wave has a frequency difference (Δf) with the resonance frequency (f0) of a quartz tuning fork (QTF). Owing to the resonance characteristic of QTF, it cannot excite QTF to the strongest response. To achieve a stronger response, a sinusoidal wave with a frequency of f0 was added to the modulation wave to compose a dual-frequency modulation. Acetylene (C2H2) was chosen as the target gas to verify the sensor performance. The proposed DFH-QEPAS improved 4.05 times of signal-to-noise ratio (SNR) compared with the traditional H-QEPAS in the same environmental conditions.

Development of continuous monitoring of ambient carbon dioxide using near-infrared quartz-enhanced photoacoustic spectroscopy

We demonstrate continuous measurement of carbon dioxide (CO2) in the atmosphere using quartz-enhanced photoacoustic spectroscopy (QEPAS) at 2.008 μm with a fiber-coupled distributed-feedback laser. The QEPAS sensor is composed of a quartz tuning fork with a resonance frequency f 0 of 12.4 kHz. The photoacoustic second-harmonic (2f) signal resulting from CO2 absorption was detected using a digital lock-in amplifier. The photoacoustic signal is proportional to the laser power. The limit of detection at an absorption line of 4980.13161 cm−1 with a line strength of 3.87 × 10−22 cm2 molecule−1 cm−1 was 12 parts per million with an integration time of 10 s. The results of the continuous monitoring of ambient CO2 using the QEPAS sensor corresponded to those obtained using a nondispersive IR sensor. The QEPAS sensor exhibited a suitable performance for field measurements of the CO2 mixing ratio.

Microfiber knot resonator augmented quartz-enhanced photoacoustic spectroscopy

In this work, a microfiber knot resonator augmented quartz-enhanced photoacoustic spectroscopy (MKR-QEPAS) was developed for the analysis of trace gases. In the proof-of-concept experiment, a 1.39 μm laser diode was utilized for the detection of H2O molecules. Based on the H2O absorption line, the optical fiber-based MKR was meticulously designed to modulate the transmission spectrum, thereby inducing constructive interference with the excitation laser wavelength. The MKR was integrated with a clamp-type tuning fork, leading to a pronounced evanescent resonance enhancement. Consequently, the detection sensitivity of the MKR-QEPAS was improved by approximately one order of magnitude. The micro ring resonator is posited to hold significant promise for hybrid integration in QEPAS on-chip sensors.

Parallel dual tuning forks for quartz-enhanced photoacoustic spectroscopy

A recently introduced approach to photoacoustic detection of trace gases utilizing dual quartz tuning forks (D-QTFs) in parallel is described in detail. It investigates the sound vibration mode that is optimal for a small acoustic module. The acoustic module was optimized using COMSOL multiphysics software. The D-QTFs sensor and a half-size resonator module platform were adopted to enhance signal strength. The advantages of the technique compared to the normal QEPAS sensor are that it uses a single resonator tube but realises a 1.25-times signal enhancement. Water vapour was chosen as the target analyte. The achieved noise of 1σ is 1.268 µV compared to the QEPAS noise of 1.196 µV. The presence of parallel forks leads to a decrease in reaction time of ∼17 %. The results of this investigation revealed that a reduced chamber volume and rapid response led to a more pronounced signal response. The sensor's performance can be further enhanced by increasing the laser power and the optical path length. It is widely used in fields with volume constraints.

Miniaturized and highly-sensitive fiber-optic photoacoustic gas sensor based on an integrated tuning fork by mechanical processing with dual-prong differential measurement

A proof-of-concept gas sensor based on a miniaturized and integrated fiber-optic photoacoustic detection module was introduced and demonstrated for the purpose of developing a custom tuning-fork (TF)-enhanced photoacoustic gas sensor. Instead of piezoelectric quartz tuning fork (QTF) in conventional quartz-enhanced photoacoustic spectroscopy (QEPAS), a low-cost custom aluminum alloy TF fabricated by mechanical processing was employed as a photoacoustic transducer and the vibration of TF was measured by fiber-optic Fabry-Pérot (FP) interferometer (FPI). The mechanical processing-based TF design scheme greatly increases the flexibility of the TF design with respect to the complex and expensive manufacture process of custom QTFs, and thus it can be better exploited to detect gases with slow vibrational-translational (V-T) relaxation rates and combine with light sources with poor beam quality. The resonance frequency and the quality factor of the designed custom TF at atmospheric pressure were experimentally determined to be 7.3 kHz and 4733, respectively. Dual-prong differential measurement method was proposed to double the photoacoustic signal and suppress the external same-direction noise. After detailed optimizing and investigating for the operating parameters by measuring H2O, the feasibility of the developed sensor for gas detection was demonstrated with a H2O minimum detection limit (MDL) of 1.2 ppm, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 3.8 × 10−8 cm−1 W/Hz1/2, which are better than the QTF-based photoacoustic sensors. The proposed gas sensing approach combined the advantages of QEPAS and fiber-optic sensing, which can greatly expand the application domains of PAS-based gas sensors.

Quartz-Enhanced Photoacoustic Sensor Based on a Multi-Laser Source for In-Sequence Detection of NO2, SO2, and NH3.

In this work, we report on the implementation of a multi-quantum cascade laser (QCL) module as an innovative light source for quartz-enhanced photoacoustic spectroscopy (QEPAS) sensing. The source is composed of three different QCLs coupled with a dichroitic beam combiner module that provides an overlapping collimated beam output for all three QCLs. The 3λ-QCL QEPAS sensor was tested for detection of NO2, SO2, and NH3 in sequence in a laboratory environment. Sensitivities of 19.99 mV/ppm, 19.39 mV/ppm, and 73.99 mV/ppm were reached for NO2, SO2, and NH3 gas detection, respectively, with ultimate detection limits of 9 ppb, 9.3 ppb, and 2.4 ppb for these three gases, respectively, at an integration time of 100 ms. The detection limits were well below the values of typical natural abundance of NO2, SO2, and NH3 in air.

Quartz-Enhanced Photoacoustic Spectroscopy Assisted by Partial Least-Squares Regression for Multi-Gas Measurements.

We report on the use of quartz-enhanced photoacoustic spectroscopy (QEPAS) for multi-gas detection. Photoacoustic (PA) spectra of mixtures of water (H2O), ammonia (NH3), and methane (CH4) were measured in the mid-infrared (MIR) wavelength range using a mid-infrared (MIR) optical parametric oscillator (OPO) light source. Highly overlapping absorption spectra are a common challenge for gas spectroscopy. To mitigate this, we used a partial least-squares regression (PLS) method to estimate the mixing ratio and concentrations of the individual gasses. The concentration range explored in the analysis varies from a few parts per million (ppm) to thousands of ppm. Spectra obtained from HITRAN and experimental single-molecule reference spectra of each of the molecular species were acquired and used as training data sets. These spectra were used to generate simulated spectra of the gas mixtures (linear combinations of the reference spectra). Here, in this proof-of-concept experiment, we demonstrate that after an absolute calibration of the QEPAS cell, the PLS analyses could be used to determine concentrations of single molecular species with a relative accuracy within a few % for mixtures of H2O, NH3, and CH4 and with an absolute sensitivity of approximately 300 (±50) ppm/V, 50 (±5) ppm/V, and 5 (±2) ppm/V for water, ammonia, and methane, respectively. This demonstrates that QEPAS assisted by PLS is a powerful approach to estimate concentrations of individual gas components with considerable spectral overlap, which is a typical scenario for real-life adoptions and applications.

Ppb-level NH3 photoacoustic sensor combining a hammer-shaped tuning fork and a 9.55 µm quantum cascade laser

We present a quartz enhanced photoacoustic spectroscopy (QEPAS) gas sensor designed for precise monitoring of ammonia (NH3) at ppb-level concentrations. The sensor is based on a novel custom quartz tuning fork (QTF) with a mid-infrared quantum cascade laser emitting at 9.55 µm. The custom QTF with a hammer-shaped prong geometry which is also modified by surface grooves is designed as the acoustic transducer, providing a low resonance frequency of 9.5 kHz and a high-quality factor of 10263 at atmospheric pressure. In addition, a temperature of 50 °C and a large gas flow rate of 260 standard cubic centimeters per minute (sccm) are applied to mitigate the adsorption and desorption effect arising from the polarized molecular of NH3. With 80-mW optical power and 300-ms lock-in integration time, the detection limit is achieved to be 2.2 ppb which is the best value reported in the literature so far for NH3 QEPAS sensors, corresponding to a normalized noise equivalent absorption coefficient of 1.4 × 10−8 W cm−1 Hz−1/2. A five-day continuous monitoring for atmospheric NH3 is performed, verifying the stability and robustness of the presented QEPAS-based NH3 sensor.

Non-Paraxial Effects in the Laser Beams Sharply Focused to Skin Revealed by Unidirectional Helmholtz Equation Approximation

Laser beams converging at significant focusing angles have diverse applications, including quartz-enhanced photoacoustic spectroscopy, high spatial resolution imaging, and profilometry. Due to the limited applicability of the paraxial approximation, which is valid solely for smooth focusing scenarios, numerical modeling becomes necessary to achieve optimal parameter optimization for imaging diagnostic systems that utilize converged laser beams. We introduce a novel methodology for the modeling of laser beams sharply focused on the turbid tissue-like scattering medium by employing the unidirectional Helmholtz equation approximation. The suggested modeling approach takes into account the intricate structure of biological tissues, showcasing its ability to effectively simulate a wide variety of random multi-layered media resembling tissue. By applying this methodology to the Gaussian-shaped laser beam with a parabolic wavefront, the prediction reveals the presence of two hotspots near the focus area. The close-to-maximal intensity hotspot area has a longitudinal size of about 3–5 μm and a transversal size of about 1–2 μm. These values are suitable for estimating spatial resolution in tissue imaging when employing sharply focused laser beams. The simulation also predicts a close-to-maximal intensity hotspot area with approximately 1 μm transversal and longitudinal sizes located just behind the focus distance for Bessel-shaped laser beams with a parabolic wavefront. The results of the simulation suggest that optical imaging methods utilizing laser beams with a wavefront produced by an axicon lens would exhibit a limited spatial resolution. The wavelength employed in the modeling studies to evaluate the sizes of the focus spot is selected within a range typical for optical coherence tomography, offering insights into the limitation of spatial resolution. The key advantage of the unidirectional Helmholtz equation approximation approach over the paraxial approximation lies in its capability to simulate the propagation of a laser beam with a non-parabolic wavefront.

Mid-infrared swept cavity-enhanced photoacoustic spectroscopy using a quartz tuning fork

We report the development of swept cavity-enhanced photoacoustic spectroscopy using a quartz tuning fork for ultra-sensitive and high-resolution molecular spectroscopy. By using a quantum cascade laser (QCL) as the mid-infrared light source, a dual-feedback Pound–Drever–Hall locking method is proposed to lock the QCL frequency to a continuously swept optical cavity. By placing an off-beam quartz-enhanced photoacoustic spectroscopy module in a 48-mm Fabry–Pérot cavity, we are able to achieve ultra-sensitive gas detection based on the doubly resonant photoacoustic effect. As a proof-of-concept, we use a distributed feedback QCL to exploit the CO line at 2190.02 cm−1, where the cavity-locked QCL is scanned over a spectral range of 10 GHz with a spectral resolution of ∼3 MHz. With the incident laser power of 7.3 mW, the optical cavity (finesse 1931) builds up the intracavity power beyond 3 W. Our photoacoustic spectrometer achieves the minimum detection limit of 375 part-per-trillion (ppt) at the averaging time of 150 s and the normalized noise equivalent absorption coefficient of 1.27 × 10−9 Wcm−1 Hz−1/2.

The intra‐cavity QEPAS sensor using fiber‐ring laser for C2H2 and CO2 detection

AbstractAn intra‐cavity quartz‐enhanced photoacoustic spectroscopy (QEPAS) sensor using fiber‐ring laser is demonstrated. The sensor includes an erbium‐doped fiber (EDF) intra‐cavity laser and a quartz tuning fork (QTF), and realizes wavelength scanning through a piezoelectric transducer (PZT). The acousto‐optic modulator (AOM) is used to modulate laser intensity at a frequency of 32.758 kHz. We use optical switches to detect acetylene (C2H2) and carbon dioxide (CO2) at 1530.37 and 1572.66 nm, respectively, based on the time division multiplexing (TDM) technology. The two‐component gas sensor showed good linearity of 0.9996 and 0.9966, and could achieve a minimum detection limit (MDL) of 180.64 ppbv and 145.45 ppmv for C2H2 and CO2 detection, respectively.

Calibration-free near-infrared methane sensor system based on BF-QEPAS

Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) is a sensitive and selective analytical technique widely used for gas sensing. To improve the response time of the methane sensing system based on QEPAS, a Beat Frequency Quartz-Enhanced Photoacoustic Spectroscopy (BF-QEPAS) system was developed. Theoretical model based on the classical spring-mass oscillator model to explain the generation of beat-frequency signal in the quartz tuning fork (QTF) was established. The BF-QEPAS system parameters were optimized and evaluated using the developed system. The performance of the system was analyzed through Allan variance and gas calibration. Experimental results showed that the BF-QEPAS system achieved a minimum detectable limit of 28.35 ppm for methane with an integration time of 114 s. The corresponding minimum detectable normalized noise equivalent absorption (NNEA) coefficient was 1.93 × 10-9 cm-1WHz1/2, which demonstrates the excellent sensitivity of the BF-QEPAS system for methane detection.

Differential quartz-enhanced photoacoustic spectroscopy

In this paper, a differential quartz-enhanced photoacoustic spectroscopy (D-QEPAS) sensor is reported. The differential photoacoustic cell (PAC) was used to generate the photoacoustic effect. Two quartz tuning forks (QTFs) with a quality factor (Q) up to 10 000 were used as the acoustic wave transducers. The signal of D-QEPAS sensor was doubly enhanced by the differential characteristic of differential PAC and resonant response of QTF. The background noise was suppressed based on the differential principle. With the help of the finite element method, the acoustic field characteristics were simulated and calculated. Wavelength modulation spectroscopy technique and second harmonic (2f) detection technique were applied to detect photoacoustic signal. Trace acetylene (C2H2) gas detection was performed to verify the D-QEPAS sensor performance. The 2f signal amplitude of differential mode was 116.03 μV, which had a 1.65 times improvement compared with the 2f signal amplitudes of QTF1. When the integration time was 334 s, the minimum detection limit of D-QEPAS sensor was about 496.7 ppb. The reported D-QEPAS provides a development and idea for the widely reported QEPAS technique.

Assessment of vibrational-translational relaxation dynamics of methane isotopologues in a wet-nitrogen matrix through QEPAS

Here we report on a study of the non-radiative relaxation dynamic of 12CH4 and 13CH4 in wet nitrogen-based matrixes by using the quartz-enhanced photoacoustic spectroscopy (QEPAS) technique. The dependence of the QEPAS signal on pressure at fixed matrix composition and on H2O concentration at fixed pressure was investigated. We demonstrated that QEPAS measurements can be used to retrieve both the effective relaxation rate in the matrix, and the V-T relaxation rate associated to collisions with nitrogen and water vapor. No significant differences in measured relaxation rates were observed between the two isotopologues.