Highly sensitive HF detection based on absorption enhanced light-induced thermoelastic spectroscopy with a quartz tuning fork of receive and shallow neural network fitting

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.

Temperature Compensation of Laser Methane Sensor Based on a Large-Scale Dataset and the ISSA-BP Neural Network

We aimed to improve the detection accuracy of laser methane sensors in expansive temperature application environments. In this paper, a large-scale dataset of the measured concentration of the sensor at different temperatures is established, and a temperature compensation model based on the ISSA-BP neural network is proposed. On the data side, a large-scale dataset of 15,810 sets of laser methane sensors with different temperatures and concentrations was established, and an Improved Isolation Forest algorithm was used to clean the large-scale data and remove the outliers in the dataset. On the modeling framework, a temperature compensation model based on the ISSA-BP neural network is proposed. The quasi-reflective learning, chameleon swarm algorithm, Lévy flight, and artificial rabbits optimization are utilized to improve the initialization of the sparrow population, explorer position, anti-predator position, and position of individual sparrows in each generation, respectively, to improve the global optimization seeking ability of the standard sparrow search algorithm. The ISSA-BP temperature compensation model far outperforms the four models, SVM, RF, BP, and PSO-BP, in model evaluation metrics such as MAE, MAPE, RMSE, and R-square for both the training and test sets. The results show that the algorithm in this paper can significantly improve the detection accuracy of the laser methane sensor under the wide temperature application environment.

End-to-end methane gas detection algorithm based on transformer and multi-layer perceptron

In this paper, an end-to-end methane gas detection algorithm based on transformer and multi-layer perceptron (MLP) for tunable diode laser absorption spectroscopy (TDLAS) is presented. It consists of a Transformer-based U-shaped Neural Network (TUNN) filtering algorithm and a concentration prediction network (CPN) based on MLP. This algorithm employs an end-to-end architectural design to extract information from noisy transmission spectra of methane and derive the CH4 concentrations from denoised spectra, without intermediate steps. The results demonstrate the superiority of the proposed TUNN filtering algorithm over other typically employed digital filters. For concentration prediction, the determination coefficient (R2) reached 99.7%. Even at low concentrations, R2 remained notably high, reaching up to 89%. The proposed algorithm results in a more efficient, convenient, and accurate spectral data processing for TDLAS-based gas sensors.

Differential integrating sphere-based photoacoustic spectroscopy gas sensing

In this Letter, a differential integrating sphere-based photoacoustic spectroscopy (PAS) gas sensor is proposed for the first time to our knowledge. The differential integrating sphere system consists of two integrating spheres and a tube. Based on differential characteristics, the photoacoustic signal of the designed differential integrating sphere was doubly enhanced and the noise was suppressed. Compared with a single channel integrating sphere, the differential integrating sphere sensing system had a 1.86 times improvement in signal level. An erbium-doped fiber amplifier (EDFA) was adopted to amplify the output of diode laser to enhance the optical excitation. The second harmonic (2f) signal of differential integrating sphere-based acetylene (C2H2) PAS sensor with an amplified 1000 mW optical output power was 104.67 mV, which was 22.80 times improved compared to the sensing system without EDFA. When the integration time was 100 s, the minimum detection limit (MDL) of the differential integrating sphere-based C2H2 PAS sensor was 416.7 ppb. The differential integrating sphere provides a new method, to the best of our knowledge, for the development of PAS sensor, which has the advantages of photoacoustic signal enhancement, strong noise immunity, and no need for optical adjustment.

Hollow-waveguide-based light-induced thermoelastic spectroscopy sensing

In this Letter, a hollow waveguide (HWG)-based light-induced thermoelastic spectroscopy (LITES) gas sensing is proposed. An HWG with a length of 65 cm and inner diameter of 4 mm was used as the light transmission medium and gas chamber. The inner wall of the HWG was coated with a silver (Ag) film to improve reflectivity. Compared with the usually used multi-pass cell (MPC), the HWG has many advantages, such as small size, simple structure and fast filling. Compared with a hollow-core anti-resonant fiber (HC-ARF), the HWG has the merits of easy optical coupling, high system stability, and wide transmission range. A diode laser with output wavelength of 1.53 µm and a quantum cascade laser (QCL) with output wavelength of 4.58 µm were selected as the sources of excitation to target acetylene (C₂H₂) and carbon monoxide (CO), respectively, to verify the performance of the HWG-based LITES sensor in the near-infrared and mid-infrared regions. The experimental results showed that the HWG-based LITES sensor had a great linear responsiveness to the target gas concentration. The minimum detection limit (MDL) for C₂H₂ and CO was 6.07 ppm and 98.66 ppb, respectively.

Highly sensitive light-induced thermoelastic spectroscopy oxygen sensor with co-coupling photoelectric and thermoelastic effect of quartz tuning fork

A light-induced thermoelastic spectroscopy (LITES) gas detection method based on CH₃NH₃PbI₃ perovskite-coated quartz tuning fork (QTF) was proposed. By coating CH₃NH₃PbI₃ thin film on the surface of ordinary QTF, a Schottky junction with silver electrodes was formed. The co-coupling of photoelectric effect and thermoelastic effect of CH₃NH₃PbI₃-QTF results in a significant improvement in detection performance. The oxygen (O₂) was select as the target analyte for measurement, and experimental results show that compared with the commercial standard QTF, the introduction of CH₃NH₃PbI₃ perovskite Schottky junction increases the 2f signal amplitude and signal-to-noise ratio (SNR) by ∼106 times and ∼114 times, respectively. The minimum detection limit (MDL) of this LITES system is 260 ppm, and the corresponding normalized noise equivalent absorption coefficient (NNEA) is 9.21 × 10^-13 cm^-1·W·Hz^-1/2. The Allan analysis of variance results indicate that when the average time is 564 s, the detection sensitivity can reach 83 ppm. This is the first time that QTF resonance detection has been combined with perovskite Schottky junctions for highly sensitive optical gas detection.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) and Beat Frequency-QEPAS techniques for air pollutants detection: A comparison in terms of sensitivity and acquisition time

In this work, a comparison between Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) and Beat Frequency-QEPAS (BF-QEPAS) techniques for environmental monitoring of pollutants is reported. A spectrophone composed of a T-shaped Quartz Tuning Fork (QTF) coupled with resonator tubes was employed as a detection module. An interband cascade laser has been used as an exciting source, allowing the targeting of two NO absorption features, located at 1900.07 cm^-1 and 1900.52 cm^-1, and a water vapor absorption feature, located at 1901.76 cm^-1. Minimum detection limits of 90 ppb and 180 ppb were achieved with QEPAS and BF-QEPAS techniques, respectively, for NO detection. The capability to detect multiple components in the same gas mixture using BF-QEPAS was also demonstrated.

T-type cell mediated photoacoustic spectroscopy for simultaneous detection of multi-component gases based on triple resonance modality

Enhancing multi-gas detectability using photoacoustic spectroscopy capable of simultaneous detection, highly selectivity and less cross-interference is essential for dissolved gas sensing application. A T-type photoacoustic cell was designed and verified to be an appropriate sensor, due to the resonant frequencies of which are determined jointly by absorption and resonant cylinders. The three designated resonance modes were investigated from both simulation and experiments to present the comparable amplitude responses by introducing excitation beam position optimization. The capability of multi-gas detection was demonstrated by measuring CO, CH4 and C2H2 simultaneously using QCL, ICL and DFB lasers as excitation sources respectively. The influence of potential cross-sensitivity towards humidity have been examined in terms of multi-gas detection. The experimentally determined minimum detection limits of CO, CH4 and C2H2 were 89ppb, 80ppb and 664ppb respectively, corresponding to the normalized noise equivalent absorption coefficients of 5.75 × 10-7 cm-1 W Hz-1/2, 1.97 × 10-8 cm-1 W Hz-1/2 and 4.23 × 10-8 cm-1 W Hz-1/2.

Near-infrared sensitive differential Helmholtz-based hydrogen sulfide photoacoustic sensors

A near-infrared (NIR) sub-ppm level photoacoustic sensor for hydrogen sulfide (H₂S) using a differential Helmholtz resonator (DHR) as the photoacoustic cell (PAC) was presented. The core detection system was composed of a NIR diode laser with a center wavelength of 1578.13 nm, an Erbium-doped optical fiber amplifier (EDFA) with an output power of ∼120 mW, and a DHR. Finite element simulation software was used to analyze the influence of the DHR parameters on the resonant frequency and acoustic pressure distribution of the system. Through simulation and comparison, the volume of the DHR was 1/16 that of the conventional H-type PAC for a similar resonant frequency. The performance of the photoacoustic sensor was evaluated after optimizing the DHR structure and modulation frequency. The experimental results showed that the sensor had an excellent linear response to the gas concentration and the minimum detection limit (MDL) for H₂S detection in differential mode can reach 460.8 ppb.

A Miniaturized 3D-Printed Quartz-Enhanced Photoacoustic Spectroscopy Sensor for Methane Detection with a High-Power Diode Laser

In this invited paper, a highly sensitive methane (CH₄) trace gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) technique using a high-power diode laser and a miniaturized 3D-printed acoustic detection unit (ADU) is demonstrated for the first time. A high-power diode laser emitting at 6057.10 cm^-1 (1650.96 nm), with the optical power up to 38 mW, was selected as the excitation source to provide a strong excitation. A 3D-printed ADU, including the optical and photoacoustic detection elements, had a dimension of 42 mm, 27 mm, and 8 mm in length, width, and height, respectively. The total weight of this 3D-printed ADU, including all elements, was 6 g. A quartz tuning fork (QTF) with a resonant frequency and Q factor of 32.749 kHz and 10,598, respectively, was used as an acoustic transducer. The performance of the high-power diode laser-based CH₄-QEPAS sensor, with 3D-printed ADU, was investigated in detail. The optimum laser wavelength modulation depth was found to be 0.302 cm^-1. The concentration response of this CH₄-QEPAS sensor was researched when the CH₄ gas sample, with different concentration samples, was adopted. The obtained results showed that this CH₄-QEPAS sensor had an outstanding linear concentration response. The minimum detection limit (MDL) was found to be 14.93 ppm. The normalized noise equivalent absorption (NNEA) coefficient was obtained as 2.20 × 10^-7 cm^-1W/Hz^-1/2. A highly sensitive CH₄-QEPAS sensor, with a small volume and light weight of ADU, is advantageous for the real applications. It can be portable and carried on some platforms, such as an unmanned aerial vehicle (UAV) and a balloon.

CH4/C2H6 dual gas sensing system using a single mid-infrared laser

Methane (CH₄) and ethane (C₂H₆) dual gas sensor with low system complexity and strong stability is proposed. The correction method based on absorbance spectrum is applied, and the cross-interference of C₂H₆ to CH₄ is eliminated. In the single gas concentration measurement, linear fitting is performed between the absorbance and concentration of CH₄ and C₂H₆, and the correlation coefficients of R² = 0.99959 and R² = 0.99994 are obtained respectively, which proves that the accuracy of the dual gas sensor is robust. In the dual gas concentration measurement, we carry out continuous measurement of five mixed gases and a long-term measurement of a mixture of gases, which verifies that our sensor has the fast response speed and strong stability. The minimum detectable column densities of 0.62 ppm·m for CH₄ and 0.1 ppm·m for C₂H₆ are achieved, respectively. The CH₄/C₂H₆ dual gas sensor assisted by the correction method has high sensitivity and strong robustness to cross-interference, and has great potential for application in various scenarios.

Development and application of an optimal three-wavelength combination for liquid film measurement with absorption spectroscopy

In the simultaneous measurement of liquid film temperature and thickness based on multi-wavelength absorption spectroscopy, selecting optimal wavelength combinations can significantly improve the measurement accuracy. In the work, the absorption spectra of e-liquid at different temperatures were measured firstly. And ten sets of two-wavelength and three-wavelength combinations were then established based on five specific wavelengths from the absorption spectra, respectively. And the measurement accuracy of all the combinations were validated with a home-made calibration system. Among them, an optimal three-wavelength combination were selected. Finally, the evaporation processes of e-liquid films at three initial thicknesses (921/780/629 μm) on a horizontal quartz plate were then investigated with the optimal combination. The variation trends of film temperature and thickness measured by the combination were consistent with imaging method and thermocouple. It was found that the optimal three-wavelength combination could achieve high accuracy in simultaneous measurement of liquid film temperature and thickness.

Highly sensitive photoacoustic acetylene detection based on differential photoacoustic cell with retro-reflection-cavity

In this paper, a highly sensitive photoacoustic spectroscopy (PAS) sensor based on retro-reflection-cavity-enhanced differential photoacoustic cell (DPAC) is demonstrated for the first time. Acetylene (C2H2) was selected as the analyte. The DPAC was designed to effectively suppress noise and increase signal level. The retro-reflection-cavity consisted of two right-angle prisms was designed to reflect the incident light to realize four passes. The photoacoustic response of the DPAC was simulated and investigated based on the finite element method. Wavelength modulation and second harmonic demodulation technologies were applied for sensitive trace gas detection. The first-order resonant frequency of the DPAC was found to be 1310 Hz. The differential characteristics were investigated and the 2f signal amplitude for this C2H2-PAS sensor based on retro-reflection-cavity-enhanced DPAC had a 3.55 times improvement compared to the system without the retro-reflection-cavity. An Allan deviation analysis was performed to investigate the long-term stability of the system. The minimum detection limit (MDL) was measured to be 15.81 ppb with an integration time of 100 s.

Development of Laser Processing Carbon-Fiber-Reinforced Plastic

Due to its exceptional advantages, such as high specific strength, high specific modulus, and good fatigue resistance, carbon-fiber-reinforced plastic (CFRP) is frequently utilized in aerospace, aviation, automotive, rail transportation, and other areas. Composite components typically need to be joined and integrated. In the equipment manufacturing industry, the most used methods for processing composite components are cutting, drilling, and surface treatment. The quality of CFRP is significantly impacted by traditional mechanical processing, causing flaws like delamination, burrs, and tears. Laser processing technology has emerged as a crucial method for processing CFRP for its high quality, non-contact, simple control, and automation features. The most recent research on the laser processing of CFRP is presented in this paper, supporting scientists and engineers who work in the field in using this unconventional manufacturing technique. This paper gives a general overview of the key features of laser processing technology and the numerous machining techniques available. The concepts and benefits of laser processing technology are discussed in terms of the material properties, mode of operation, and laser characteristics, as well as the methods to achieve high efficiency, low damage, and high precision. This paper reviews the research development of laser processing of carbon-fiber-reinforced plastics, and a summary of the factors affecting the quality of CFRP laser processing. Therefore, the research content of this article can be used as a theoretical basis for reducing thermal damage and improving the processing quality of laser-processed composite materials, while, on this basis, we analyze the development trend of CFRP laser processing technology.

Quartz tuning fork-based high sensitive photodetector by co-coupling photoelectric and the thermoelastic effect of perovskite

This paper reports a new strategy for enhancing the photoresponse of a quartz tuning fork (QTF). A deposited light absorbing layer on the surface of QTF could improve the performance only to a certain extent. Herein, a novel strategy is proposed to construct a Schottky junction on the QTF. The Schottky junction presented here consists of a silver-perovskite, which has extremely high light absorption coefficient and dramatically high power conversion efficiency. The co-coupling of the perovskite's photoelectric effect and its related QTF thermoelastic effect leads to a dramatic improvement in the radiation detection performance. Experimental results indicate that the CH₃NH₃PbI₃-QTF obtains two orders of magnitude enhancement in sensitivity and SNR, and the 1σ detection limit was calculated to be 1.9 µW. It was the first time that the QTF resonance detection and perovskite Schottky junction was combined for optical detection. The presented design could be used in photoacoustic spectroscopy and thermoelastic spectroscopy for trace gas sensing.

The Development of a Novel Headspace O2 Concentration Measurement Sensor for Vials

In the process of manufacture and transportation, vials are prone to breakage and cracks. Oxygen (O₂) in the air entering vials can lead to the deterioration of medicine and a reduction in pesticide effects, threatening the life of patients. Therefore, accurate measurement of the headspace O₂ concentration for vials is crucial to ensure pharmaceutical quality. In this invited paper, a novel headspace oxygen concentration measurement (HOCM) sensor for vials was developed based on tunable diode laser absorption spectroscopy (TDLAS). First, a long-optical-path multi-pass cell was designed by optimizing the original system. Moreover, vials with different O₂ concentrations (0%, 5%, 10%, 15%, 20%, and 25%) were measured with this optimized system in order to study the relationship between the leakage coefficient and O₂ concentration; the root mean square error of the fitting was 0.13. Moreover, the measurement accuracy indicates that the novel HOCM sensor achieved an average percentage error of 1.9%. Sealed vials with different leakage holes (4, 6, 8, and 10 mm) were prepared to investigate the variation in the headspace O₂ concentration with time. The results show that the novel HOCM sensor is non-invasive and has a fast response and high accuracy, with prospects in applications for online quality supervision and management of production lines.

Multi-MEMS-microphone schemes in a miniature photoacoustic cell for acetylene trace gas measurement

Dissolved gas analysis is a strong tool for online health monitoring of electrical power equipment. The industry's large-scale deployment of photoacoustic (PA) sensors is still constrained by cost and sensitivity, despite the great accuracy achieved with a mid-infrared light source or optical sensors. We provide a low-cost PA sensor for ppb-level trace gas sensing based on a near-infrared distributed feedback laser source, miniature gas cell, and multiple microelectromechanical system (MEMS) microphones. Five multi-MEMS-microphones schemes are modeled. The simulation indicates that the sensor, including two MEMS microphones in the center of the resonator, is the most cost-efficient option. The experiments that present this scheme can be realized easily by modifying a traditional single microphone PA cell and with ppb-level sensitivity.

Characterization of H2S QEPAS detection in methane-based gas leaks dispersed into environment

The increase in fatal accidents and chronic illnesses caused by hydrogen sulfide (H2S) exposure occurring in various workplaces is pushing the development of sensing systems for continuous and in-field monitoring of this hazardous gas. We report here on the design and realization of a Near-IR quartz-enhanced photoacoustic sensor (QEPAS) for H2S leaks detection. H2S QEPAS signal was measured in matrixes containing up to 1 % of methane (CH4) and nitrogen (N2) which were chosen as the laboratory model environment for leakages from oil and gas wells or various industrial processes where H2S and CH4 can leak simultaneously. An investigation of the influence of CH4 on H2S relaxation and photoacoustic generation was proposed in this work and the sensor performances were carefully assessed with respect to CH4 content in the mixture. We demonstrated the high selectivity, with no cross talk between H2S, H2O and CH4 absorption lines, high sensitivity, and fast response time of the developed sensor, achieving a minimum detection limit (MDL) of 2.5 ppm for H2S with 2 s lock-in integration time. The employed 2.6 µm laser allowed us to employ the sensor also for CH4 detection, achieving an MDL of 85 ppm. The realized QEPAS sensor lends itself to the development of a portable and compact device for industrial monitoring.

Parts-per-billion-level detection of hydrogen sulfide based on doubly resonant photoacoustic spectroscopy with line-locking

We report on the development of a highly sensitive hydrogen sulfide (H₂S) gas sensor exploiting the doubly resonant photoacoustic spectroscopy technique and using a near-infrared laser emitting at 1578.128 nm. By targeting the R(4) transition of H₂S, we achieved a minimum detection limit of 10 part per billion in concentration and a normalized noise equivalent absorption coefficient of 8.9 × 10^-12 W cm^-1 Hz^-1/2. A laser-cavity-molecule locking strategy is proposed to enhance the sensor stability for fast measurement when dealing with external disturbances. A comparison among the state-of-the-art H₂S sensors using various spectroscopic techniques confirmed the record sensitivity achieved in this work.

Super tiny quartz-tuning-fork-based light-induced thermoelastic spectroscopy sensing

In this Letter, a sensitive light-induced thermoelastic spectroscopy (LITES)-based trace gas sensor by exploiting a super tiny quartz tuning fork (QTF) was demonstrated. The prong length and width of this QTF are 3500 µm and 90 µm, respectively, which determines a resonant frequency of 6.5 kHz. The low resonant frequency is beneficial to increase the energy accumulation time in a LITES sensor. The geometric dimension of QTF on the micrometer scale is advantageous to obtain a great thermal expansion and thus can produce a strong piezoelectric signal. The temperature gradient distribution of the super tiny QTF was simulated based on the finite element analysis and is higher than that of the commercial QTF with 32.768 kHz. Acetylene (C₂H₂) was used as the analyte. Under the same conditions, the use of the super tiny QTF achieved a 1.64-times signal improvement compared with the commercial QTF. The system shows excellent long-term stability according to the Allan deviation analysis, and a minimum detection limit (MDL) would reach 190 ppb with an integration time of 220 s.

Pre-Shaped Burst-Mode Hybrid MOPA Laser System at 10 kHz Pulse Frequency

A temporal pre-shaped burst-mode hybrid fiber-bulk laser system was illustrated at a 10 kHz rate with a narrow spectral linewidth. A theoretical model was proposed to counteract the temporal profile distortion and compensate for the desired one, based on reverse process of amplification. For uniformly modulated injection, amplified shapes were recorded and investigated in series for their varied pulse duration, envelope width and amplification delay, respectively. The pre-shaped output effectively realized a uniform distribution on a time scale for both the burst envelope and pulse shape under the action of the established theoretical method. Compared with previous amplification delay methods, this model possesses the capacity to extend itself for applications in burst-mode shaping with variable parameters and characteristics. The maximum pulse energy was enlarged up to 9.68 mJ, 8.94 mJ and 6.57 mJ with a 300 ns pulse duration over envelope widths of 2 ms to 4 ms. Moreover, the time-averaged spectral bandwidths were measured and characterized with Lonrentz fits of 68.3 MHz, 67.2 MHz and 67.7 MHz when the pulse duration varied from 100 ns to 300 ns.