Photoacoustic Spectroscopy System Research Articles

High performance filtering and high-sensitivity concentration retrieval of methane in photoacoustic spectroscopy utilizing deep learning residual networks

A novel method is introduced to improve the detection performance of photoacoustic spectroscopy for trace gas detection. For effectively suppressing various types of noise, this method integrates photoacoustic spectroscopy with residual networks model which encompasses a total of 40 weighted layers. Firstly, this approach was employed to accurately retrieve methane concentrations at various levels. Secondly, the analysis of the signal-to-noise ratio (SNR) of multiple sets of photoacoustic spectroscopy signals revealed significant enhancement. The SNR was improved from 21 to 805, 52–962, 98–944, 188–933, 310–941, and 587–936 across the different concentrations, respectively, as a result of the application of the residual networks. Finally, further exploration for the measurement precision and stability of photoacoustic spectroscopy system utilizing residual networks was carried out. The measurement precision of 0.0626 ppm was obtained and the minimum detectable limit was found to be 1.47 ppb. Compared to traditional photoacoustic spectroscopy method, an approximately 46-fold improvement in detection limit and 69-fold enhancement in measurement precision were achieved, respectively. This method not only advances the measurement precision and stability of trace gas detection but also highlights the potential of deep learning algorithms in spectroscopy detection.

High sensitivity and stability cavity-enhanced photoacoustic spectroscopy with dual-locking scheme

We present a high sensitivity and long-term stability cavity-enhanced photoacoustic spectroscopy (CE-PAS) system with optical cavity and acoustic frequency dual-locking scheme for trace acetylene (C2H2) detection. The first mechanism involves locking the optical wavelength to the cavity resonance by comparing the phase-sensitive component of the reflected light with a reference signal in a feedback loop. The second locking mechanism controls the gas proportion inside the photoacoustic cell to lock the acoustic frequency, suppressing the drift caused by environmental temperature variation. By minimizing amplitude fluctuations through dual-locking, the sensor achieves improved stability with reduced fluctuations from ±10.12 % to ±1.16 %. The linear responsivity and excellent linearity of the sensor are demonstrated over a concentration variation spanning four orders of magnitude. Experimental results showcase a minimum detection limit of 1.2 parts-per-billion (ppb) at an integration time of 400 s, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 1.37×10−11 cm−1·W· Hz−1/2 for C2H2 detection. Long-term stability is within ±2 % over a 15-day period. The combination of high sensitivity and long-term stability make this CE-PAS sensor suitable for a wide range of applications in environmental monitoring, industrial process control, and gas leak detection.

A dual‐resonance enhanced photoacoustic spectroscopy gas sensor based on a fiber optic cantilever beam microphone and a spherical photoacoustic cell

AbstractWe propose a novel high‐performance dual‐resonance enhanced photoacoustic spectroscopy (DRE‐PAS) gas sensor based on a highly sensitive fiber optic cantilever beam microphone and a high‐Q spherical photoacoustic cell (PAC). The first‐order resonant frequency (FORF) of the spherical PAC is analyzed by finite element analysis to match the FORF of the cantilever microphone for the double resonance enhancement of the photoacoustic signal. The photoacoustic spectroscopy (PAS) system, including the DRE‐PAS sensor, a 1532.8 nm distributed feedback laser, and a high‐speed spectrometer, has been successfully exploited for trace acetylene (C2H2) detection. The experimental results show that the limit of detection (LOD) is 106.8 parts‐per‐billion (ppb) with an integral time of 1 s, and the LOD can be further reduced to 11.03 ppb by Allan‐Werle deviation for 100 s integral time. The normalized noise equivalent absorption coefficient can be obtained as 2.44 × 10−8 cm−1 WHz−1/2. The reported DRE‐PAS gas sensor has the superior characteristics of photoacoustic signal enhancement, high sensitivity, and strong antielectromagnetic interference capability, which can provide a new solution for PAS development.

Fiber optic photoacoustic gas sensor enhanced by multi-pass cell with overlapping phantom spots

A neotype fiber optic photoacoustic gas sensor with overlapping phantom spots based multi-pass is proposed, which greatly improves the reflection times and the length of gas absorption path. Moreover, the detection accuracy of photoacoustic spectroscopy (PAS) system is independent of the optical interference effect caused by overlapping spots, which is different from tunable diode laser absorption spectroscopy (TDLAS) technology. A mathematical model including light beam motion space, spot size and overlap discrimination of multi-pass structure based on overlapping phantom spots is established. The incident light is reflected up to dozens of times in the resonator with a diameter of only 8 mm. The emergent light does not need to converge on a photodetector, which reduces the difficulty of adjusting the optical path and promotes the realization of overlapping phantom spots. The experimental results show that the amplitude of photoacoustic signal is improved by about 40 times compared with the single spot structure. Appling high-sensitivity optical fiber cantilever to detect the sound pressure signal. The detection limit of C2H2 gas reaches 8.2 ppt. The normalized noise equivalent absorption (NNEA) coefficient of the gas sensor is achieved to be 9.3×10−11 cm−1WHz−1/2.

TT-type resonator-based differential photoacoustic spectroscopy for trace gas detection.

A novel TT-type resonator was proposed for the first time, to our knowledge, to realize differential photoacoustic (PA) detection for trace gas measurement. The special design of the TT-type resonator allows us to install the microphone at the resonant center of the acoustic field to maximize the use of the absorption-induced PA signal. To meet the requirement of low gas consumption and easy integration, the TT-type resonator-based PA cell was fabricated as a fiber-coupled module with an inner volume of only 1.1 ml. For validation, the TT-type PA cell was integrated to a photoacoustic spectroscopy (PAS) system for acetylene detection. As a result, a linearity of 0.99999 was achieved in a concentration range from 0 to 5000 ppm with a noise equivalent sensitivity of 101 ppb. The proposed TT-type resonator contributes a new style of PA cell structure to the field of PAS gas detection, combining the advantages of easy integration, low gas consumption, differential detection, and photoacoustic enhancement together.

All-optical non-resonant photoacoustic spectroscopy for multicomponent gas detection based on aseismic photoacoustic cell

An all-optical non-resonant photoacoustic spectroscopy system for multicomponent gas detection based on a silicon cantilever optical microphone (SCOM) and an aseismic photoacoustic cell is proposed and demonstrated. The SCOM has a high sensitivity of over 96.25 rad/Pa with sensitivity fluctuation less than ± 1.56 dB between 5 Hz and 250 Hz. Besides, the minimal detectable pressure (MDP) of the sensor is 0.55 μPa·Hz−1/2 at 200 Hz, which indicates that the fabricated sensor has high sensitivity and low noise level. Six different gases of CO2, CO, CH4, C2H6, C2H4, C2H2 are detected at the frequency of 10 Hz, whose detection limits (3σ) are 62.66 ppb, 929.11 ppb, 1494.97 ppb, 212.94 ppb, 1153.36 ppb and 417.61 ppb, respectively. The system achieves high sensitivity and low detection limits for trace gas detection. In addition, the system exhibits seismic performance with suppressing vibration noise by 4.5 times, and achieves long-term stable operation. The proposed non-resonant all-optical PAS multi-component gas detection system exhibits the advantages of anti-vibration performance, low gas consumption and long term stability, which provides a solution for working in complex environments with inherently safe.

Feasibility study of detecting some milk adulterations using a LED-based Vis-SWNIR photoacoustic spectroscopy system.

The aim of this study is to evaluate a previousely developed photoacoustic spectroscopy system with light sources of visible to short-wave near infrared (Vis-SWNIR, 395-940nm) for detection of adulterations in cow's milk including formalin, urea, hydrogen peroxide, starch, sodium hypochlorite, and detergent powder. The results of principal component analysis (PCA) showed a very good visual differentiation of different adulterations. The artificial neural networks (ANN) showed the highest classification accuracy (97.6%) in detection of adulteration type and adulteration level (nearly 100%). It can be generally concluded that the Vis-SWNIR photoacoustic spectroscopy system is a reliable and potent instrument for detecting various types of milk adulterations. Further studies are suggested with including cow's milk of different sources with probable variations in composition to generalize the findings of the present study. With the extension of the light sources to the range of long-wave NIR, the system can be applied as a diagnostic tool for quality evaluation of other liquid foods.

Highly sensitive trace gas detection based on a miniaturized 3D-printed Y-type resonant photoacoustic cell.

We report, what we believe to be, a novel miniaturized 3D-printed Y-type resonant photoacoustic cell (YRPAC) consisting of a frustum of cone-type buffer chamber and a cylindrical resonant chamber. The volume of the designed YRPAC is about 7.0 cm3, which is only about a half of the T-resonant photoacoustic cell (TRPAC). The finite element simulation of the sound field distribution of the TRPAC and YRPAC based on COMSOL shows that the photoacoustic signal is enhanced with the shape of the buffer chamber changing from the traditional cylinder to a frustum of cone. The photoacoustic spectroscopy (PAS) system, utilizing the YRPAC and TRPAC as the photoacoustic reaction units, a 1653.7 nm distributed feedback (DFB) laser as the excitation light source, a cantilever beam acoustic sensor as the acoustic sensing unit, and a high-speed spectrometer as the demodulation unit, has been successfully developed for high-sensitivity trace CH4 sensing. When the CH4 concentration is 1000 ppm, the 2f signal of YRPAC in the first-order resonance mode is 2.3 nm, which is 1.7 times higher than the 2f signal amplitude of TRPAC. The detection sensitivity and minimum detection limit for the PAS system are 2.29 pm/ppm and 52.8 parts per billion (ppb) at 100 s of averaging time. The reported YRPAC has higher sensitivity, smaller size, and faster response time compared to the conventional TRPAC, which can provide a new solution for PAS development.

Simultaneous detection of greenhouse gases CH4 and CO2 based on a dual differential photoacoustic spectroscopy system.

In addition to the atmospheric measurement, detection of dissolved carbon oxides and hydrocarbons in a water region is also an important aspect of greenhouse gas monitoring, such as CH4 and CO2. The first step of measuring dissolved gases is the separation process of water and gases. However, slow degassing efficiency is a big challenge which requires the gas detection technology itself with low gas consumption. Photoacoustic spectroscopy (PAS) is a good choice with advantages of high sensitivity, low gas consumption, and zero background, which has been rapidly developed in recent years and is expected to be applied in the field of dissolved gas detection. In this study, a miniaturized differential photoacoustic cell with a volume of 7.9 mL is designed for CH4 and CO2 detection, and a dual differential method with four microphones is proposed to enhance the photoacoustic signal. What we believe to be a new method increases photoacoustic signal by 4 times and improves the signal to noise ratio (SNR) over 10 times compared with the conventional single-microphone mode. Two distributed feedback (DFB) lasers at 1651 nm and 2004nm are employed to construct the PAS system for CH4 and CO2 detection respectively. Wavelength modulation spectroscopy (WMS) and 2nd harmonic demodulation techniques are applied to further improve the SNR. As a result, sensitivity of 0.44 ppm and 7.39 ppm for CH4 and CO2 are achieved respectively with an integration time of 10 s. Allan deviation analysis indicates that the sensitivity can be further improved to 42 ppb (NNEA=4.7×10-10cm-1WHz-1/2) for CH4 and 0.86 ppm (NNEA=5.3×10-10cm-1WHz-1/2) for CO2 when the integration time is extended to 1000 s.

Mid-infrared photoacoustic spectroscopy based on ultrasound detection for blood component analysis.

For the non-invasive measurement of biological tissue, a piezoelectric photoacoustic spectroscopy (PZT-PAS) system that detects a single frequency of ultrasound induced by the irradiation of pulse-modulated mid-infrared laser light was developed. PA spectra of the optical phantom and biological samples were obtained, and the relationship between the PA signal intensity and optical absorbance in the fingerprint region (930-1,200 cm-1) was analyzed to estimate the optical absorbance. The resonance vibration of the induced ultrasound was utilized to further increase the signal strength for biological tissue measurement. Consequently, PA spectrum reflecting the absorption of components in biological tissues was obtained.

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.

NIR and THz spectroscopy: An experimental investigation toward nicotine-related devices.

This study examined the content of nicotine-delivery products using terahertz time-domain spectroscopy (THz-TDS) and breath ethylene investigated with CO2 laser photoacoustic spectroscopy (CO2 LPAS) system as a biomarker of oxidative stress after smoking. The THz-TDS method provided valuable information on the transmission spectra of tobacco and nicotine in smoking products. From the CO2 LPAS data it was observed that in cigarette (TC) smoking the mean breath ethylene was 687 parts per billion (ppb), while in electronic cigarettes and tobacco heating devices smoking the mean ethylene was 56 ppb and 48 ppb, respectively. The main finding was that TC showed higher transmission in the THz region producing a higher oxidative stress on the body.

Near-infrared quartz-enhanced photoacoustic spectroscopy system for ppb-level methane detection

In this paper we present a quartz-enhanced photoacoustic spectroscopy (QEPAS) of methane near 1651 nm. QEPAS is a high-sensitivity gas sensing method that relies on detecting acoustic waves generated by gas molecules. The sensor setup consists of a bismuth-doped fiber amplifier (BDFA) operating at 1651 nm that is used to enhance the amplitude of the QEPAS signal and increase the detection sensitivity. With the BDFA delivering ∼250 mW of optical power to the sample, the minimum detection limit of ∼11 ppb was achieved for the integration time of 150 s.

On the measurement of local vapor concentration around sessile water droplet with high spatiotemporal resolution

Droplet evaporation is a seemingly simple heat and mass transfer process, while of which high-fidelity characterization of the vapor field remains a long-standing challenge. In this brief communication, we report the first experimental observation of the local vapor concentration of the sessile water droplet with unprecedently high spatiotemporal resolution. The experimental measurement was enabled by a tailored quartz-enhanced photoacoustic spectroscopy (QPEAS) system. In this system, a near-infrared, continuous-wave, distributed feedback laser at 1392 nm was utilized to excite the photoacoustic signal of water vapor above the sessile droplet. The photoacoustic signal between the prongs of a tiny quartz tuning fork (QTF) was detected and further interpreted as the vapor concentration information. The feasibility of QEPAS for droplet evaporation characterization was carefully verified. As a demonstration, we measured the time histories of water vapor concentration above the evaporating droplet in both open and closed chambers at the spatial resolution of ∼30 micrometers and temporal resolution of 0.1 ms. Interestingly, we found the vapor concentration above the evaporating droplet in open ambient featured distinct fluctuations indiscernible to the conventional photography method. Such fluctuation has never been reported before and further corroborated the insufficiency of quasi-steady approximation for droplet evaporation. This work not only extends the application of QEPAS to more complicated physical processes, but also advances the state of the art in the experimental characterization of sessile water droplet evaporation, which opens up a new possibility to unravel the intricate mechanism of the vapor-mediated droplet motions.

Ozone detection based on an nitrogen oxide photoacoustic spectroscopy system and chemical reaction

Ozone detection based on an nitrogen oxide photoacoustic spectroscopy system and chemical reaction

Compact QEPAS CO2 sensor system using a quartz tuning fork-embedded and in-plane configuration

A quartz-enhanced photoacoustic spectroscopy (QEPAS) system was developed based on a novel quartz tuning fork-embedded, and in-plane QEPAS technique, while increasing the efficiency of assembly alignment with the aid of 3D printing technology. In this paper, it was verified that the simplified numerical model enabled rapid optimization of the dimensions of the acoustic micro-resonator (AmR), and this was validated by experimental results. The optimized results are as follows: outer diameter = 3.2 mm, inner diameter = 0.5 mm, length = 6.77 mm. The acoustic transducer used a commercial quartz tuning fork with a resonance frequency of 30.69 kHz. After optimizing the laser wavelength modulation amplitude and the dimensions of the AmR, a minimum CO2 detection limit of 1.06 ppm was obtained, corresponding to a normalized noise equivalent absorption of 5.5 × 10−9 cm−1 WHz−1/2.

Measuring ammonia concentrations by an infrared photo-acoustic multi-gas analyser in an open dairy barn: Repetitions planning strategy

The infrared photoacoustic spectroscopy is a common gas sensing technique used for research activities to perform continuous measurement of gas concentrations. Few research works have investigated how to collect data and obtain reliable measurements in open and semi-open dairy houses for gas concentration estimation.In the present study, measurements of ammonia (NH3) concentrations were carried out in a semi-open free-stall dairy barn by using a commercial photo-acoustic spectroscopy system (INNOVA by LumaSense Technologies A/S, Denmark), widely used for scientific purposes. Several repetitions for each sampling location (SLs) were investigated in in-field conditions to study the variability of the measurement in each repetition. Based on acquired data, one-way analyses of variance (ANOVA) were carried out for different groups of data. The achieved results showed that the measurement strategy depended on the number of repetitions with effect on the variability of each measurement. In detail, the number of repetitions performed had a significant influence on data collection (P < 0.001). From the outcomes of the research, it is recommended to: perform three repetitions, or at most, four repetitions for each SLs; exclude the first repetition from the data analyses because it reduces measurement variability; consider the mean value of the second and third repetitions as the NH3 concentration value determined at each SL; up to four repetitions (that would take a five-minutes acquisition) are recommended because measurement changed in time when a higher number of repetitions were carried out. The measurement design should be planned based on the number of repetitions and the number of SLs in order to perform more than one SL measurement cycle within an hour. Since the measurement cycles are strictly related to the number of SLs in the barn, they depend on the barn typology and dimensions.The application of a specific measurement strategy is valuable to obtain reliable data that could be used in the research activities focused on estimation of gas concentrations for monitoring air quality, improving animal welfare, assessing mitigation strategies, or estimating emissions in open and semi-open free-stall dairy barns.

Analysis of the Horizontal Distribution of Sampling Points for Gas Concentrations Monitoring in an Open-Sided Dairy Barn.

In the livestock sector, the monitoring of gas concentrations plays a relevant role in the implementation of mitigation strategies, as well as the improvement in the indoor air quality for both workers and animals in barns. In the present study, measurements of gas concentrations (NH3, CH4, and CO2) at different locations were carried out in an open dairy barn using a commercial photo-acoustic spectroscopy system. Measurement variability for different datasets was computed to contribute to the evaluation of the sampling strategy in the measurement campaign. The results showed that the position of sampling locations (SLs) significantly influenced (p < 0.001) the variability in the gas distribution. Specifically, the SLs located on the perimeter of the building had the highest variability. The number of SLs utilised for determining the mean value of gas concentration in the barn had a significant influence on NH3 (p < 0.001), CO2 (p < 0.001), and CH4 (p < 0.001) in both the central zone and the perimeter one. At least three SLs were necessary to obtain a mean value of gas concentration that reduced the variability to under the 10% in the central zone and 16% in the perimeter one. Moreover, the NH3 concentrations obtained as the mean value of the gas measurements at two SLs with a 10 m distance had a lower variability compared to those recorded at two SLs located at 5 m.

High-Sensitivity Multitrace Gas Simultaneous Detection Based on an All-Optical Miniaturized Photoacoustic Sensor.

We propose an all-optical miniaturized multigas simultaneous detection photoacoustic (PA) sensor, which is primarily composed of a copper tube, a silica cantilever, and four single-mode fibers. Three single-mode fibers are used as excitation fibers to transmit lasers of different wavelengths, and the remaining one is used as a probe fiber. The volumes of the PA cell (PAC) and the sensor are 36 μL and 71 cubic millimeters, respectively. A laser photoacoustic spectroscopy (PAS) system, using the all-optical miniaturized PA sensor as a detector, 1532.8, 1576.3, and 1653.7 nm distributed feedback (DFB) lasers as the excitation sources for acetylene (C2H2), hydrogen sulfide (H2S), and methane (CH4) gases, and a high-speed spectrometer as a demodulator, has been developed for multigas simultaneous measurements. The minimum detection limits of 4.8, 162, and 16.6 parts per billion (ppb) have been achieved for C2H2, H2S, and CH4, respectively, with an integration time of 100 s. The reported sensor shows a potential for high-sensitivity multigas simultaneous measurements in cubic millimeter-scale space.

Simultaneous measurement of NO and NO2 by dual-channel cavity photoacoustic spectroscopy technique

The levels of nitric oxide (NO) and nitrogen oxide (NO2) are trace gases in the order of ppb, which are the main products of fossil fuel, anthropogenic combustion, natural lightning and microbial emissions, and are important for further analysis of air quality. Photoacoustic spectroscopy (PAS) technique, which is based on absorption at the wavelength of 403.65 nm, has been used for NO2 measurements. In this study, a dual-channel PAS system for NO2 and NOx detection is reported. The PAS method is capable of measuring species with good sensitivity and accuracy. The calibrated slopes of photoacoustic cavity A and B are 0.040 μv/ppb and 0.039 μv/ppb, respectively. And the detection limits of the developed PAS system for NO2 and NOx measurements are estimated to be about 1.47 ppb (3σ, 60 s) and 1.50 ppb (3σ, 60 s), respectively. In order to reduce the influence of local noise, the photoacoustic cavity with multiple buffer partitions is designed and the corresponding photoacoustic frequency of the photoacoustic pool is analyzed. The sound pressure value and resonance frequency of the photoacoustic pool gradually decrease with the increase of the number of buffer partitions, but the amplitude is small. The relationship between the gas absorption cross section, volume fraction, laser power and photoacoustic excitation signal is derived. After deducting the influence of local noise, the photoacoustic response gradients of dual-channel cavities are obtained, the linear fitting of the response gradient of both cavities is good, the correlation R2 is greater than 0.99, and the fitting slope is 0.040 and 0.039 respectively. The influence of O3 generated by photolysis of the mercury lamp is considered under different NO level conditions. An intercomparison between the system and a cavity ring-down system (CRDS) independently developed by our research group showed a good agreement for measuring the ambient atmosphere NO2 gas. The results of NO2 showed a linear correction factor (R2) of 0.943 in a slope of 0.944 ± 0.002, with an offset of (0.636 ± 0.021) ppb. The accuracy of NO2 measurement by the photoacoustic spectroscopy system was verified. In addition, the simultaneous measurement of NO and NO2 by dual-channel cavity photoacoustic spectroscopy technique was deployed. These observations indicate that NO2 and NO concentrations can be effectively observed with the dual-channel PAS instrument.