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• The baseline signal is corrected and the absorbance signal is denoised by using the fitting neural network. • Based on a simple system structure, it is possible to increase the SNR of the absorbance signal to about 20 dB. • Line shape function is used for feature selection to reduce the training difficulty of the neural network. We designed a methane/ethane (CH 4 /C 2 H 6) dual-gas sensor on the basis of direct absorption spectroscopy (DAS) technology and explored the potential for low power consumption and miniaturization of the detection system using the effective optical range is only 25 cm for absorption cell and an uncooled photodetector. The problem of baseline drift introduced by uncooled photodetectors has been improved by using of a fitting neural network approach, and absorbance spectrum denoising has been realized. We established the mathematical relationship between absorbance and concentration for each gas of CH 4 /C 2 H 6 through experimental detection, which shows strong linearity. And the correlation coefficients (R2) reached 0.99995 and 0.99998, demonstrating the robustness of the sensor. Through 3 h of measured and evaluated system, we obtained the minimum detectable column densities of 0.588 ppm⋅m and 0.128 ppm⋅m for CH 4 and C 2 H 6 , respectively, which proved that the sensor has high sensitivity, strong stability, and has broad application prospects. [ABSTRACT FROM AUTHOR]

• In this paper, a second harmonic noise suppression method based on SSA-VMD-WTD is proposed, which can extract useful signals under the condition of low SNR. • This paper firstly describes the principle and steps of the method theoretically, and verifies the feasibility of the method through simulation and experiment. • This is an adaptive denoising method that does not require analyzing the characteristics of the noise in advance. Aiming at the problem that second harmonics generated in gas measurement by photoacoustic spectroscopy (PAS) technology based on photoacoustic effect are susceptible to noise. In this paper, a method based on Sparrow Search Algorithm (SSA) optimized Variational Mode Decomposition (VMD) combined with Wavelet Threshold Denoising (SSA-VMD-WTD) was proposed to identify and suppress the noise. Firstly, the SSA algorithm optimizes the VMD parameters, and then the second harmonic signal with noise is decomposed by VMD. Secondly, the Intrinsic Mode Function (IMF) with a smaller Variance Contribution Rate (VCR) is removed, and the optimal mode function is selected to reconstruct the signal. Finally, the reconstructed signal is denoised by WTD, and the final denoised signal is obtained. The algorithm is applied to process simulated and experimental second harmonic signals separately. In the processing results of the simulated signal, it is observed that compared to three different algorithms, namely wavelet filtering, Savitzky-Golay (S-G) filtering, and complete ensemble empirical mode decomposition with adaptive noise and WTD (CEEMDAN-WTD), the proposed algorithm demonstrates more effective identification and noise suppression in the second harmonic signal. In the processing results of the experimental signal, the correlation coefficient (R2) between gas concentration and the maximum value of the second harmonic signal has been improved from the original value of 0.98144 to 0.99773. With the SSA-VMD-WTD enhancement, the optimized acetylene (C 2 H 2) sensor features precision and stability in real-time measurements and achieves the minimum detection limit (MDL) of 7.82 ppm (1σ). The results indicate that the proposed denoising method can more effectively, stably, and accurately process noisy signals. [ABSTRACT FROM AUTHOR]

An intra-cavity photoacoustic spectroscopy dual-gas detection system using dual wavelength fiber laser has been developed and experimentally demonstrated. The output wavelength of the fiber laser can be tuned within the spectral range of 1520–1600 nm. Using a single fiber laser, the detection of different gases can be easily achieved by changing the wide fiber Bragg grating (FBG) and seed laser. Photoacoustic spectroscopy with the laser intra-cavity technique is employed to achieve high detection sensitivity. The mixture of carbon dioxide (CO 2) and acetylene (C 2 H 2) is selected as target gas for evaluating the detection characteristics of the dual-gas detection system. The minimum detection limits (MDLs) of ∼71.82 ppm for monitoring CO 2 and ∼924.24 ppb for monitoring C 2 H 2 are realized. The concentrations of CO 2 and C 2 H 2 are linearly fitted with the photoacoustic signal, with R-squared values of 0.99859 and 0.99732, respectively, proving the accuracy and robustness of the dual-gas detection system. [Display omitted] • An intra-cavity photoacoustic spectroscopy dual-gas detection system using dual wavelength fiber laser has been proposed. • C 2 H 2 , CO 2 , CO, NH 3 , and H 2 S can be detected by selecting suitable seed laser and wide FBG. • The experimental results show that the MDL of CO 2 and C 2 H 2 are ∼71.82 ppm and ∼924.24 ppb, respectively. [ABSTRACT FROM AUTHOR]

A simple laser wavelength calibration technique, based on second harmonic signal, is demonstrated in this paper to improve the performance of quartz enhanced photoacoustic spectroscopy (QEPAS) gas sensing system, e.g. improving the signal to noise ratio (SNR), detection limit and long-term stability. Constant current, corresponding to the gas absorption line, combining f /2 frequency sinusoidal signal are used to drive the laser (constant driving mode), a software based real-time wavelength calibration technique is developed to eliminate the wavelength drift due to ambient fluctuations. Compared to conventional wavelength modulation spectroscopy (WMS), this method allows lower filtering bandwidth and averaging algorithm applied to QEPAS system, improving SNR and detection limit. In addition, the real-time wavelength calibration technique guarantees the laser output is modulated steadily at gas absorption line. Water vapor is chosen as an objective gas to evaluate its performance compared to constant driving mode and conventional WMS system. The water vapor sensor was designed insensitive to the incoherent external acoustic noise by the numerical averaging technique. As a result, the SNR increases 12.87 times in wavelength calibration technique based system compared to conventional WMS system. The new system achieved a better linear response ( R 2 = 0 . 9995 ) in concentration range from 300 to 2000 ppmv, and achieved a minimum detection limit (MDL) of 630 ppbv. [ABSTRACT FROM AUTHOR]

We proposed a new method for gas detection in photoacoustic spectroscopy based on acousto-optic Q-switched fiber laser by merging a transmission PAS cell (resonant frequency f 0 = 5.3 kHz) inside the fiber laser cavity. The Q-switching was achieved by an acousto-optic modulator, achieving a peak pulse power of ~679 mW in the case of the acousto-optic modulation signal with an optimized duty ratio of 10%. We used a custom-made fiber Bragg grating with a central wavelength of 1530.37 nm (the absorption peak of C2H2) to select the laser wavelength. The system achieved a linear response (R2 = 0.9941) in a concentration range from 400 to 7000 ppmv, and the minimum detection limit compared to that of a conventional intensity modulation system was enhanced by 94.2 times. [ABSTRACT FROM AUTHOR]

• A long-distance free space QEPTS gas detection system is proposed. • An integral MDL of 3.79 ppb·m is achieved at 502 s integration time. • The system has a good ability to monitor CH 4 leakage. Based on the quartz tuning fork enhanced photothermal spectroscopy (QEPTS) technology, a long-distance free space gas detection system was proposed to monitor CH 4 leakage. A CH 4 absorption line at 1650.961 nm was selected for this gas detection system to reduce the interference of water vapor and carbon dioxide in the atmosphere. The Allan deviation analysis showed that the integral detection limit of this long-distance free space gas detection system for CH 4 detection could reach 3.79 ppb·m at the optimal integration time of 502 s. And the linear response of the proposed sensor was evaluated, with yielded R-square value of 0.997 being obtained. In addition, a CH 4 cylinder with concentrations of 5% was placed in different positions to simulate the leakage of CH 4 in free space, and the experimental results showed that the long-distance free space with a QEPTS gas detection system could meet the need for CH 4 leakage detection in the practical engineering application. [ABSTRACT FROM AUTHOR]

• A long-path QEPTS gas sensor using a high power Q-switched fiber laser was proposed. • Long optical path and high power are beneficial to the QEPTS sensor. • A minimum detection limit (MDL) of 6.1 ppbv at the 48 s integration time was achieved. A long-path quartz tuning fork enhanced photothermal spectroscopy (QEPTS) gas sensor using a Q-switched fiber laser is proposed. A theoretical model is used to describe the process of the long-path QEPTS sensor and the factors that affect its performance. We can conclude that long optical path and high power are beneficial to the QEPTS sensor from the theoretical analysis. And we use our long-path QEPTS sensor for C 2 H 2 detection at the wavelength of 1531.59 nm. With the scanning time of 4 s and the peak pulse power of 127.5 mW, an Allan deviation analysis shows that our sensor has a minimum detection limit (MDL) of 6.1 ppbv at the 48 s integration time and a good linear response (R-square = 0.99796). [ABSTRACT FROM AUTHOR]

In this paper, we reported a new multi-point sensor for C 2 H 2 detection based on photoacoustic spectroscopy and acousto-optic Q-switched fiber laser. Three transmission-type PAS cells were placed inside the fiber laser cavity and used for C 2 H 2 detection simultaneously to demonstrate the potential of spatially resolved measurements. A maximum pulse peak power of ∼544 mW was generated by the acousto-optic Q-switched fiber laser. The signal-to-noise ratios (SNR) calculated from the experimental results was 57.7 dB, 45.1 dB and 57.2 dB. This resulted in a minimum detection limit (MDL) for C 2 H 2 of 1.73 ppmv, 7.82 ppmv and 1.93 ppmv for PAS cell 1, PAS cell 2 and PAS cell 3, respectively. [ABSTRACT FROM AUTHOR]

• A scanned-wavelength intra-cavity QEPAS sensor with injection seeding technique and Q-switched technique was reported. • The Q-switched fiber laser has a peak pulse power of 0.88 W. • A minimum detection limit (MDL) of 507 ppbv was achieved. A high sensitivity scanned-wavelength intra-cavity quartz enhanced photoacoustic spectroscopy (QEPAS) system, based on injection seeding technique and Q-switched technique, was proposed in this paper. In this system, the gas cell was placed in the acousto-optic Q-switched fiber laser cavity, and the high power Q-switched pulses were used to acoustic wave excitation. A distributed feedback (DFB) diode laser with a wavelength scanning range from 1532.63 nm to 1533.03 nm was used as the seed laser to select the laser wavelength. The laser power emitted by the seed laser operating at an injection current of 56 mA (the gas absorption peak) was 3 mW. A custom-made broadband fiber Bragg grating (FBG) with a bandwidth of 500 pm (picometer) was used to stabilize the cavity operation and suppress the amplified spontaneous emission (ASE) spectrum. With an intra-cavity peak pulse power of 0.88 W, the system achieved a minimum detection limit (MDL) of 507 ppbv at atmospheric pressure and room temperature, corresponding to a normalized noise-equivalent absorption (NNEA) of 1.616 × 10−8 W cm−1 Hz−1/2. [ABSTRACT FROM AUTHOR]

• A QTF-self-difference QEPTS sensor for C 2 H 2 detection was proposed. • The SNR could be increased 5.1 times by using the QTF-self-difference technique. • The system achieved a MDL of 723 ppbv, corresponding to a NNEA of 7.85 × 10−10 cm−1 W/Hz1/2. A novel self-difference technique based on quartz tuning fork (QTF) is proposed in this manuscript. C 2 H 2 is selected as the target analyte to verify the effectiveness of this technique. The measurement technique consists of two laser beams, one laser beam passes through the gas cell as the probe laser beam and the other beam is used as the reference laser beam. The vibration direction of the QTF caused by the two laser beams is opposite by adjusting the position where the two laser beams illuminate QTF. The two quartz tuning fork enhanced photothermal spectroscopy (QEPTS) signals are subtracted by utilizing the mechanical vibration characteristics of the QTF. As a result, the SNR increases 5.1 times in QTF-self-difference based system compared to a traditional differential gas detection system. Finally, the system achieved a minimum detection limit (MDL) of 723 ppbv, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 7.85 × 10−10 cm−1 W/Hz 1/2. [ABSTRACT FROM AUTHOR]

It is of great significance to analyze the calorific value and carbon dioxide emission level of coal with different degrees of coalification for energy conservation and emission reduction in coal-fired power plants, and elemental carbon and fixed carbon are closely related to the carbon dioxide emission level and calorific value. Due to the complexity of matrix effects and differences in regression tasks, modeling only all coal samples will limit the quantitative performance of laser-induced breakdown spectroscopy (LIBS). In this work, we combine a dual-cycle variable selection mechanism with competitive adaptive reweighted sampling to optimize partial least squares regression (PLSR). According to the difference between coal samples in coal quality characteristics and prediction tasks, the matrix correction of elemental carbon and fixed carbon is realized by qualitative classification and by using generalized spectra (GS), respectively. The coefficient of determination (R2) of the training set and test set of elemental carbon was improved from 0.7178 and 0.7095 to 0.9534 and 0.9515, respectively. The root-mean-square error of cross-validation (RMSECV) and the root-mean-square error of prediction (RMSEP) decreased from 1.1723 wt% and 1.2488 wt% to 0.4504 wt% and 0.4871 wt%, respectively. The R2 of the training set and test set of fixed carbon was also improved from 0.8252 and 0.8236 to 0.9814 and 0.9749. The RMSECV and RMSEP decreased from 1.2274 wt% and 1.3548 wt% to 0.4421 wt% and 0.4711 wt%, respectively. According to Chinese national standards and IPCC guidelines, the maximum error in CO2 emission factors for all coal samples during the accounting period is 1.7581%. Taking into account the level of CO2 emissions and the net calorific value, the selection of coal with different degrees of coalification can guide the combustion of mixed coal in power plants to achieve high efficiency and clean power generation. This work demonstrates the great potential of in situ LIBS for online applications in coal-fired power plants. [ABSTRACT FROM AUTHOR]

A technique for elimination of residual amplitude modulation (ERAM) in photoacoustic spectroscopy based on dual path lock-in was proposed and experimentally demonstrated. There are two lock-in amplifiers, one is for gas concentration demodulation and another for residual amplitude modulation (RAM) measurement by tuning the reference signal in different phases, and then a dual path lock-in technique based on subtraction is applied to RAM removal, improving the second harmonic profile significantly. In this system, the signal to noise ratio (SNR) increases about two times based on our dual path lock-in technique compared to one distributed feedback laser diode (DFB-LD). The system achieved a good linear response (R-square = 0.99887) in a concentration range from 100 ppmv to 2400 ppmv and a minimum detection limit (MDL) of 1.47 ppmv. [ABSTRACT FROM AUTHOR]

An 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. [ABSTRACT FROM AUTHOR]

• Scanning cycle is optimized in detail for photoacoustic energy accumulation. • Optimum modulation index is verified to be ∼5 by experiments instead of 2.2. • Frequency and efficiency of QTF are evaluated in a long-term (∼8 months) experiment. • Photoacoustic spectrophones are discussed and compared in their optimum geometric dimensions. Efforts were made in this study to evaluate a series of pivotal techniques in standard quartz tuning fork (QTF) based quartz enhanced photoacoustic spectroscopy (QEPAS) systems for engineering applications. In current QEPAS systems, a scanning ramp combined with high frequency sinusoidal signal is used to drive the light source for photoacoustic excitation, the scanning cycle was optimized for the first time in experiments guaranteeing that photoacoustic energy accumulation finished to achieve the largest photoacoustic signal, and the optimum modulation index was verified by experiments to be ∼5 instead of the simulated result 2.2. As an important step for engineering applications, a long-term (∼8 months) experiment was also carried out to evaluate the stability of QTF's property. In addition, several photoacoustic spectrophone configurations were compared in the optimum geometry parameters, dual-tubes on beam was verified in experiments to be the most efficient one compared with half on-beam and off-beam structures in standard QTF based systems, which is consist with the summary of published papers. [ABSTRACT FROM AUTHOR]

• A novel single quartz tuning fork (QTF) compact integrated dual-spectroscopy gas detection structure based on quartz-enhanced photoacoustic spectroscopy (QEPAS) and quartz-enhanced photothermal spectroscopy (QEPTS) was designed for improving detection intensity and reducing sensor size. • In the structure, an off-beam acoustic micro-resonator (AMR) was used to enhance the photoacoustic signal. After passing the AMR, the laser beam was turned back by the right-angle prism and incident on the side of the QTF fork again to generate the photothermal signal. • The silver layer on the QTF surface was chemically etched to further enhance the photothermal signal. For the traditional off-beam photoacoustic excitation, the photothermal signal is excited by the way of laser retracting. The superposition of the dual-spectroscopy detection technology on a single QTF is realized. • Under the same conditions, photoacoustic signal, photothermal signal and dual-spectroscopy signal were measured respectively. The measured dual-spectroscopy signal was approximately the superposition value of photoacoustic signal and photothermal signal, which showed the effective signal lifting effect. • With C 2 H 2 as the target gas, the experimental results showed that the signal value of the dual-spectroscopy structure was 1.94 times that of QEPAS structure, 2.18 times that of QEPTS structure. And the minimum detection limit at atmospheric pressure was 6.64 ppm. The integrated dual-spectroscopy structure was compact, and was suitable for small space measurement, mobile measurement, and other fields. If the acetylene (C 2 H 2) content in the oxygen production device exceeds the standard, it is highly susceptible to explosion. It is necessary to detect C 2 H 2 gas and reach the detection limit of ppm. A novel single quartz tuning fork (QTF) compact integrated dual-spectroscopy gas detection structure based on quartz-enhanced photoacoustic spectroscopy (QEPAS) and quartz-enhanced photothermal spectroscopy (QEPTS) was designed for improving detection intensity and reducing sensor size. The detection structure (40 mm × 90 mm × 50 mm) includes gas buffer, gas detection structure, and preamplifier circuit. In the structure, an off-beam acoustic micro-resonator (AMR) was used to enhance the photoacoustic signal. After passing the AMR, the laser beam was turned back by the right-angle prism and incident on the side of the QTF fork again to generate the photothermal signal. The silver layer on the QTF surface was chemically etched to further enhance the photothermal signal. For the traditional off-beam photoacoustic excitation, the photothermal signal is excited by the way of laser retracting. The superposition of the dual-spectroscopy detection technology on a single QTF is realized. And the structure is finely integrated which is convenient to carry and measure. Under the same conditions, photoacoustic signal, photothermal signal and dual-spectroscopy signal were measured respectively. The measured dual-spectroscopy signal was approximately the superposition value of photoacoustic signal and photothermal signal, which showed the effective signal lifting effect. With C 2 H 2 as the target gas, the experimental results showed that the signal value of the dual-spectroscopy structure was 1.94 times that of QEPAS structure, 2.18 times that of QEPTS structure. And the minimum detection limit at atmospheric pressure was 6.64 ppm. The integrated dual-spectroscopy structure was compact, and was suitable for small space measurement, mobile measurement, and other fields. [ABSTRACT FROM AUTHOR]

• In this paper, a two-component gas sensing system in the resonant cavity of the fiber ring laser based on photoacoustic spectroscopy (PAS) is designed by embedding the PAS cell into the laser resonant cavity. • A ring cavity erbium-doped fiber (EDF) laser is built in this system, combined with a longitudinal resonant PAS cell to detect the PAS signal, and a custom-designed wavelength modulator uses the fiber bragg grating (FBG) to paste the piezoelectric transducer (PZT) for scanning and modulating the gas absorption wavelength. • The gas concentration is calculated by the second harmonic (2f) PAS signal. Based on time division multiplexing (TDM) technology, the acetylene (C 2 H 2) and carbon dioxide (CO 2) is detected respectively by the time-sharing operation of C-band and L-band EDFs for different wavelengths output. • The system can improve the laser power and eliminate the mode competition between the two output laser bands. • The two-component gas sensing system also showed superior gas sensing capability for trace gas detection and could achieve a minimum detection limit of 728 ppb and 503 ppm for C 2 H 2 and CO 2 detection, respectively. In this paper, a two-component gas sensing system in the resonant cavity of the fiber ring laser based on photoacoustic spectroscopy (PAS) is designed by embedding the PAS cell into the laser resonant cavity. A ring cavity erbium-doped fiber (EDF) laser is built in this system, combined with a longitudinal resonant PAS cell to detect the PAS signal, and a custom-designed wavelength modulator uses the fiber bragg grating (FBG) to paste the piezoelectric transducer (PZT) for scanning and modulating the gas absorption wavelength. The gas concentration is calculated by the second harmonic (2f) PAS signal. Based on time division multiplexing (TDM) technology, the acetylene (C 2 H 2) and carbon dioxide (CO 2) is detected respectively by the time-sharing operation of C-band and L-band EDFs for different wavelengths output. The time-sharing operation of the two wavelength laser is realized through optical switches, which effectively avoids the laser power loss and the instability of each wavelength caused by the mode competition phenomenon of multiple wavelengths. The detection performance of C 2 H 2 and CO 2 is implemented under normal temperature and pressure. The PAS signal intensity is further improved by optimizing the wavelength modulation amplitude and incident laser power. The linear fit (R2) and minimum detection limit (MDL) of C 2 H 2 and CO 2 are 0.99821, 728 ppbv, 0.9994 and 503 ppmv respectively. It shows that the two-component gas sensing system designed in this paper has a good linear response and high sensitivity, and plays an important role on the multi-component gas detection. [ABSTRACT FROM AUTHOR]

[Display omitted] • An improved WMS-Y 1f method is suitable for the full range of concentration. • The interference of background gas is well eliminated under low modulation depth. • The relative errors are all less than 1%. At present, gas sensors are extremely susceptible to interference from background gases in the field environment, which leads to greatly reduced accuracy. For this reason, we propose an improved method of recovering integral absorbance (IA) using Y component of first harmonic to achieve accurate prediction of the full range of concentration (not reaching absorption saturation). This approach can eliminate the interference of background gas at a low modulation depth (m < 0.25). When the background gas is pure nitrogen and a mixture of nitrogen and carbon dioxide, the recovery effect of this method on methane is both close to the theoretical value when the background gas is air. The linear fitting coefficients for the methane concentration range of 2000–7000 ppm are all greater than 0.999. The prediction effect is satisfactory regardless of the background gas, with a relative error of less than 1%. In summary, this method has considerable application prospects. [ABSTRACT FROM AUTHOR]

Multi-gas tunable diode laser absorption spectroscopy (TDLAS) system based on time division multiplexing (TDM) demodulation technology which combine with frequency division multiplexing (FDM) technology is proposed, called F-TDM (frequency- time division multiplexing) system. In addition, traditional TDM technology and FDM technology are introduced to analyze and compare their performance. As for the performance of F-TDM system, multi-gas detection of CH4 and C2H2 has been accomplished with minimum detection limits of 10.24 ppmv for CH4 at 1653.72 nm and 0.763 ppmv for C2H2 at 1532.83 nm respectively. The relationships (CH4: R-square = 0.9989; C2H2: R-square = 0.9995) between gas concentration and second harmonic signal amplitude are proved as a good linear response. It is consistent with the data obtained by traditional TDM and FDM demodulation methods, indicating that the F-TDM system is feasible. By comparison, it is known that different methods have their advantages in different applications. The TDM system uses less equipment and is the most cost-effective. The system based on FDM technology is the most time-saving. The F-TDM system is a compromise method that combines the advantages of both and has no obvious disadvantage at the detection limit. [ABSTRACT FROM AUTHOR]

[Display omitted] • The CH 4 /C 2 H 6 dual gas sensor using a single mid-infrared laser is developed. • The cross-interference of C 2 H 6 on CH 4 is eliminated. • High sensitivity, strong stability, fast response speed and simple system structure. Methane (CH 4) and ethane (C 2 H 6) 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 C 2 H 6 to CH 4 is eliminated. In the single gas concentration measurement, linear fitting is performed between the absorbance and concentration of CH 4 and C 2 H 6 , 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 CH 4 and 0.1 ppm·m for C 2 H 6 are achieved, respectively. The CH 4 /C 2 H 6 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. [ABSTRACT FROM AUTHOR]

Optical projection tomography (OPT) is an advanced three-dimensional (3D) imaging technology, which uses the filtered backprojection (FBP) algorithm to recover the 3D volume with sufficient number of projections. As to in vivo imaging, it is urgent to reduce the number of projections because the acquisition time could be minimized. However, reconstructing from undersampled OPT data can lead to artifacts and the decline in image quality. The simultaneous iterative reconstruction technique (SIRT) is introduced to remove artifacts and improve the image quality. The image qualities reconstructed from FBP and SIRT separately are compared, and the structural similarity and peak signal-to-noise ratio are calculated. Through simulated phantoms and OPT data of in vivo zebrafish embryo, SIRT consistently outperforms FBP in terms of reduced artifacts and enhanced image contrast especially when the projection numbers are reduced. SIRT method can provide high-quality reconstruction with 50 or fewer projections, thereby significantly reducing the minimum acquisition time and light dose while maintaining reconstruction quality. Through optimization and GPU acceleration, the SIRT algorithm can converge faster so as to reduce the image processing time. To our knowledge, this is the first time one SIRT algorithm is used for reconstruction of sparse OPT data. The experimental results show that SIRT algorithm outperform FBP especially when the number of projections is reduced. In addition, SIRT performs better in the preservation of vascular signal, which is significant for the monitoring of angiogenesis. [ABSTRACT FROM AUTHOR]

Carbon monoxide and methane dual gas sensor with low system complexity and high stability is proposed in this study. A neural separator based on deep learning is developed to solve the cross-interference problem from the ultra-high spectral overlap between CO and CH 4 molecules. A large amount of simulated overlapping spectra of different concentrations CO and CH 4 are used to construct, train, tune and test the neural separator, instead of collecting data from onerous experiments. The linear fitting is performed between the predicted concentrations and preset concentrations of CH 4 and CO, determination coefficients of R2 = 0.99960 and R2 = 0.99301 are achieved respectively which proves the accuracy of the dual gas sensor is robust and greatly enhanced by the neural separator. In addition, the minimum detection limits of 120.86 ppm (CH 4) and 0.5 ppm (CO) are achieved in real-time simultaneous detection of CO and CH 4 overlapping environment. This is a successful attempt to apply deep learning method to tunable diode laser absorption spectroscopy (TDLAS) gas sensors to solve the problem of spectral cross-interference, which provides an alternative direction for the realization of simultaneous measurement of multi-component gases. • The dual gas sensor based on neural separator is developed. • The cross-interference problem with the ultra-high spectral overlap is solved. • The minimum detection limits of 120.86 ppm (CH 4) and 0.5 ppm (CO) are achieved. [ABSTRACT FROM AUTHOR]

• 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. • A QTF was used as a photodetector and an acoustic wave transducer to detect the TDLAS and QEPAS signal simultaneously for 2f signal enhancement. • A gas absorption cell with 3 m optical path length was utilized to generate and enhance the TDALS signal. We tested the feasibility of the proposed dual-spectroscopy detection technique by detecting acetylene (C 2 H 2) at 1532.83 nm. • The signal of the dual-spectroscopy technique was approximately 1.13 and 8.71 times higher than that of the QEPAS and TDLAS techniques based on QTF. 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. 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 (C 2 H 2) 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. [ABSTRACT FROM AUTHOR]