Spectroscopy|18 Article(s)
Terahertz Nondestructive Testing Signal Recognition Based on PSO-BP Neural Network
Meihui JIA, Lijuan LI, Jiaojiao REN, Jian GU, Dandan ZHANG, Jiyang ZHANG, and Weihua XIONG
Terahertz time domain spectroscopy technique was used to detect the defects of high temperature resistant composite materials with multi-bonded structures. In order to identify the debonding defects in both upper and lower layers at the same location, the terahertz signal waveforms in the non-defect area, upper and lower debonding areas were analyzed. The characteristic peak-to-peak, skewness, minimum value, peak-to-valley value, waveform factor and absolute mean value of signal amplitude were taken as the input of BP neural network.The initial weight and threshold value of BP neural network were optimized by Particle Swarm Optimization (PSO), which solved the problem that BP neural network was easy to fall into local optimum. The optimized PSO-BP neural network could realize the identification of the debonding defects of upper 100 μm and lower 100 μm, with the accuracy of 90.71% and 86.92%.
Acta Photonica Sinica
  • Publication Date: Sep. 25, 2021
  • Vol. 50, Issue 9, 0930004 (2021)
Intracavity Gas Sensing System Based on Thulium Fiber Laser
Yafan LI, Kun LIU, Tiegen LIU, Junfeng JIANG, and Jifang SHAN
In order to expand the detection range, the 2 μm band laser generated by thulium-doped fiber laser is used to conduct intracavity gas sensing of water vapor in human breathing gas. Firstly, the direct absorption gas sensing technology is analyzed theoretically. Secondly, the characteristics of thulium-doped fiber pumped by a 1 570 nm laser are studied, and the spontaneous emission spectrum is mainly distributed in the band of 1.85~2.05 μm. Then, an all-fiber thulium-doped fiber ring laser is built, further using a tunable filter, the wavelength output in the range of 1 927.5~1 985 nm is achieved and the laser linewidth of 0.05 nm is obtained, which shows the advantages of single longitudinal mode, narrow linewidth and stable output. Finally, combined with wavelength sweep technique, the spectral scanning of water vapor in the range of 1928~1938 nm is realized and eight absorption lines are resolved, which are consistent with the simulated spectrum based on the HITRAN spectrum database. The absolute error of wavelength positioning is less than 0.03 nm. The results show that the intracavity gas sensing system is suitable for 2 μm band gas sensing detection.
Acta Photonica Sinica
  • Publication Date: Sep. 25, 2021
  • Vol. 50, Issue 9, 0930003 (2021)
Sensing System of Dissolved Methane in Water Based on Cavity-enhanced Laser Spectroscopy
Wenwen CHEN, Kaiyuan ZHENG, Yanwei CAO, Yixi CHEN, Chuantao ZHENG, and Yiding WANG
In order to detect the concentration of methane in water, a sensor system of dissolved methane in water based on off-axis integrated cavity output spectroscopy is developed in this paper. A distributed feedback laser (center wavelength, 1 653 nm), a laser temperature control module, a laser current drive module, a resonator cavity/gas chamber, a photoelectric detector, a data acquisition module, a data processing module and a gas-liquid separation module were included in the system. Experiments including calibration of effective optical path length, calibration of direct absorption signal, and stability test were carried out by using methane gas samples and pure nitrogen (N2). The effective optical path length of the cavity is calibrated using a methane sample with a concentration level of 10×10-6, which is determined to be 1 906 m. Using pure nitrogen as the target gas, the stability of the system is measured. The results of Allan variance analysis show that when the averaging time is 2 s, the detection sensitivity is 92.8×10-9, and the sensitivity of the system can be improved to 13.2×10-9 with an averaging time of 134 s. The system was used to measure the concentration of the dissolved methane in tap water, rainwater and lake water, and the results confirmed the engineering practical value of the technique and system. The research work and related results have laid a good foundation for water quality detection and exploration of clean energy such as natural gas hydrate.
Acta Photonica Sinica
  • Publication Date: Sep. 25, 2021
  • Vol. 50, Issue 9, 0930002 (2021)
Establishment and Optimization of Photoacoustic Cell Model in Photoacoustic Spectrum Detection System
Nan ZHAO, Yang LIU, Ningyang ZHAO, Longge MA, Meicong LI, Jingjing JIAO, and Chenfei DING
The photoacoustic cell, as the core component of the photoacoustic spectroscopy gas detection system, directly affects the detection accuracy of the system. Based on the classic cylindrical photoacoustic cell, the finite element analysis software is used to combine the two physics of pressure acoustics and thermoviscous acoustics and build a model to simulate the acoustic-thermal coupling process in the photoacoustic cell. The effect of different geometric parameters of the resonant cavity and buffer cavity on the performance of the photoacoustic cell is compared through simulation, and then, the optimal size is determined. The simulation results show that the length and radius of the resonant cavity and the buffer cavity will affect the resonance frequency and acoustic pressure. After fully considering the comparison results and the difficulty of industrial manufacturing, the optimal length of the resonant cavity is selected as 120 mm, the optimal radius is 3 mm, and the buffer cavity radius is 35 mm. On this basis, a photoacoustic cell with a rounded connection between the resonant cavity and the buffer cavity is designed. Compared with the right-angle photoacoustic cell with the same size, the round-corner photoacoustic cell has more advantages in improving the photoacoustic signal and reducing the flow noise interference. The quality factor is increased to 1.109 times, the cell constant is increased to 3 635.1 Pa·cm/W, and the acoustic pressure is increased to 1.26×10-5 Pa. In the concentration detection of methane gas, the system sensitivity can reach 0.87 ppm, and the detection result is ideal, which meets the requirements of high sensitivity. Therefore, the performance of the round-corner photoacoustic cell has been significantly improved, which can provide a reference for the optimal design of the photoacoustic cell.
Acta Photonica Sinica
  • Publication Date: Jul. 25, 2021
  • Vol. 50, Issue 7, 238 (2021)
Cross Interference Characteristics of Photoacoustic Spectroscopy Multi-gas Analyzer
Shuai YUAN, Guangzhen WANG, Dehui FU, Ke CHEN, Ran AN, Bo ZHANG, Min GUO, and Guangyin ZHANG
A photoacoustic spectroscopy multi-gas detection system based on infrared heat radiation light source was developed. The broadband mid-infrared thermals radiation source and band-pass filter were used to generate the photoacoustic excitation light. Combined with a small-volume non-resonant photoacoustic cell, the time-sharing measurement of multi-component gas concentration was realized. The parameters of the mid-infrared bandpass filter were determined by analyzing the main factors of cross-interference among multi-component gases and the infrared absorption spectrum of target gases. To determine the quantitative relationship of cross interference among the gases to be measured, the photoacoustic spectrometer system was calibrated by using the standard gas, and a humidifier was used to analyze the interference from water vapor. The experimental results showed that the interference levels of C2H2 to CH4 and CH4 to C2H6 reached 10.49 μV/(μL/L) and 18.66 μV/(μL/L) respectively, and the interference between other hydrocarbon gases can be ignored. The responsiveness of CO2 to CO, CH4, C2H2 and C2H4 interference was 1.615 μV/(μL/L), 0.055 μV/(μL/L), 0.130 μV/(μL/L) and 0.016 μV/(μL/L), respectively. In addition, water vapor will cause certain interference to C2H2, CH4, C2H6, C2H4, CO and CO2, and the responsiveness of the interference was 0.591 μV/(μL/L), 0.421 μV/(μL/L), 0.071 μV/( μL/L), 0.007 μV/(μL/L), 0.051 μV/(μL/L) and 0.055 μV/(μL/L). The experimental results indicated that there was a high level of interference when detecting CH4 in C2H2 background, C2H6 in CH4 background, CO in CO2 background, and other target gases in high concentration water vapor background, which should be considered in the measurement process.
Acta Photonica Sinica
  • Publication Date: Apr. 25, 2021
  • Vol. 50, Issue 4, 198 (2021)
Improvement of LVF-based NIR Spectral Sensor on Both Spatial and Time Domains
Xuquan WANG, Songlei HUANG, Pengyu KE, Mengxuan LIU, Zhenli ZHAO, and Jiaxiong FANG
Acta Photonica Sinica
  • Publication Date: Apr. 25, 2021
  • Vol. 50, Issue 4, 191 (2021)
Calculation and Evaluation of Acoustic and Flow Field Characteristics of Trapezoid Compound Photoacoustic Cell
Gang CHENG, Yanan CAO, Xing TIAN, Kun LIU, and Jiajin CHENG
To improve the detection performance of the photoacoustic cell, a trapezoid compound photoacoustic cell is proposed and analyzed. Taking the traditional cylindrical resonant photoacoustic cell as the reference model, the reliability of the simulation method is verified by comparing the results of analysis and simulation. Based on the simulation method, the influence of the radius, length, and the number of the stepped cavity on the acoustic field and flow field characteristics of the trapezoidal composite photoacoustic cell is calculated. The results show that reducing the stepped cavity radius of the trapezoidal compound photoacoustic cell can enhance the photoacoustic signal, and the optimal length of the stepped cavity can make the photoacoustic signal to be the strongest, and the number of the stepped cavity should be one. In terms of the flow field, the configuration characteristics of the trapezoidal composite photoacoustic cell improve the condition of gas vortex return to the cavity. If the transition to the cavity is furthering rounded or chamfered, the velocity gradient in the cavity will become more stable. Select a set of design parameters, the calculation results show that the volume of the trapezoid compound photoacoustic cell cavity is reduced to 39.7% of the corresponding cylindrical resonant photoacoustic cell, and the photoacoustic signal is increased by about 18.7%. In addition, its frequency response bandwidth is narrowed, and its quality factor is improved. The overall results show that the acoustic field and flow field characteristics of the trapezoidal composite photoacoustic cell are better than that of the corresponding cylindrical resonant photoacoustic cell. The research content can provide reference for the structural optimization and improvement of the photoacoustic spectral photoacoustic cell.
Acta Photonica Sinica
  • Publication Date: Feb. 25, 2021
  • Vol. 50, Issue 2, 192 (2021)
Unsupervised Nearest Regularized Subspace Based on Spectral Space Reconstruction for Hyperspectral Anomaly Detection
Zhi-wei WANG, Kun TAN, Xue WANG, Jian-wei DING, and Yu CHEN
The high dimension and huge data volume of hyperspectral remote sensing images and the complexity of surface feature lead to difficulty in distinguishing the anomaly pixel from the background. To solve these problems, an unsupervised nearest regularized subspace anomaly detection algorithm based on spectral space reconstruction is proposed. Firstly, in the process of band selection based on structure tensor, noise pixels are removed to obtain more effective bands. Then, the spectral space reconstruction is utilized to increase the absolute spectral distance between the background and the anomaly. Finally, to take full advantage of the spatial similarity information between background dictionaries, the spatial distance weight is introduced into the unsupervised nearest regularized subspace algorithm to improve the accuracy of linear representation.To validate the effectiveness of the proposed algorithm, experiments on four sets of real hyperspectral data are conducted, and the infulence of different parameters on the detection results is studied. Experimental results demonstrate that the proposed algorithm has a better detective performance than other anomaly detection algorithms.
Acta Photonica Sinica
  • Publication Date: Jun. 25, 2020
  • Vol. 49, Issue 6, 0630004 (2020)
Algorithm for Broadband Brillouin Spectra Analysis Based on Thomae's Function
Xu-sheng XIA, Xiang-long CAI, Zhong-hui LI, Chen-cheng SHEN, Wan-fa LIU, Yu-qi JIN, Feng-ting SANG, and Jing-wei GUO
As the seed-injected lasers widely used in Brillouin Lidar area are too expensive and huge, the algorithm for a broadband Brillouin signal should be developed, which could accelerate the application of compact and economic diode lasers. A variation of Thomae's function was found during the signal processing of broadband Brillouin lidar, and the property of this function-reaching minimum value at the Brillouin frequency shift was used to recover the original signal spectrum and the corresponding frequency shift from a 1:1 superposition of pump light and Brillouin light spectra. Experiments using test data show nearly 100% accuracies for ideal cases; but for non-ideal cases, only when the noise level is less than -30 dB and the ratio of pump light to Brillouin light is less than 1.05 can this algorithm obtain accurate result.
Acta Photonica Sinica
  • Publication Date: Jun. 25, 2020
  • Vol. 49, Issue 6, 0630003 (2020)