To address the impact of noise in spectral signals on gas detection accuracy and sensitivity, this paper carries out a study of a singular value decomposition denoising algorithm based on a genetic algorithm and fast Fourier transform. Aiming at the optimization problem of two key parameters (structure of Toeplitz matrix and effective singular value order) in the smooth filtering algorithm of singular value decomposition, this paper uses a genetic algorithm to optimize the number of rows of the reconstructed Toeplitz matrix and determines the effective singular value order by performing fast Fourier transform on the constructed differential spectral signal. To test the effectiveness of the self-established singular value decomposition denoising algorithm, a detailed filtering and noise reduction analysis study is carried out on the simulated spectrum and experimentally measured signals with the absorption spectrum of atmospheric oxygen molecules as the analysis object, and compared with the traditional S-G filtering algorithm, wavelet transform denoising and principal component analysis denoising algorithm. The results show that the singular value decomposition denoising algorithm established in this paper has obvious superiority in suppressing noise, which can more effectively improve the signal-to-noise ratio and reduce the root-mean-square error of the spectral signal, thus improving the gas detection accuracy and sensitivity of the spectral experimental system.
Global environment and climate change are the focus and frontier subjects of ecology, biogeochemistry and environmental science. Simultaneous in-situ monitoring of multiple atmospheric pollution components base on laser absorption spectroscopy technology has become an effective way for in-depth analysis and accurate identification of atmospheric pollution sources by analyzing the correlation of their concentration data. In view of the wide variety of actual atmospheric pollutants, the source, transformation mechanism, and transportation process of each component are extremely complex and overlap phenomenon, the in-depth analysis of atmospheric pollution sources is an important challenge. The depth analysis of air pollution sources is the key basis for scientific control of the air pollution. It is of great significance to carry out in situ monitoring techniques and analytical algorithms for various pollutants. In this paper, carbon monoxide (CO), a typical pollutant in the atmosphere, is firstly selected and analyzed by using HYSPLIT backward trajectory model, and the reliability of the proposed algorithm is verified. In addition, with PM2.5 as the analysis target, the backward trajectory of air mass during 72 h of pollution was simulated by HYSPLIT model for Beijing's heavy pollution during January 26-28, 2020. The potential transport channels and pollution contribution sources of PM2.5 in different areas to Beijing were analyzed by daily trajectory clustering analysis.
Photoacoustic spectroscopy (PAS) has been a powerful tool in terms of sensitivity, selectivity and simplicity. In this paper, a resonant PAS sensor system based on a distributed feed-back (DFB) diode laser near 1653 nm for methane (CH4) detection is introduced. Wavelength modulation spectroscopy (WMS) with second harmonic (2f) method is combined with the PAS detection strategy for further sensitivity improvement. A PA cell working in the second-order longitudinal resonant mode was fabricated and used for experimental test of CH4 spectroscopy measurement. Photoacoustic signal dependence on gas pressure and modulation parameters were studied to optimize the sensor performance. In addition, potential approaches and detection schemes on improving sensor performance are also discussed.
Incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) is increasingly finding applications in atmosphere chemical, environmental monitoring, and medical diagnostics, etc. The paper describes the application of IBBCEAS for measurements of atmospheric NO2 using a blue LED centered at the wavelength of about 445 nm. The reflectivity of the cavity mirror is calibrated based on the difference between the Rayleigh scattering cross sections of N2 and He. The detection limit for NO2 detection is related to the integration time and averaging number. This issue is addressed by performing experimental measurements under different integration time and averaging number to determine the optimum condition. The measurement results are useful for the design of a spectroscopic sensor for monitoring atmosphere NO2.
Methane (CH4), a key greenhouse by influencing tropospheric ozone (O3) and stratospheric water vapor (H2O), plays an important role on the process of global warming and climate change. In this paper, we report on the development of a gas sensing technique based on high sensitive tunable diode laser absorption spectroscopy (TDLAS). A room-temperature distributed-feedback (DFB) diode laser near 1653 nm is used to excite the rotational-vibrational absorption line of CH4 molecule. Direct absorption spectroscopy and wavelength modulation spectroscopy were used for investigating methane molecular line parameters and sensing gas concentration, respectively. Moreover, potential strategies for sensitivity improvement are also discussed.
An adaptive polarized interferometer is proposed for optical free form surfaces metrology without auxiliary devices such as the wavefront sensors and deflectometry systems for deformable mirror (DM) monitoring. In this method, the DM surface monitoring and free form surface measurement are achieved simultaneously in only one interferometer by the polarizing design. The polarizing design divide the interferometer into two partial common path interferometric system, which provide the null test for tested free form surface and non-null test for the DM surface. The final figure error of the free form surface is extracted by ray tracing. Experiments proving the feasibility of this interferometer is shown.
KEYWORDS: Quantum cascade lasers, Sensors, Chemical analysis, Mid-IR, Gas sensors, Chemical detection, Gases, Spectroscopy, Signal processing, Algorithm development
Mid-infrared (MIR) laser spectroscopy is a powerful analytical tool for trace gases sensing, since a number of atmospheric pollutants and greenhouse gases have strong fundamental vibrational transitions within this spectral range. Here, we report the development of mid-infrared spectroscopy techniques coupled with a broadband tunable external-cavity (EC) mode-hop-free quantum cascade laser (QCL) operating between 6.96 and 8.85 μm. The ECQCL sensor was evaluated for quantitative and qualitative analysis of volatile organic compound (VOC) components. For signal processing, a self-established spectral analysis model integrated with various algorithms was developed for VOC spectral analysis. A good agreement was obtained between the experimentally observed spectra and the standard reference spectra taken from Pacific Northwest National Laboratory (PNNL) database.
Quartz enhanced photoacoustic spectroscopy (QEPAS) is an extremely effective tool for the detection and quantification of trace gases, which offers advantages of fast response, high sensitivity and high resolution. In this paper, a gas sensor based on quartz-enhanced photoacoustic detection and an external cavity quantum cascade laser (ECQCL) was realized and characterized for acetone measurement. Photoacoustic signal dependence on gas pressure and laser operating parameters were studied to optimize sensor performance. In addition, potential approaches and detection schemes on improving sensor performance were also discussed.
An adaptive optics based non-null interferometry (ANI) is proposed for optical free form surfaces testing, in which an open-loop deformable mirror (DM) is employed as a reflective compensator, to compensate various low-order aberrations flexibly. The residual wavefront aberration is treated by the multi-configuration ray tracing (MCRT) algorithm. The MCRT algorithm based on the simultaneous ray tracing for multiple system models, in which each model has different DM surface deformation. With the MCRT algorithm, the final figure error can be extracted together with the surface misalignment aberration correction after the initial system calibration. The flexible test for free form surface is achieved with high accuracy, without auxiliary device for DM deformation monitoring. Experiments proving the feasibility, repeatability and high accuracy of the ANI were carried out to test a bi-conic surface and a paraboloidal surface, with a high stable ALPAOTM DM88. The accuracy of the final test result of the paraboloidal surface was better than 1/20 Μ PV value. It is a successful attempt in research of flexible optical free form surface metrology and would have enormous potential in future application with the development of the DM technology.
A new type of tunable diode spectroscopy sensor based on an external cavity quantum cascade laser (ECQCL) and a quartz crystal tuning fork (QCTF) were used for quantitative analysis of volatile organic compounds. In this work, the sensor system had been tested on different gasoline sample analysis. For signal processing, the self-established interpolation algorithm and multiple linear regression algorithm model were used for quantitative analysis of major volatile organic compounds in gasoline samples. The results were very consistent with that of the standard spectra taken from the Pacific Northwest National Laboratory (PNNL) database. In future, The ECQCL sensor will be used for trace explosive, chemical warfare agent, and toxic industrial chemical detection and spectroscopic analysis, etc.
Breath analysis is an attractive method for disease diagnosis and therapeutic monitoring, due to its high potential for non-invasive medical diagnostics. Among numerous analysis techniques, tunable diode laser-based absorption spectroscopy (TDLAS) is an excellent method for detection of gas concentration, since it presents advantages of high sensitivity, good selectivity, fast response and high temporal resolution. In this study, state-of-the-art quantum cascade laser based gas sensor is demonstrated as a promising new tool for noninvasive, real-time identification and quantification of trace gases in human breath for clinical uses. Details of selection of spectroscopic parameters and primary lab studies conducted on CO, H2O and N2O molecules in exhaled breath are presented, together with suggestions on the future direction of this challenging analytical field.
A tunable diode laser absorption spectroscopy (TDLAS) system based on a broad band external cavity quantum cascade laser (ECQCL) near 7.78 μm was used to study volatile organic compounds (VOCs) measurements. Instead of using a standard infrared mercury cadmium telluride (MCT) detector, a quartz crystal tuning fork (QCTF) as a light detector was successfully used for laser signal detection. Fast Fourier transform (FFT) was used to extract vibration intensity information of QCTF. Primary results indicate that the new developed system has a good reproducibility, and a good agreement was obtained by comparing with data taken from standard spectroscopic database.
Mid-infrared laser spectroscopy provides an ideal platform for trace gas sensing applications. Despite this potential, early MIR sensing applications were limited due to the size of the involved optical components, e.g. light sources and sample cells. A potential solution to this demand is the integration of hollow fiber waveguide with novelty quantum cascade lasers.Recently QCLs had great improvements in power, efficiency and wavelength range, which made the miniaturized platforms for gas sensing maintaining or even enhancing the achievable sensitivity conceivable. So that the miniaturization of QCLs and HWGs can be evolved into a mini sensor, which may be tailored to a variety of real-time and in situ applications ranging from environmental monitoring to workplace safety surveillance. In this article, we introduce QCLs and HWGs, display the applications of HWG based on QCL gas sensing and discuss future strategies for hollow fiber coupled quantum cascade laser gas sensor technology.
A sensitive open-path gas sensor employing a continuous-wave (CW) distributed feedback (DFB) quantum cascade laser (QCL) and direct absorption spectroscopy (DAS) was demonstrated for simultaneously measurements of atmospheric CO and N2O. Two interference free absorption lines located at 2190.0175 cm-1 and 2190.3498 cm-1 were selected for CO and N2O concentration measurements, respectively. The Allan variance analysis technique was performed to investigate the long-term performance of the QCL sensor system. The results indicate that a detection limit of 9.92 ppb for CO and 7.7 ppb for N2O with 1-s integration time were achieved, which can be further improved to 1.5 ppb and 1.1 ppb by increasing the average time up to 80 s.
The temperature dependent of the ratio between second and first harmonic signals in wavelength modulation spectroscopy was experimentally measured. The results show the ratio decrease with the temperature increase the temperature increase from 30°C to 50°C the ratio decrease about 5%, the results is consistent with theoretical calculation. As increasing the temperature of the CO gas the ratio between second and first harmonic signals is the straight line parallel displaces downwards. Therefore the measuring concentration can be corrected by using a calibrated temperature curve in practice sensor application. The results are useful for the application of the diode laser sensor ofthe ratio detection type.
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