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Laser-Spectroscopy Based Sensing Technologies

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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Optical Sensors".

Deadline for manuscript submissions: closed (31 July 2022) | Viewed by 35507

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Special Issue Editors

Dr. Simona M. Cristescu

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Guest Editor

Life Science Trace Gas Facility, Radboud University, 6525 AJ Nijmegen, The Netherlands
Interests: laser-based spectroscopy; mid-infrared lasers; gas detection; real-time analysis; online sampling; quantitative measurements; volatile biomarkers in biomedical research

Prof. Dr. Gábor Galbács

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Guest Editor

Department of Inorganic and Analytical Chemistry, University of Szeged, Szeged, Hungary
Interests: laser-induced breakdown spectroscopy (LIBS); diode laser atomic spectroscopy; fluorescence spectroscopy; nanoparticle characterization; molecular probes; plasma diagnostics; inductively coupled plasma spectroscopy (ICP-OES, ICP-MS); qualitative sample discrimination; spatial heterodyne spectroscopy; portable spectroscopy instrumentation

Dr. Miklos Veres

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Guest Editor

Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, P.O. Box 49, H-1525 Budapest, Hungary
Interests: Raman spectroscopy
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The interest towards the development of spectroscopy sensors based on laser excitation remains intense. To a great extent, this is due to the exceptional selectivity and sensitivity, fast and noncontact features provided by these sensors/sensor systems, but the continuous development in associated areas like quantum electronics, fiber optics, laser sources, and data mining also greatly helps the advancement of this research. Applications of laser-based spectroscopy in various, multidisciplinary fields, such as life sciences, industrial process control, screening of goods, space exploration, and environmental sciences are constantly intensifying.

This Special Issue intends to collect papers describing the newest scientific developments and applications of laser-based spectroscopic sensors for in situ, real-time analysis of important biological, chemical or physical components/parameters. It is open for contributions from all branches of this field, including research on sensors working in the absorption, emission, fluorescence (luminescence), scattering or interferometric mode. Contributions focusing on methodology, system design, and data evaluation are all welcome.

Dr. Simona M. Cristescu
Prof. Dr. Gábor Galbács
Dr. Miklos Veres
Guest Editors

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Keywords

Process analytical sensing technology
Remote/stand-off analysis
Medical sensors
Hyperspectral imaging
Gas and aerosol sensing
Fluorescent molecular probes
Multivariate evaluation of data from laser spectroscopy sensors
Compact spectrometers
Optochemical sensors with laser excitation

Published Papers (14 papers)

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Research

16 pages, 8105 KiB

Open AccessArticle

Towards Automated Classification of Zooplankton Using Combination of Laser Spectral Techniques and Advanced Chemometrics

by Nikolai I. Sushkov, Gábor Galbács, Patrick Janovszky, Nikolay V. Lobus and Timur A. Labutin

Sensors 2022, 22(21), 8234; https://doi.org/10.3390/s22218234 - 27 Oct 2022

Cited by 3 | Viewed by 1321

Abstract

Zooplankton identification has been the subject of many studies. They are mainly based on the analysis of photographs (computer vision). However, spectroscopic techniques can be a good alternative due to the valuable additional information that they provide. We tested the performance of several chemometric techniques (principal component analysis (PCA), non-negative matrix factorisation (NMF), and common dimensions and specific weights analysis (CCSWA of ComDim)) for the unsupervised classification of zooplankton species based on their spectra. The spectra were obtained using laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy. It was convenient to assess the discriminative power in terms of silhouette metrics (Sil). The LIBS data were substantially more useful for the task than the Raman spectra, although the best results were achieved for the combined LIBS + Raman dataset (best Sil = 0.67). Although NMF (Sil = 0.63) and ComDim (Sil = 0.39) gave interesting information in the loadings, PCA was generally enough for the discrimination based on the score graphs. The distinguishing between Calanoida and Euphausiacea crustaceans and Limacina helicina sea snails has proved possible, probably because of their different mineral compositions. Conversely, arrow worms (Parasagitta elegans) usually fell into the same class with Calanoida despite the differences in their Raman spectra. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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11 pages, 1270 KiB

Open AccessArticle

Intrinsic Performance of Monte Carlo Calibration-Free Algorithm for Laser-Induced Breakdown Spectroscopy

by Igor B. Gornushkin and Tobias Völker

Sensors 2022, 22(19), 7149; https://doi.org/10.3390/s22197149 - 21 Sep 2022

Cited by 1 | Viewed by 1326

Abstract

The performance of the Monte Carlo (MC) algorithm for calibration-free LIBS was studied on the example of a simulated spectrum that mimics a metallurgical slag sample. The underlying model is that of a uniform, isothermal, and stationary plasma in local thermodynamical equilibrium. Based on the model, the algorithm generates from hundreds of thousands to several millions of simultaneous configurations of plasma parameters and the corresponding number of spectra. The parameters are temperature, plasma size, and concentrations of species. They are iterated until a cost function, which indicates a difference between synthetic and simulated slag spectra, reaches its minimum. After finding the minimum, the concentrations of species are read from the model and compared to the certified values. The algorithm is parallelized on a graphical processing unit (GPU) to reduce computational time. The minimization of the cost function takes several minutes on the GPU NVIDIA Tesla K40 card and depends on the number of elements to be iterated. The intrinsic accuracy of the MC calibration-free method is found to be around 1% for the eight elements tested. For a real experimental spectrum, however, the efficiency may turn out to be worse due to the idealistic nature of the model, as well as incorrectly chosen experimental conditions. Factors influencing the performance of the method are discussed. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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13 pages, 2325 KiB

Open AccessArticle

Rapid Identification of Beached Marine Plastics Pellets Using Laser-Induced Breakdown Spectroscopy: A Promising Tool for the Quantification of Coastal Pollution

by Roberta Giugliano, Bruno Cocciaro, Francesco Poggialini, Stefano Legnaioli, Vincenzo Palleschi, Marina Locritani and Silvia Merlino

Sensors 2022, 22(18), 6910; https://doi.org/10.3390/s22186910 - 13 Sep 2022

Cited by 11 | Viewed by 1909

Abstract

The rapid identification of beached marine micro-plastics is essential for the determination of the source of pollution and for planning the most effective strategies for remediation. In this paper, we present the results obtained by applying the laser-induced breakdown spectroscopy (LIBS) technique on a large sample of different kinds of plastics that can be found in a marine environment. The use of chemometric analytical tools allowed a rapid classification of the pellets with an accuracy greater than 80%. The LIBS spectrum and statistical tests proved their worth to quickly identify polymers, and in particular, to distinguish C-O from C-C backbone pellets, and PE from PP ones. In addition, the PCA analysis revealed a correlation between appearance (surface pellets roughness) and color (yellowing), as reported by other recent studies. The preliminary results on the analysis of metals accumulated on the surface of the pellets are also reported. The implication of these results is discussed in view of the possibility of frequent monitoring of the marine plastic pollution on the seacoast. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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11 pages, 2320 KiB

Open AccessArticle

Laser-Induced Breakdown Spectroscopy Combined with Nonlinear Manifold Learning for Improvement Aluminum Alloy Classification Accuracy

by Edward Harefa and Weidong Zhou

Sensors 2022, 22(9), 3129; https://doi.org/10.3390/s22093129 - 20 Apr 2022

Cited by 6 | Viewed by 2490

Abstract

Laser-induced breakdown spectroscopy (LIBS) spectra often include many intensity lines, and obtaining meaningful information from the input dataset and condensing the dimensions of the original data has become a significant challenge in LIBS applications. This study was conducted to classify five different types of aluminum alloys rapidly and noninvasively, utilizing the manifold dimensionality reduction technique and a support vector machine (SVM) classifier model integrated with LIBS technology. The augmented partial residual plot was used to determine the nonlinearity of the LIBS spectra dataset. To circumvent the curse of dimensionality, nonlinear manifold learning techniques, such as local tangent space alignment (LTSA), local linear embedding (LLE), isometric mapping (Isomap), and Laplacian eigenmaps (LE) were used. The performance of linear techniques, such as principal component analysis (PCA) and multidimensional scaling (MDS), was also investigated compared to nonlinear techniques. The reduced dimensions of the dataset were assigned as input datasets in the SVM classifier. The prediction labels indicated that the Isomap-SVM model had the best classification performance with the classification accuracy, the number of dimensions and the number of nearest neighbors being 96.67%, 11, and 18, respectively. These findings demonstrate that the combination of nonlinear manifold learning and multivariate analysis has the potential to classify the samples based on LIBS with reasonable accuracy. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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16 pages, 3496 KiB

Open AccessArticle

Qualitative Analysis of Glass Microfragments Using the Combination of Laser-Induced Breakdown Spectroscopy and Refractive Index Data

by Dávid Jenő Palásti, Judit Kopniczky, Tamás Vörös, Anikó Metzinger and Gábor Galbács

Sensors 2022, 22(8), 3045; https://doi.org/10.3390/s22083045 - 15 Apr 2022

Cited by 4 | Viewed by 2329

Abstract

We have successfully demonstrated that although there are significant analytical challenges involved in the qualitative discrimination analysis of sub-mm sized (microfragment) glass samples, the task can be solved with very good accuracy and reliability with the multivariate chemometric evaluation of laser-induced breakdown spectroscopy (LIBS) data or in combination with pre-screening based on refractive index (RI) data. In total, 127 glass samples of four types (fused silica, flint, borosilicate and soda–lime) were involved in the tests. Four multivariate chemometric data evaluation methods (linear discrimination analysis, quadratic discrimination analysis, classification tree and random forest) for LIBS data were evaluated with and without data compression (principal component analysis). Classification tree and random forest methods were found to give the most consistent and most accurate results, with classifications/identifications correct in 92 to 99% of the cases for soda–lime glasses. The developed methods can be used in forensic analysis. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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18 pages, 6120 KiB

Open AccessArticle

Highly Sensitive Sphere-Tube Coupled Photoacoustic Cell Suitable for Detection of a Variety of Trace Gases: NO₂ as an Example

by Zhengang Li, Ganshang Si, Zhiqiang Ning, Jiaxiang Liu, Yonghua Fang, Beibei Si, Zhen Cheng and Changping Yang

Sensors 2022, 22(1), 281; https://doi.org/10.3390/s22010281 - 30 Dec 2021

Cited by 15 | Viewed by 2270

Abstract

The concentration of trace gases in the atmospheric environment is extremely low, but it has a great impact on the living environment of organisms. Photoacoustic spectroscopy has attracted extensive attention in the field of trace gas detection because of its high sensitivity, good selectivity, and fast response. As the core of a photoacoustic detection setup, the photoacoustic cell has a significant impact on detection performance. To improve detection sensitivity, a sphere-tube coupled photoacoustic cell (STPAC) was developed, which was mainly composed of a diffuse-reflective sphere and an acoustic resonance tube. Modulated light was reflected multiple times in the sphere to increase optical path, and photoacoustic (PA) signals were further amplified by the tube. Based on STPAC, a PA gas detection setup was built with a laser diode (LD) at 450 nm as the light source. The experimental results showed that the minimum detection limit (noise equivalent concentration, NEC) of NO₂ was ~0.7 parts per billion (ppb). Compared with the T-type PA cell (TPAC) in which the modulated light passed through the sphere, the signal-to-noise ratio of STPAC was increased by an order of magnitude at the same concentration of the NO₂ sample. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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11 pages, 3143 KiB

Open AccessArticle

Development and Field Tests of a Deep-Sea Laser-Induced Breakdown Spectroscopy (LIBS) System for Solid Sample Analysis in Seawater

by Chunhao Liu, Jinjia Guo, Ye Tian, Chao Zhang, Kai Cheng, Wangquan Ye and Ronger Zheng

Sensors 2020, 20(24), 7341; https://doi.org/10.3390/s20247341 - 21 Dec 2020

Cited by 19 | Viewed by 3183

Abstract

In recent years, the investigation and exploitation of hydrothermal region and polymetallic mineral areas has become a hot topic. The emergence of underwater vehicle platforms has made it possible for new chemical sensors to be applied in marine in-situ detection. Laser-induced breakdown spectroscopy (LIBS), with its advantages of rapid real-time analysis, sampling without pretreatment, simultaneous multi-element detection and stand-off detection, has great potential in marine applications. In this paper, a newly more compact and lighter underwater LIBS system based on the LIBSea system named LIBSea II was developed and tested both in the laboratory and sea trials. The system consists of a Nd:YAG single-pulse laser at 1064 nm, a fiber spectrometer, optical layout, a power supply module and an internal environment sensor. The system is encapsulated in a pressure vessel (Φ 190 mm × L 588 mm) with an optical window on the end cap. Experimental parameters of the system including laser energy and delay time were firstly optimized in the laboratory. Then, field test of the system in nearshore was performed with various samples, including pure metal and alloy samples as well as a manganese nodule sample from deep sea, to verify the detection performance of the LIBSea II system. In 2019, the system was deployed on a remotely operated vehicle (ROV) of Haima for deep sea trial, and atomic lines of K, Na, Ca and strong molecular bands of CaOH from a carbonate rock sample were obtained for the first time at depths of 1400 m. These results show that the LIBSea II system has great potential to be used in deep-sea geological exploration. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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20 pages, 3190 KiB

Open AccessArticle

Advanced Fiber-Coupled Diode Laser Sensor for Calibration-Free 1f-WMS Determination of an Absorption Line Intensity

by Vladimir Liger, Vladimir Mironenko, Yury Kuritsyn and Mikhail Bolshov

Sensors 2020, 20(21), 6286; https://doi.org/10.3390/s20216286 - 04 Nov 2020

Cited by 5 | Viewed by 2564

Abstract

A new scheme for a calibration-free diode laser absorption spectroscopy (DLAS) sensor for measuring the parameters of harsh zones is proposed. The key element of the scheme is a micro-prism retroreflector (MPRR). The MPRR facilitates an increase in the mechanical stability of the sensor and a decrease in the background thermal radiation in the hot areas of a tested zone. Reduction in the broadband thermal emission allowed the application of a differential logarithmic conversion (LC) technique for elimination of the residual amplitude modulation and other sources of non-selective attenuation of the probing laser beam. LC allows the use of a 1f-wavelength modulation spectroscopy (WMS) detection scheme. Combination of LC and a 1f-WMS algorithm provided a new modification of calibration-free DLAS, which could be particularly useful for probing harsh zones with pronounced strong turbulence and high levels of acoustic and electrical noise. The influence of the experimental parameters and characteristics of the main electronic components of the recording and processing system on the accuracy of the integral line intensity determination is investigated theoretically and experimentally. The proposed optical scheme of a DLAS sensor and algorithm for the data processing allowed the integral intensity of an absorption line to be obtained. The potential for the scheme was exemplified with a single water vapor absorption line at 7185.6 cm⁻¹. Simultaneous detection of several absorption lines and data processing using the developed algorithm provides the final goal of a DLAS sensor—determination of temperature and partial pressure of a test molecule in a probed gas volume. The developed scheme allows the spatial multiplexing of the radiation of different diode lasers (DLs), which can be used if various test molecules are to be detected, or absorption lines of a test molecule are detected over different wavelength intervals. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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15 pages, 3645 KiB

Open AccessArticle

An Optically Tunable THz Modulator Based on Nanostructures of Silicon Substrates

by Chen Mo, Jingbo Liu, Dongshan Wei, Honglei Wu, Qiye Wen and Dongxiong Ling

Sensors 2020, 20(8), 2198; https://doi.org/10.3390/s20082198 - 13 Apr 2020

Cited by 1 | Viewed by 2298

Abstract

Nanostructures can induce light multireflection, enabling strong light absorption and efficient photocarrier generation. In this work, silicon nanostructures, including nanocylinders, nanotips, and nanoholes, were proposed as all-optical broadband THz modulators. The modulation properties of these modulators were simulated and compared with finite element method calculations. It is interesting to note that the light reflectance values from all nanostructure were greatly suppressed, showing values of 26.22%, 21.04%, and 0.63% for nanocylinder, nanohole, and nanotip structures, respectively, at 2 THz. The calculated results show that under 808 nm illumination light, the best modulation performance is achieved in the nanotip modulator, which displays a modulation depth of 91.63% with a pumping power of 60 mW/mm² at 2 THz. However, under shorter illumination wavelengths, such as 532 nm, the modulation performance for all modulators deteriorates and the best performance is found with the nanohole-based modulator rather than the nanotip-based one. To further clarify the effects of the nanostructure and wavelength on the THz modulation, a graded index layer model was established and the simulation results were explained. This work may provide a further theoretical guide for the design of optically tunable broadband THz modulators. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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15 pages, 5741 KiB

Open AccessArticle

Compact Open-Path Sensor for Fast Measurements of CO₂ and H₂O using Scanned-Wavelength Modulation Spectroscopy with 1f-Phase Method

by Xiang Li, Feng Yuan, Mai Hu, Bin Chen, Yabai He, Chenguang Yang, Lifang Shi and Ruifeng Kan

Sensors 2020, 20(7), 1910; https://doi.org/10.3390/s20071910 - 30 Mar 2020

Cited by 7 | Viewed by 3234

Abstract

We report here the development of a compact, open-path CO₂ and H₂O sensor based on the newly introduced scanned-wavelength modulation spectroscopy with the first harmonic phase angle (scanned-WMS-θ1f) method for high-sensitivity, high temporal resolution, ground-based measurements. The considerable advantage of the sensor, compared with existing commercial ones, lies in its fast response of 500 Hz that makes this instrument ideal for resolving details of high-frequency turbulent motion in exceptionally dynamic coastal regions. The good agreement with a commercial nondispersive infrared analyzer supports the utility and accuracy of the sensor. Allan variance analysis shows that the concentration measurement sensitivities can reach 62 ppb CO₂ in 0.06 s and 0.89 ppm H₂O vapor in 0.26 s averaging time. Autonomous field operation for 15-day continuous measurements of greenhouse gases (CO₂/H₂O) was performed on a shore-based monitoring tower in Daya Bay, demonstrating the sensor’s long-term performance. The capability for high-quality fast turbulent atmospheric gas observations allow the potential for better characterization of oceanographic processes. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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10 pages, 2672 KiB

Open AccessArticle

Segment-Resolved Gas Concentration Measurements by a Time Domain Multiplexed Dual Comb Method

by Xinyi Chen, Weipeng Zhang, Yujia Zhang, Minjian Lu, Yan Li and Haoyun Wei

Sensors 2020, 20(6), 1566; https://doi.org/10.3390/s20061566 - 11 Mar 2020

Cited by 4 | Viewed by 2873

Abstract

Locating gas concentration changes in widespread locations can be conducive to environmental atmospheric detection, gas emissions monitoring, production process control, etc. A time domain multiplexed dual-comb system for segment-resolved gas concentration measurement is reported in this work. Both absorption spectra and path lengths for multiple path-segments in a target path can be derived from the time domain separated interferograms and then the equivalent gas concentrations in each segment can be retrieved separately. A benchtop experiment aiming at a target path with three path-segments of different gases has been demonstrated. The relative deviation of gas concentration retrieval is 1.08% in 1 s. Besides, additional numerical simulations prove that the crosstalk between the interference signals affects the spectrum analysis by no more than 0.1% for a kilometer-long atmospheric absorption detection. Therefore, achieving a gridded measurement of regional gas concentration in the open air can be foreseen using this method. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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13 pages, 11027 KiB

Open AccessArticle

A Wide-Range and Calibration-Free Spectrometer Which Combines Wavelength Modulation and Direct Absorption Spectroscopy with Cavity Ringdown Spectroscopy

by Zhen Wang, Yanjun Du, Yanjun Ding and Zhimin Peng

Sensors 2020, 20(3), 585; https://doi.org/10.3390/s20030585 - 21 Jan 2020

Cited by 8 | Viewed by 3323

Abstract

A wide-range, calibration-free tunable diode laser spectrometer is established by combining wavelength modulation and direct absorption spectroscopy (WM-DAS) with continuous wave cavity ringdown spectroscopy (CW-CRDS). This spectrometer combines the benefits of absolute concentration measurements, wide range, and high speed, using WM-DAS with enhanced noise reduction in CW-CRDS. The accurate baseline ringdown time, τ₀, is calculated by the absorption peak (measured by WM-DAS) and the ringdown time containing gas absorption information (measured by CW-CRDS at the center wavelength of the spectral line). The gas concentration is obtained without measuring τ₀ in real time, thus, greatly improving the measuring speed. A WM-DAS/CW-CRDS spectrometer at 1.57 μm for CO detection was assembled for experimental validation of the multiplexing scheme over a concentration ranging from 4 ppm to 1.09% (0.1 MPa, 298 K). The measured concentration of CO at 6374.406 cm⁻¹ shows that the dynamic range of this tunable diode laser absorption spectrometer is extendable up to five orders of magnitude and the corresponding precision is improved. The measurement speed of this spectrometer can extend up to 10 ms, and the detection limit can reach 35 ppb within 25 s. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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9 pages, 1450 KiB

Open AccessArticle

The Effect of Light Source Line Width on the Spectrum Resolution of Dual-Frequency Coherent Detection Signals

by Jianying Ren, Huayan Sun, Laixian Zhang and Yanzhong Zhao

Sensors 2019, 19(23), 5264; https://doi.org/10.3390/s19235264 - 29 Nov 2019

Viewed by 2120

Abstract

In this paper, the power spectrum resolution problem of dual-frequency coherent mixing signals is analyzed when the Doppler frequency difference is small. The power spectrum function formula of the four optical coherent mixing signals is obtained using statistical theory and the Wiener–Khinchin theorem. The influence of delay time and light source line width on the power spectrum of dual-frequency coherent signals is analyzed using this formula. The results show that delay time only affects the peak of the power spectrum of the coherent signal. An increase in the line width of the light source broadens the signal power spectrum and reduces the peak value. The necessary condition for distinguishing the Doppler frequency difference is that the theoretical Doppler frequency difference is greater than 1/5 times the line width of the light source. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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12 pages, 4795 KiB

Open AccessArticle

Influence of Spatial Inhomogeneity of Detector Temporal Responses on the Spectral Fidelity in Continuous Wave Cavity Ringdown Spectroscopy

by Zhensong Cao, Zhixin Li, Fei Xu, Yongqian Wu, Zixin Zhou, Zhaomin Tong, Weiguang Ma and Wenyue Zhu

Sensors 2019, 19(23), 5232; https://doi.org/10.3390/s19235232 - 28 Nov 2019

Cited by 2 | Viewed by 2839

Abstract

Due to their advantages of having a wide bandwidth, low cost, and being easy to obtain, traditional photodetectors (PDs) are being widely applied in measurements of transient signals. The spatial inhomogeneity of such PD temporal responses was measured directly to account for the PD spatial effect of decay rate due to poor alignment in continuous wave cavity ringdown spectroscopy (CW-CRDS) experiments. Based on the measurements of three PDs (i.e., model 1611 (Newport), model 1811 (Newport), and model PDA10CF-EC (Thorlabs)), all the temporal responses followed a tendency of declining first and then rising, and steady platforms existed for the last two PDs. Moreover, as we expected, the closer the PD center was, the faster the response. On the other hand, the initial shut-off amplitude generally reached a larger value for a faster temporal response. As a result, the spatial effect can strongly influence the spectral line shape and value, which will introduce more errors into the precise measurements of spectral parameters using the CRDS technique if this effect is not considered. The defined effective detection area (EDA) of the PDs, which was close to the active area given by manufacturers, was the key parameter that should be paid more attention by researchers. Therefore, the PD should be aligned perfectly to make sure that the EDA covers the laser spot completely. Full article

(This article belongs to the Special Issue Laser-Spectroscopy Based Sensing Technologies)

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