Tunable Laser Absorption Spectroscopy Research Articles

Study of Mid-Pressure Ar Radiofrequency Plasma Used in Plasma-Enhanced Atomic Layer Deposition of α-Al2O3

This study investigated the characteristics of radiofrequency, middle-pressure argon plasma used in the atomic layer deposition (ALD) of Al2O3 films. Based on the electrical characteristics—the current, voltage, and phase shift between them—and the stability of the plasma plume, the optimum plasma power, allowing reliable switching on of the plasma for any step of an ALD cycle, was determined. Spectral measurements were performed to determine the gas temperature and reactive species that could be important in the ALD process. The density of metastable argon atoms was estimated using tunable laser absorption spectroscopy. It was concluded that plasma heating of substrates did not affect film growth. The crystallization-enhancing effect of plasma observed in these experiments was due to the action of OH radicals produced in the plasma.

Advancing thermochemical diagnostics in kilogram-scale explosive fireballs via laser absorption spectroscopy

This article presents methodological advances in the state-of-the-art for making time-dependent, thermochemical measurements within kilogram-scale explosive post-detonation fireballs utilizing tunable laser absorption spectroscopy. This measurement capability is critical for validating multi-scale, multi-physics models of post-detonation dynamics. The technique is based on hardened gauges built around rapidly-tunable lasers and custom post-processing algorithms that provide quantitative thermochemical data interior to large and opaque explosive fireballs. The authors present a holistic overview of the technique including gauge design, the laser absorption diagnostic, and the custom data processing algorithms. Additionally, fielding high-bandwidth laser absorption probes at stand-off ranges presents new challenges in data processing that must compensate for long distance signal transmission effects. We highlight representative data from a hardened gauge measurement at 0.81 m stand-off from a 2.78 kg LX-14 explosive charge detonated in an outdoor test arena. We discuss progress in all-optical measurement of temperature, pressure, and water vapor number density at a 100 kHz repetition rate during the first 10 ms of the fireball evolution. We conclude the article with a brief discussion on our current approach for comparing hardened gauge measurements with computational fluid dynamic simulations.

Methane gas spectral imaging method based on dual wedge scanning mirrors

With the increase in the number of oil and gas pipelines laid in China, more attention needs to be paid to pipeline maintenance work. At present, the main methods of detecting natural gas leaks in oil and gas transmission stations include manual inspections, opposing natural gas detection equipment, and cloud desktop natural gas detection equipment. Hand held natural gas detection equipment is used for manual inspection, which requires regular manual inspection. However, the response speed is poor and gas leaks cannot be detected in a timely manner. The opposed laser gas detection method can only detect the presence of gas on the beam path. If a larger area of leakage detection is desired, more equipment needs to be installed, resulting in a greatly increase in hardware costs. The existing cloud desktop laser gas detection method controls the deflection of the laser beam through the cloud platform to achieve leak detection at various points in the area to be tested. However, the rotation speed of the cloud platform is slow, and a complete detection cycle takes dozens of minutes, and only the presence of gas can be detected. For accurate leak location, manual on-site survey is also required to further determine the leak location. In order to meet the needs of the real-time monitoring and rapid positioning of oil and gas pipeline leaks, in this work, a fast and accurately controlled dual wedge scanning mirror system is designed, which combines tunable semiconductor laser absorption spectroscopy technology to convert the gas measurement laser beam from point measurement to surface measurement, thereby obtaining the two-dimensional distribution of gas, which is conducive to subsequent analysis and positioning of gas leakage sources. By using the inverse solution iterative optimization algorithm, the angle of the wedge mirror is controlled to obtain an efficient and uniform beam scanning trajectory. The deflection direction and detection position of the laser beam are fused with the corresponding methane concentration information, and a methane concentration data containing position information is constructed. In order to quantitatively verify the measurement accuracy and spatial resolution in the experiment, a standard air bag is used to simulate the methane leakage distribution. The results show that the minimum detection limit of the system can be lower than 5×10<sup>–4</sup> m, and the spatial resolution can be less than 6 cm. At the same time, this method can adjust the scanning step node based on the measurement distance of the oil from gas station, thereby achieving adjustable imaging resolution. This imaging method provides a new idea for accurately positioning and detecting the methane leakage location and amount.

A Portable Laser Spectroscopic System for Measuring Nitrous Oxide Emissions on Fertilized Cropland.

Nitrous oxide (laughing gas, N2O) is a relevant greenhouse gas. Agriculture contributes significantly to its emissions. As nitrogen fertilization has been identified as one of the main sources of N2O, controlled application and reduction of the amount of fertilizer adapted to crop demand is essential to reduce N2O emissions. This requires detailed studies of the local distribution of the N2O emission fluxes on different croplands. Consequently, frequent spatially resolved field measurements of N2O concentrations are needed. A precision in the ppb range close to the ambient N2O level of 333 ppb is necessary. Tunable laser absorption spectroscopy using quantum-cascade lasers (QCL) as a light source is an established technique for the measurement of N2O traces. We present the development and validation of a compact portable setup for on-site measurement of N2O emissions from the soil. The setup differs from previous solutions by using an interband cascade laser (ICL), which has significantly lower power consumption compared to a QCL. The portable measurement setup allows N2O emission fluxes to be determined with a precision of 3.5% with a measuring duration of 10 min. The developed system enables the detection of increased N2O emissions because of the fertilization of fields. High N2O emission fluxes are indicators of the overfertilization of the field. Directly after fertilization, N2O fluxes between 2.9 and 5.3 µL m-2 min-1 depending on the gas acquisition site are measured during the field tests. Over time, the fluxes decrease. The obtained results compare well with data from more precise but also more complex and maintenance-intensive instruments for atmospheric research. With this system, the soil moisture as well as the air humidity and air temperature are recorded. Strong influences on N2O fluxes by soil moisture were observed. The presented measurement system is a contribution to the establishment of mobile N2O screening systems that are robust in the field and suitable for comprehensive and routine detection of N2O emissions from soil.

Line positions and effective line strengths of trans-HONO near 1280 cm−1

The measurement of the line positions and effective line strengths of the ν3 fundamental band of trans-nitrous acid (trans-HONO) near 1280 cm−1 (7.8 µm) by tunable laser absorption spectroscopy (TLAS) utilizing a room temperature continuous-wave quantum cascade laser (cw-QCL) was reported. The effective line strengths of 30 well-resolved trans-HONO absorption lines in the range of 1279.8–1282.2 cm−1 were determined using the HONO line strength at 1280.3841 cm−1 as a scale. The maximum measurement uncertainty of 7.64% in the line strengths is mainly determined by the uncertainty of the referenced line strength, while the measurement precision of the line positions is better than 5.56 * 10−3 cm−1. The line positions and strengths of the trans-HONO absorption lines obtained in this work provide a reference for continuous gas monitoring and analysis of the sources and sinks of atmospheric HONO.

Temperature Compensation Algorithm of Air Quality Monitoring Equipment Based on TDLAS

When tunable diode laser absorption spectroscopy is used to measure the concentration of gas, the second harmonic signal of demodulation is changed due to the influence of temperature change, and the error in concentration measurement is great. In order to solve the problem of large errors in atmospheric quality monitoring equipment due to the change in gas temperature, this paper, based on the tunable semiconductor laser absorption spectroscopy (TDLAS) theory, measured methane gas with 1000 ppm standard gas as the target and selected the central absorption wavelength of 1650 nm. The influence of temperature change on gas injection and the laser absorption spectrometer is studied. A temperature compensation algorithm based on an empirical formula is designed. Firstly, by analyzing the variable temperature test data of the detection module, it is proposed to divide the influence factors of temperature into two parts and study the influence of injection gas temperature and detector temperature, respectively. Secondly, the temperature compensation is carried out by polynomial fitting the concentration inversion results. Finally, according to the compensation effect, a scheme was proposed to compensate the measured gas by applying a constant temperature treatment to the detector at 313 K. After compensation, the average error of the system measurement is reduced from 8.4% to 1.08% when the gas temperature changes from 233 K to 343 K, which effectively reduces the deviation of the measured value caused by the abrupt temperature change. It further improves the accuracy and reliability of measuring gas concentration when gas inspection equipment is working outdoors and has strong practicability.

Comparison of excitation and kinetic temperatures in a laser-produced plasma using absorption spectroscopy.

High-resolution tunable laser absorption spectroscopy is used to measure time-resolved absorption spectra for six neutral uranium transitions in a laser-produced plasma. Analysis of the spectra shows that kinetic temperatures are similar for all six transitions, but excitation temperatures are higher than kinetic temperatures from 10-100 μs, indicating departures from local thermodynamic equilibrium.

Vapor concentration and temperature field measurement of an evaporating sessile drop by tomographic laser absorption spectroscopy

Drop evaporation is a ubiquitous phenomenon that has been studied for over a century. However, the surrounding gas-phase field including the temperature and vapor concentration distribution is not sufficiently studied experimentally. In this paper, a sensor based on tunable laser absorption spectroscopy is designed to study the vapor-phase temperature and concentration distribution of evaporating sessile drops, and data processing method involving data pre-processing and tomographic reconstruction is proposed to realize high-precision, spatially resolved measurement, which was realized by scanning the mechanical galvanometer in the horizontal direction. With free-knot splines smoothing and “denucleated” onion-peeling algorithm, temperature and H2O concentration distributions surrounding the evaporated drop at three different substrate plate temperatures are observed. The concentration and temperature in close vicinity to the gas–liquid interface are reconstructed accurately despite the high-gradient changes. A spatial resolution of under 100 μm with a temporal resolution of 10 s has been realized. Quantitative depiction of the temperature and concentration fields shows evidence of convection and indicates that while the concentration level sharply peaks at the interface, temperature in the close vicinity to the drop shows flattening or even dipping trends. The in situ laser measurement results are validated against contact measurement, theoretical prediction with saturated vapor pressure, and model simulation of COMSOL. Uncertainties have been evaluated based on both repeated measurements and model prediction of input uncertainty propagation. Temperature and concentration measurement uncertainties are estimated to be <1.5% and <3.5%, respectively, even though all experiments were performed in open air with non-negligible buoyancy-induced convection.

Methyl methacrylate thermal decomposition: Modeling and laser spectroscopy of species time-histories behind reflected shock waves

The thermal decomposition of methyl methacrylate (MMA) was studied through species time-history measurements of formaldehyde (CH2O), carbon monoxide (CO), and carbon dioxide (CO2) behind reflected shock waves over a temperature range of 1200–1600 K near 1 atm. Tunable laser absorption spectroscopy was employed to spectrally and temporally-resolve a cluster of rovibrational lines in the Q-branches of the v1 fundamental band and the v2+v4 combination band of CH2O near 3.60μm, three rovibrational transitions in the P-branch of the fundamental band of CO near 4.98μm, and a transition in the R-branch of the (0100→0101) v3 band of CO2 near 4.19μm. Spectral fitting procedures are subsequently used to infer CO, CO2, and CH2O mole fraction during the pyrolysis of shock-heated mixtures of MMA in argon. These data provided valuable experimental constraints on MMA pyrolysis chemical kinetic models. Sensitivity analysis of a detailed chemical model for MMA decomposition identified specific reactions likely to account for differences observed between the species measurements and simulations of the test conditions. Modified reaction rate parameters for select MMA decomposition reactions are proposed, determined via a genetic algorithm optimization procedure anchored to the speciation data.

Time-resolved CO2, CO, and N2 vibrational population measurements in Ns pulse discharge plasmas

Time-resolved CO2 and N2 vibrational populations and translational-rotational temperature are measured in a CO2–N2 plasma sustained by a ns pulse discharge burst in plane-to-plane geometry. Time-resolved, absolute number density of CO generated in the plasma is also inferred from the experimental data. CO2 and CO vibrational populations are measured by mid-IR, tunable quantum cascade laser absorption spectroscopy, and N2 vibrational populations are measured by the ns broadband vibrational CARS. Transient excitation of N2 and CO2 asymmetric stretch vibrational energy modes is detected during the discharge burst. The time-resolved rate of CO generation does not correlate with N2 or CO2(ν 3) vibrational temperatures, indicating that CO2 dissociation via the vibrational excitation is insignificant at the present conditions. The rate of CO generation decreases gradually during the discharge burst. The estimated specific energy cost of the CO product is close to that of N atoms in pure nitrogen, measured previously at similar operating conditions. Comparison of the experimental data with the kinetic modeling analysis indicates that CO2 dissociation in collisions with electronically excited N2 molecules is the dominant channel of CO generation at the present conditions, although the inferred CO yield in these processes is significantly lower than 1. The effect of vibrational energy transfer between N2 and CO2 on the plasma chemical processes is insignificant. The kinetic model underpredicts a rapid reduction of the N2 and CO2(ν 3) vibrational temperatures during the later half of the discharge burst and in the afterglow. V–T relaxation of N2 by N and O atoms generated in the ns pulse discharge plasma does not affect the vibrational relaxation rate in a significant way. However, rapid V–T relaxation of CO2 by O atoms has a significant effect on the relaxation rate. The difference between the experimental data and the modeling predictions may be due to the unknown scaling of the CO2–O V–T rates with the vibrational quantum number.

Tunable Laser Absorption Imaging for 2D Gas Measurement With an Electronic Rolling Shutter Camera

High spatial resolution two-dimensional (2D) imaging of gas flow is essential for better understanding and characterization of a combustion field. We propose a method for 2D gas measurement with high spatial resolution by combining tunable laser absorption spectroscopy and an imager with an electronic rolling shutter through their common feature of time scanning. The method was demonstrated by oxygen measurement using a 764nm vertical cavity surface emitting laser and a low-cost CMOS camera. 2D imaging of the oxygen concentration distribution were obtained at a frame rate of 15fps with a horizontal spatial resolution of about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$28\mu \text{m}$ </tex-math></inline-formula> . This method enjoys simple structure, low cost, high spatial resolution and shows potential in the 2D and 3D reconstruction of the combustion field.

Measurement of CO, HCN, and NO productions in atmospheric reaction induced by femtosecond laser filament

It is proved that the chemical reaction induced by femtosecond laser filament in the atmosphere produces CO, HCN, and NO, and the production CO and HCN are observed for the first time. The concentrations of the products are measured by mid-infrared tunable laser absorption spectroscopy. In the reduced pressure air, the decomposition of CO2 is enhanced by vibration excitation induced by laser filament, resulting in the enhanced production of CO and HCN. At the same time, the CO and HCN generated from the atmosphere suffer rotation excitation induced by laser filament, enhancing their absorption spectra. It is found that NO, CO, and HCN accumulate to 134 ppm, 80 ppm, and 1.6 ppm in sealed air after sufficient reaction time. The atmospheric chemical reaction induced by laser filament opens the way to changing the air composition while maintaining environmental benefits.

Temperature measurement of turbulent flame using CT-TDLAS (computed tomography-tunable diode laser absorption spectroscopy)

In order to satisfy the requirements of high-quality and high-performance optimal material manufacturing process, it is essential to control the gas system of the manufacturing process. In the actual industry, the quality of products is improved by controlling various gases in the manufacturing process. The tunable laser absorption spectroscopy (TDLAS) technique can be measured by the temperature and concentration of target gas simultaneously. Among the more advanced technologies, CT-TDLAS is the most crucial technique for measuring temperature and concentration distributions across two-dimensional planes. This study suggests a three-dimensional useful measurement of irregular flow or exhaust gases. Furthermore, an optical measurement method has been adopted to measure temperature distribution at a cross-section of the Methane Air premixed turbulent flame. The equivalence ratio of fuel can control this system. The burner system consists of two sections (main flame, sub-flame) for a turbulent flame. In the CT-TDLAS technique, it is essential to set the wavelength for target gases. There is a limit to high-temperature measurement in temperature estimation using a single laser-specific wavelength. Therefore, compared measurement performance was made using a mixed type laser (1388 nm, 1343 nm) and a single type laser (1388 nm). The temperatures obtained by using the optical measurement results were relatively evaluated with those obtained by the thermocouples. It was confirmed that the relative error of the temperature occurred at the central position of the burner. It was about 13.33% by the mixed laser system and 38.33% by the single laser system in most close to point from the burner.

Early- and late-time dynamics of laser-produced plasmas by combining emission and absorption spectroscopy

Optical emission spectroscopy and laser absorption spectroscopy are two highly complementary techniques for diagnosing plasmas, specifically determining important parameters such as temperature and density. In this work, we characterized a laser-produced plasma by combining emission and tunable laser absorption spectroscopy. Plasmas are produced by focusing pulses from a frequency quadrupled Nd:YAG laser onto a glass target containing a minor concentration of aluminum in an ambient atmosphere of ~100 Torr N2. Emission spectroscopy is used to investigate the line behavior of Al and Ca, electron density, and excitation temperature at times ≤ 5 μs, while absorption spectroscopy is used to investigate the line behavior of Al and Ca, species density, and kinetic temperature at times ≥ 5–100 μs. A comparison is made between the excitation temperature measured from emission using the Boltzmann plot method and the kinetic temperature measured from absorption; the implications of this comparison on local thermodynamic equilibrium and line-of-sight averaging are discussed. Combining the two methods of optical emission and laser absorption spectroscopy, a comprehensive characterization of the plasma over the entire lifetime of its evolution is possible, leading to a greater understanding of plasma behavior.

Time-resolved absorption spectroscopic characterization of ultrafast laser-produced plasmas under varying background pressures.

Time-resolved tunable laser absorption spectroscopy is used to characterize the physical properties of ultrafast laser-produced plasmas. The plasmas were produced from an Inconel target, with ≤0.4wt% Al, using ∼35fs, ∼800nm, ∼5mJ laser pulses at varying Ar background pressures from 1 to 100 Torr. The absorption spectrum of atomic Al is measured with high spectral and temporal resolution when the probe laser is stepped across the selected Al transition at 394.4 nm. Spectral fitting is used to infer linewidths, kinetic temperature, Al column density, and pressure broadening coefficient. The late time physical properties of plasmas are compared for various pressure levels. Our studies highlight that a significant lower state population exists even at early times of ultrafast laser-produced plasma evolution, and lower state population persistence decreases with increasing ambient pressure. We also show that the fundamental optical properties, such as pressure broadening, can be measured using ultrafast laser-produced plasmas combined with laser absorption spectroscopy.

Standoff Chemical Detection Using Laser Absorption Spectroscopy: A Review

Remote chemical detection in the atmosphere or some specific space has always been of great interest in many applications for environmental protection and safety. Laser absorption spectroscopy (LAS) is a highly desirable technology, benefiting from high measurement sensitivity, improved spectral selectivity or resolution, fast response and capability of good spatial resolution, multi-species and standoff detection with a non-cooperative target. Numerous LAS-based standoff detection techniques have seen rapid development recently and are reviewed herein, including differential absorption LiDAR, tunable laser absorption spectroscopy, laser photoacoustic spectroscopy, dual comb spectroscopy, laser heterodyne radiometry and active coherent laser absorption spectroscopy. An update of the current status of these various methods is presented, covering their principles, system compositions, features, developments and applications for standoff chemical detection over the last decade. In addition, a performance comparison together with the challenges and opportunities analysis is presented that describes the broad LAS-based techniques within the framework of remote sensing research and their directions of development for meeting potential practical use.

High-resolution and -precision spectra of acetonitrile at the ν5-band for laser remote sensing

Ultrahigh-resolution and -precision air-broadened acetonitrile (CH3CN) spectra of the v5-band in the region of 3039.0–3041.5 cm−1 were recorded and analyzed using tunable laser absorption spectroscopy (TLAS) technology with a distributed feedback interband cascade laser. The spectral line profile of methane (CH4) was used as the standard to correct the nonlinearity and baseline drift in the CH3CN spectra. The recorded spectra are very close to that in the Pacific Northwest National Laboratory (PNNL) database, however it has a much lower absorbance uncertainty of 1.02%. The air-broadened CH3CN absorption at the v5-band with a spectral resolution of 2.15 × 10−5 cm−1 (~23 fm/637 kHz) and an estimated uncertainty of 1.1% was derived at a temperature of 296.75 K and a pressure of 1 atm. The ultrahigh-resolution and -precision spectra of CH3CN provide reference data for absolute spectral measurements, laser remote sensing, and atmospheric inversion.

Midinfrared Sensor System Based on Tunable Laser Absorption Spectroscopy for Dissolved Carbon Dioxide Analysis in the South China Sea: System-Level Integration and Deployment.

System-level integration of a midinfrared carbon dioxide (CO2) sensor system based on tunable laser absorption spectroscopy (TLAS) was realized for the analysis of dissolved CO2 in seawater, employing an interband cascade laser (ICL) centered at 4319 nm and a multipass cell (MPC) with an optical path length of 29.8 m. At a low measurement pressure of 30 Torr, three absorption lines of 12CO2 were selected to realize different measurement ranges and a 13CO2 absorption line was targeted for simultaneous isotopic abundance analysis of δ13CO2. The sensor system was compactly integrated into a standalone system with automatic operation for underwater field deployment, and the working process was controlled by a specially designed electrical system. A gas-liquid separator system was developed for CO2 extraction from water, and a pressure-control mechanism with two operation modes (i.e., static and dynamic modes) was proposed to make the sensor system applicable under a deep-sea environment. A series of experiments were carried out in the laboratory for performance assessment of the developed sensor system employed for the analysis of dissolved CO2 in water. The sensor was deployed for a field test for natural gas hydrates exploration at an underwater depth of 0-2000 m in the South China Sea, with the sensor operating normally during the deployment.

A study on two-dimensional temperature and concentration distribution of Propane-Air premixed flame using CT-TDLAS

To use supplying gases and energy resources efficiently, accurate measurement of irregular gas is necessary. The TDLAS (Tunable laser absorption spectroscopy) technique can be used to control and monitor the supplying gas conditions and combustion of industrial processes. Recently, CT-TDLAS (Computed tomography-tunable diode laser absorption spectroscopy) has been developed to measure the temperature and concentration field of gases. In this study, the 2-dimensional temperature distribution of the Propane-Air premixed flame in several mixing conditions of fuel has been measured by the constructed CT-TDLAS system. 2-Dimensional temperature distributions are measured by 16 path cells. Further, the third-order polynomial regression analysis was applied to resolve the absorption spectra from the incident and transmitted light for a particular gas. The SMART (simultaneous multiplicative algebraic reconstruction technique) algorithm has been adopted for reconstructing the absorption coefficients on the detecting area. As a result of comparing the temperature for the 2-dimensional detecting area using the thermocouple and CT-TDLAS technique, it has been verified that the relative error for the temperatures measured by the thermocouples and calculated by the CT-TDLAS was up to 8%.

Ultrahigh resolution spectroscopy for dimethyl sulfide at the ν1- and ν8-bands by a distributed feedback interband cascade laser

The ultrahigh-resolution, air-broadened mid-infrared absorption spectrum of dimethyl sulfide (DMS) was recorded in the spectral region of 2994.17–2999.03 cm−1, where the strong ν1-/ν8-bands are located. Benefitting from the tunable laser absorption spectrometry with a distributed feedback interband cascade laser, the spectral resolution achieved 5.69 × 10−6 cm−1. The recorded spectra of DMS are very close to that in the Pacific Northwest National Laboratory (PNNL) database, however it has a much lower absorbance uncertainty of 0.38%. And the maximum relative difference of the full width at half maximum and the integrated area of spectral peaks between the recorded spectra and the PNNL data are as large as 16.7% and 27.9%, respectively, which could mainly come from the difference of the spectral resolution. The more accurate reference spectrum is hopeful to be applied to absolute measurement and remote sensing for gaseous DMS.