Quantum Cascade Laser Absorption Spectroscopy as a Plasma Diagnostic Tool: An Overview

Welzel, S.; Hempel, F.; Hübner, M.; Lang, N.; Davies, Paul B.; Röpcke, J.

doi:10.3390/s100706861

Cited by 62 publications

(48 citation statements)

References 89 publications

(146 reference statements)

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“…These effects appeared more pronounced at higher laser intensities, i.e., well above threshold, and for strong absorption features [79]. A compilation of the main spectroscopic issues encountered when using pulsed QCLs for plasma diagnostic purposes has been given in reference [3]. In particular, under low pressure conditions, non-linear absorption effects may appear.…”

Section: General Spectroscopic Issuesmentioning

confidence: 99%

“…While short pulse spectrometers may approach the Fourier limit for extremely short pulses of the order of 5 ns [76] increased pulse widths lead to a chirp limited resolution (70 ns, 2.79 GHz [77]). Furthermore, several studies reported observing absorption features with asymmetric line shapes when employing both the short pulse and the long pulse mode (see [3] and references therein). Additionally, the latter method suffers from non-linear absorption effects [78].…”

Section: General Spectroscopic Issuesmentioning

confidence: 99%

“…This quantum cascade laser absorption spectroscopy (QCLAS) method is known as the intra pulse mode [71,72]. From the middle of the last decade QCLAS has been successfully implemented for a variety of plasma diagnostics purposes (see [3] and references therein), although specific obstacles such as pulse-to-pulse fluctuations inherent to pulsed operation had to be overcome. Recently, the so called intermittent operation of cw QCLs, using a rectangular pulse for tuning, has been proposed [73].…”

Section: General Spectroscopic Issuesmentioning

confidence: 99%

“…Broadly speaking, the commercial availability of different types of DFB-and EC-QCLs along with their convenient operating conditions and performance has led to the rapid development of MIR-LAS from a niche position to a standard diagnostic technique [3,6]. A topical review on the application of MIR-LAS in plasma diagnostics was published in 2012 [6].…”

Section: Introductionmentioning

confidence: 99%

“…Over the last two decades, chemical sensing using mid infrared laser absorption spectroscopy (MIR-LAS) in the molecular fingerprint region from 3 to 20 µm, which contains strong ro-vibrational absorption bands of a large variety of gaseous species, has been established as a powerful in situ diagnostic tool for molecular plasmas [1][2][3][4][5][6]. Quantum cascade lasers (QCL) in particular have played a central role in this field so much so that they have become the infrared light sources of choice for plasma diagnostics in the mid infrared.…”

Section: Introductionmentioning

confidence: 99%

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Applying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics

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Abstract:The considerably higher power and wider frequency coverage available from quantum cascade lasers (QCLs) in comparison to lead salt diode lasers has led to substantial advances when QCLs are used in pure and applied infrared spectroscopy. Furthermore, they can be used in both pulsed and continuous wave (cw) operation, opening up new possibilities in quantitative time resolved applications in plasmas both in the laboratory and in industry as shown in this article. However, in order to determine absolute concentrations accurately using pulsed QCLs, careful attention has to be paid to features like power saturation phenomena. Hence, we begin with a discussion of the non-linear effects which must be considered when using short or long pulse mode operation. More recently, cw QCLs have been introduced which have the advantage of higher power, better spectral resolution and lower fluctuations in light intensity compared to pulsed devices. They have proved particularly useful in sensing applications in plasmas when very low concentrations have to be monitored. Finally, the use of cw external cavity QCLs (EC-QCLs) for multi species detection is described, using a diagnostics study of a methane/nitrogen plasma as an example. The wide frequency coverage of this type of QCL laser, which is significantly broader than from a distributed feedback QCL (DFB-QCL), is a substantial advantage for multi species detection. Therefore, cw EC-QCLs are state of the art devices and have enormous potential for future plasma diagnostic studies.

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Section: General Spectroscopic Issuesmentioning

confidence: 99%

Section: General Spectroscopic Issuesmentioning

confidence: 99%

Section: General Spectroscopic Issuesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Applying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics

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Laser‐Based Combustion Diagnostics*Update based on original article by Jürgen Wolfrum, Thomas Dreier, Volker Ebert, and Christof Schulz, Encyclopedia of Analytical Chemistry, ©2000, John Wiley & Sons Ltd.

Dreier

Ebert

Schulz

2011

Encyclopedia of Analytical Chemistry

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In laser spectroscopy, the interaction of light emitted from various types of laser sources — tunable or nontunable in their output frequency — with the atomic or molecular species of interest is used to probe the sample through a variety of spectral responses. In order to perform laser spectroscopy, suitable laser sources must be selected, which meet the requirements of the chosen spectroscopic method. This means that the laser has to provide radiation in the wavelength range of interest, has the appropriate emission characteristics (lineshape), and has a suitable energy to perform the measurements. Further requirements are pulse length (milliseconds to femtoseconds or continuous wave), repetition rate, and beam profile. Nowadays, laser radiation can be generated with most of the required parameters necessary for the respective spectroscopic application, either directly or by generating new radiation frequencies by frequency mixing of one or several laser beams in a nonlinear medium (gas, liquid, and solid). As an example, the most direct interrogation technique is absorption of laser radiation (LAS, laser absorption spectroscopy) by suitable spectroscopically allowed transitions in atoms or molecules, which are known from conventional spectroscopic methods. The increase or decrease in the laser radiation transmitted through the sample is then a measure of the amount of substance probed in the sample, which characteristically absorbs at the required wavelength. Laser light scattering methods, elastic (Rayleigh scattering (RS)) and inelastic (spontaneous Raman scattering (SRS)), are other techniques to probe the medium. In the first method, which is not species specific, the density of the medium can be interrogated, whereas the second is able to probe all species with Raman‐active vibration‐rotation transitions. There are several advantages in using laser spectroscopy instead of conventional spectroscopic techniques using conventional thermal light sources. The spectral brightness of laser beams is many orders of magnitude higher than that of thermal radiation sources, which correspondingly increase the detection sensitivity of laser spectroscopic techniques. In addition, the small linewidth of the emitted radiation dramatically increases the spectral resolution such that minor details of the spectroscopic branch investigated can be resolved. This enables more quantitative interpretations of all parameters influencing the lineshape and line intensity of the probed transition, and as such the physical and chemical environments of the probed species: temperature, pressure, velocity, chemical species, and so on. It makes laser spectroscopic techniques much more selective than conventional methods, which often are not able to separate closely spaced spectral features from different species. A third advantage of laser spectroscopic techniques is connected with the variable pulse duration and repetition frequency of lasers: the very short pulse lengths can be used successfully to probe the sample within time periods that are short compared to any other physical or chemical time development — flow, chemical reaction, pressure changes, and so on. Finally, the small spatial regions that can be probed by focusing diffraction‐limited laser beams makes laser spectroscopic techniques ideally suited for applications where high spatial resolution is required. All these advantages of laser spectroscopy are beneficial when the various techniques are applied as a diagnostic tool in combustion processes: flames constitute a complex interaction of fast chemistry with flow fields and surfaces, and therefore, a detailed understanding of combustion events often needs in situ, species‐specific optical diagnostics with high spatial and temporal resolution. In many recent applications, laser spectroscopy has become a developed technique that can even be performed by nonlaser specialists. However, numerous laser spectroscopic techniques require detailed theoretical knowledge of the spectroscopy underlying the respective technique and the use of sophisticated equipment in order to obtain meaningful results. Future development is aimed toward simplifying experimental set‐ups, data evaluation, and maintenance. This development runs parallel to the breathtaking development in laser technology that continuously increases available wavelength ranges, simplicity of use, pulse power, and repetition rates.

show abstract

Laser‐Based Combustion Diagnostics

Wolfrum

Dreier

Ebert

et al. 2016

Encyclopedia of Analytical Chemistry

View full text Add to dashboard Cite

In laser spectroscopy, the interaction of light emitted from various types of laser sources – tunable or nontunable in their output frequency – with the atomic or molecular species of interest is used to probe the sample through a variety of spectral responses. In order to perform laser spectroscopy, suitable laser sources must be selected, which meet the requirements of the chosen spectroscopic method. This means that the laser has to provide radiation in the wavelength range of interest, has the appropriate emission characteristics (lineshape), and has a suitable energy to perform the measurements. Further requirements are pulse length (milliseconds to femtoseconds or continuous wave), repetition rate, and beam profile. Nowadays, laser radiation can be generated with most of the required parameters necessary for the respective spectroscopic application, either directly or by generating new radiation frequencies by frequency mixing of one or several laser beams in a nonlinear medium (gas, liquid, or solid). As an example, the most direct interrogation technique is absorption of laser radiation (LAS, laser absorption spectroscopy) by suitable spectroscopically allowed transitions in atoms or molecules, which are known from conventional spectroscopic methods. The increase or decrease in the laser radiation transmitted through the sample is then a measure of the amount of substance probed in the sample, which characteristically absorbs at the required wavelength. Laser light‐scattering methods, elastic (Rayleigh scattering (RS)), and inelastic (spontaneous Raman scattering (SRS)) are other techniques to probe the medium. In the first method, which is not species specific, the density of the medium can be interrogated, whereas the second is able to probe all species with Raman‐active vibration–rotation transitions. There are several advantages in using laser spectroscopy instead of conventional spectroscopic techniques using conventional thermal light sources. The spectral brightness of laser beams is many orders of magnitude higher than that of thermal radiation sources, which correspondingly increase the detection sensitivity of laser spectroscopic techniques. In addition, the small linewidth of the emitted radiation dramatically increases the spectral resolution such that minor details of the spectroscopic branch investigated can be resolved. This enables more quantitative interpretations of all parameters influencing the lineshape and line intensity of the probed transition, and as such the physical and chemical environments of the probed species: temperature, pressure, velocity, chemical species, and so on. It makes laser spectroscopic techniques much more selective than conventional methods, which often are not able to separate closely spaced spectral features from different species. A third advantage of laser spectroscopic techniques is connected with the variable pulse duration and repetition frequency of lasers: the very short pulse lengths can be used successfully to probe the sample within time periods that are short compared to any other physical or chemical time development – flow, chemical reaction, pressure changes, and so on. Finally, the small spatial regions that can be probed by focusing diffraction‐limited laser beams makes laser spectroscopic techniques ideally suited for applications where high spatial resolution is required. All these advantages of laser spectroscopy are beneficial when the various techniques are applied as a diagnostic tool in combustion processes: flames constitute a complex interaction of fast chemistry with flow fields and surfaces, and therefore, a detailed understanding of combustion events often needs in situ, species‐specific optical diagnostics with high spatial and temporal resolution. In many recent applications, laser spectroscopy has become a developed technique that can even be performed by nonlaser specialists. However, numerous laser spectroscopic techniques require detailed theoretical knowledge of the spectroscopy underlying the respective technique and the use of sophisticated equipment in order to obtain meaningful results. Future development is aimed toward simplifying experimental set‐ups, data evaluation, and maintenance. This development runs parallel to the breathtaking development in laser technology that continuously increases available wavelength ranges, simplicity of use, pulse power, and repetition rates.

show abstract

Quantum Cascade Laser Absorption Spectroscopy as a Plasma Diagnostic Tool: An Overview

Cited by 62 publications

References 89 publications

Applying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics

Applying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics

Laser‐Based Combustion Diagnostics*Update based on original article by Jürgen Wolfrum, Thomas Dreier, Volker Ebert, and Christof Schulz, Encyclopedia of Analytical Chemistry, ©2000, John Wiley & Sons Ltd.

Laser‐Based Combustion Diagnostics

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