Applying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics

Abstract

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.

Carbon Monoxide in Plasma and Agriculture: Just a Foe or a Potential Friend?

“…IR light can interact with the CO molecule and therefore be measured by means of an absorption method that is based on the Beer-Lambert law. [131][132][133] Fourier-transform IR (FTIR) absorption spectroscopy and laser absorption, both based on this law, use, respectively, a broadband light and a laser as light source. FTIR absorption yields broad absorption spectra with low sensitivity, 132 whereas laser absorption can measure a very small amount of CO (< 1 ppm) on a very narrow spectral range.…”

“…FTIR absorption yields broad absorption spectra with low sensitivity, 132 whereas laser absorption can measure a very small amount of CO (< 1 ppm) on a very narrow spectral range. A quantum cascade laser is generally used instead of its ancestor, the lead salt laser, 131 and a temporal resolution down to 100 ns is, in principle, feasible. Usually, it is hardly possible to use these methods in situ to measure CO density due to the small dimensions and the nonhomogeneity of atmospheric pressure plasmas.…”

An Overview on Quantum Cascade Lasers: Origins and Development

“…This observation can be done by using absorption spectroscopy methods in the mid-infrared spectral region. Recently, a QCL-based absorption spectroscopic system, the quantum cascade laser measurement and control system (Q-MACS), was developed and used to study industrial process plasmas and for environmental studies, and its applicability for monitoring online processes has been proven [54].…”

Broadband Time-Resolved Absorption and Dispersion Spectroscopy of Methane and Ethane in A Plasma Using a Mid-Infrared Dual-Comb Spectrometer

“…Alternatively, in pulsed broadband operation they can be used in combination with a Fourier Transform Spectrometer (FTS) [17] or a dispersive based spectrometer [18,19] to simultaneously record broadband spectra. The use of these mid-infrared sources for plasma diagnostics is comprehensively discussed in [20,21].…”