Laser‐Based Combustion Diagnostics

Abstract

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.

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