FiberLaserIntracavityAbsorption Spectroscopy(FLICAS)ofCO/CO2 mixture. This experiment will expose you to tools and approaches, common in modern laser spectroscopy. During the following weeks we will cover several important chapters in the field of modern optics and experimental laser spectroscopy: Basic principles of laser operation. Fiber optics. Spectrographs and monochromators. Intracavity Laser Absorption Spectroscopy (ICLAS). Fiber Laser Intracavity Absorption Spectroscopy (FLICAS).
Introduction Laser Diagnostics in Combustion and Environmental Research The availability and high reliability of lasers, laser spectroscopy is assuming an ever-expanding role in the diagnostic probing of combustion and environmental processes. Laser based techniques supply the researcher with the capability for remote, nonintrusive, in-situ, spatially and temporally precise measurements of important chemical parameters. Laser diagnostics are facilitating improved understanding of a wide variety of combustion phenomena that, in turn, will lead to improved efficiency and cleanliness in the energy conversion devices so vital to modern day life. ICLAS: the methodology of the experiment Direct absorption spectroscopy of atoms and molecules in the gas phase, yielding both quantitative absolute concentrations as well as absolute frequency-dependent cross-sections, is a very powerful tool in analytical chemistry and physical chemistry. This absoluteness is the reason why sensitive absorption spectroscopy techniques have gained renewed interest, even in research fields where more sophisticated laser-based diagnostic techniques are commonly applied. Among the various direct absorption techniques, the Intracavity Laser Absorption Spectroscopy (ICLAS) and its fiber-laser based version (FLICAS) have proven to be valuable addition, since they combine a good sensitivity with a relatively simple and straightforward experimental set-up. In a `conventional absorption experiment, one measures the amount of light that is transmitted through a sample. If the light source is monochromatic (e.g. a laser), one can record an absorption spectrum of the sample by recording the transmitted intensity as a function of the frequency. Alternatively, a broad light source can be used when the incident light or the transmitted light is spectrally dispersed. A drawback of direct absorption might be its limited sensitivity. A small attenuation in transmitted light has to be measured on top of a large background. High sensitivity is obtained by using modulation schemes and by increasing the absorption path length. Alternatively, other experimental spectroscopy techniques can be used which are based on the detection of phenomena which are induced by absorption of light, such as pressure changes in photoacoustic spectroscopy, fluorescence in laser-induced fluorescence (LIF), or ions in resonant enhanced multiphoton ionization (REMPI). The great advantage of these techniques is that they are background free. A disadvantage is the
sometimes difficult calibration procedure which is needed to make these techniques absolute (i.e. these techniques are not self-calibrating). ICLAS spectroscopy is a sensitive absorption technique, in which the absorbing species are placed inside the cavity of a broadband laser. Figure 1 illustrates the schematics of the intracavity absorption experiment. The sample is either confined in an absorption cell, or just placed in an open part of the cavity. Fig1: Schematic description of intracavity absorption experiment When the linewidth of the absorber is narrower than homogeneous spectral broadening of the laser gain medium, then even minute absorber quantities lead to spectral holes in the laser output due to the positive feedback mechanism in lasers. Additional inhomogeneous broadening of the gain is beneficial, since the spectral range for absorption measurements with a particular laser is broadened. Experimental procedure During the following weeks you will use the Fiber Laser Intracavity Absorption Spectroscopy (FLICAS) to monitor CO/CO2 mixture. CO/CO2 ratio is an important measure of completeness of combustion and is among the major environmental and performance characteristics of any combustion device. Specifically, you will perform the following tasks: 1. Alignment of the optical layout 2. Measurement of the absorption Spectrum vs. at different pressures of CO/CO2 mixture 3. Measurement of the absorption Spectrum vs. at different temperatures of CO/CO2 mixture 4. Obtain the mixture temperature and quantum-state dependent
Demtröder, Laser Spectroscopy (Vol.1, Vol.2, 4th edition) Baev V. M. et. al, APPLIED PHYSICS B-LASERS AND OPTICS Volume: 69 Issue: 3 Pages: 171-202 DOI: 10.1007/s003400050793 Published: SEP 1999