Improved species concentration measurements using a species‐specific weighting procedure on rotational CARS spectra

Abstract

AbstractRotational coherent anti‐Stokes Raman spectroscopy (CARS) is used for measurements of temperatures and relative species concentrations. The evaluation of a rotational CARS spectrum is normally performed using a least‐squares fitting algorithm to find the best‐fit theoretical spectrum in a library of spectra calculated at different temperatures and relative concentrations. A general problem is that species with weak spectral features have a minor influence on the results in the standard evaluation procedure. A species‐specific weighting (SSW) procedure enhancing the influence of weak spectral features in the spectral fitting algorithm has therefore been developed. The SSW procedure was tested on the evaluation of low concentrations of O2 in mixtures with N2, and it is shown that both the accuracy and precision of single‐shot spectra concentration measurements are improved by use of the SSW evaluation procedure. Also, a comparison with previously used weighting procedures shows that this approach is superior. The application of the technique to oxygen concentration measurements in the product gas of a fuel‐lean flame is demonstrated. In addition, a test was made using the developed weighting procedure for thermometry, by applying spectral weighting to the rotational spectrum originating from the first thermally excited vibrational state. The precision of the temperature measurements was then slightly improved. Copyright © 2005 John Wiley & Sons, Ltd.