Measuring CO Concentration and Temperature in a CH4/air Premixed Flame with Broadband Coherent Anti-Stokes Raman Spectroscopy

Abstract

Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a diagnostic tool for probing the temperature and species concentration of gas phase samples. CARS concentration measurements are typically performed either from the integration1,2 or from the shape of a CARS spectrum.3, 4 If the resonant signal of CARS is much bigger than the nonresonant background, integration of the CARS spectrum depends linearly on the squares of the concentration of the species. In practice, this method can be used to measure the concentration of major species. When the fraction of a species goes down to the order of a few percentage, the nonresonant background becomes comparable to the resonant signal of CARS and interferes with the resonant signal. In such a case, the portion of the nonresonant background included in the integration of the CARS spectrum leads to a large error in the concentration measurement. The interference of the nonresonant background and the real term of the resonant signal of CARS form modulation dips on the spectrum that are sensitive to the concentration. Recently Hahn et al.5 proposed a new technique for measuring the minor species concentration from the modulation dip of nonresonant background of broadband CARS spectrum.