CARS Concentration Sensitivity With and Without Nonresonant Background Suppression

Abstract

—Species detectivity is limited in conventional CARS approaches with aligned laser polarizations by the presence of a background, nonresonant electronic contribution to the CARS signal. At low species densities, a modulated CARS spectrum results which can be used to perform concentration measurements from the spectral shape, a unique feature of CARS. When the species concentration becomes very low, the “signal” from the species of interest, i.e. the modulation, essentially disappears into the dispersionless nonresonant background and the specie is no longer detectable. By proper orientation of the polarizations of the pump and Stokes laser fields and the CARS signal polarization analyzer, the nonresonant background can be suppressed, leaving the pure CARS spectrum of the specie being probed. Unfortunately, a large reduction in signal strength accompanies the elimination of the background. Investigations of background suppression from CO in flames are described using polarization sensitive CARS with spectrally broadband Stokes waves. The studies were conducted in flat CO-air diffusion flames employing nonplanar, crossed-beam phase matching, i.e. folded BOXCARS. The relative concentration sensitivity of CARS with and without background suppression was compared at both high and low temperatures. In all cases, the detection sensitivity of CARS with the nonresonant background present was found to be better, typically by a factor of five to ten, than with the background suppressed.