Concentration Measurements in CO/N2 and Ar/N_2 Gas Mixtures using Femtosecond Coherent Anti-Stokes Raman Scattering

摘要

Single-laser-shot concentration measurements in CO/N_2 and Ar/N_2 gaseous mixtures are performed using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS) with a chirped probe pulse. Measurements are performed in a heated gas cell at temperatures of 300, 600, and 900 K for the complete range of binary gas mixtures. Due to the broadband nature of the ultrafast pulses, Raman transitions of CO (2145 cm~(-1)) and N_2 (2330 cm~(-1)) are probed simultaneously. A theoretical model is developed to extract temperature and concentration measurements from the experimental data. The best agreement between theory and experiment was found for probe-pulse time delays from 0 to 2 picoseconds (ps). Preliminary concentration measurement results are promising when the lasers are tuned to the minor species Raman transition frequency. Measurements performed in Ar/N_2 mixtures are used to explore detection limits in environments with strong nonresonant backgrounds. Experimental data is also reported utilizing polarization suppression of the nonresonant background and further analysis will be performed in the future. These measurements will help to determine the effect of resonant and nonresonant gases on temperature measurements based on the N_2 spectral shape.