Enhanced single – shot flame thermometry with femtosecond / picosecond CARS using a phase – characterized picosecond probe

Abstract

Hybrid fs/ps coherent anti-Stokes Raman CARS is a powerful laser diagnostic technique for thermometry and concentration measurements in chemically-reacting flows. The use of SHBC to generate the ps probe pulse is simple, robust, and relatively efficient. However, the presence of spectral sidebands in this pulse determines an additional modulation of the CARS spectrum, preventing comparisons to synthetic models. A solution is to employ a 4F-filter, to clean the spectrum, but dumps ~70% of the probe pulse energy. Fs/ps CARS thermometry can greatly benefit from an investigation of the spectrochronographic properties of the ps pulses, removing the filter. The probe pulse was characterized through frequency- and time-correlation measurements: its spectrum is discretized and the relative phase of each spectral segment is determined by measuring its arrival time. The fs pump/Stokes pulse is employed as an active temporal gate to perform time-correlation measurements. The phase information is further tuned using a genetic algorithm optimization. The resulting model is capable of rendering the additional modulation of experimental CARS spectra.