Simultaneous detection of resonant and non-resonant femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (CARS) signals has been developed as a viable technique to provide in-situ referencing of the impulsive excitation efficiency for temperature assessments in flames. In the framework of CARS thermometry, the occurrence of both a resonant and a non-resonant contribution to the third-order susceptibility is well known. While the resonant part conceives the useful spectral information for deriving temperature and species concentrations in the probed volume, the non-resonant part is often disregarded. It nonetheless serves the CARS technique as an essential reference to map the finite bandwidth of the laser excitation fields and the transmission characteristics of the signal along the detection path. Hence, the standard protocols for CARS flame measurements include the time-averaged recording of the non-resonant signal, to be performed sequentially to the experiment. In the present work we present the successful single-shot recordings of both the resonant and non-resonant CARS signals, split on the same detector frame, realizing the in-situ referencing of the impulsive excitation efficiency. We demonstrate the use of this technique on one-dimensional CARS imaging spectra, acquired across the flame front of a laminar premixed methane/air flame. The effect of pulse dispersion on the laser excitation fields, while propagating in the participating medium, is proved to result, if not accounted for, in an ~1.3% systematic bias of the CARS-evaluated temperature in the oxidation region of the flame.
- Combustion diagnostics
- Femtosecond laser excitation
- Gas-phase thermometry
- Non-linear optical spectroscopy