Femtosecond Coherent Anti-Stokes Raman Scattering: Recent Applications and the Surprisingly Beneficial Effects of Moderate Pump and Stokes Chirp
13:30 - 14:00
Level 0 lecture hall between Al-Jazri and Al-Kindi (buildings 4 and 5)
The use of ultrafast laser radiation for femtosecond anti-Stokes Raman scattering (fs-CARS) for high-repetition-rate temperature measurements in turbulent flames is discussed. We have recently performed 5 kHz single-laser-shot CPP fs CARS measurements in highly turbulent, liquid-fueled, pilot-stabilized jet flames with a spatial resolution of approximately 500 microns. The results of measurements performed for various levels of liquid loading and interferences from the interaction of the femtosecond laser beams with liquid droplets are discussed. Measurements were also performed in heavily sooting ethylene/nitrogen jet diffusion flames.
The effects of moderate levels of chirp in the pump and Stokes pulses on CPP fs CARS were also investigated. Frequency chirp in the pump and Stokes pulses was introduced by placing SF11 glass disks with thicknesses of 10, 20 or 30 mm in the optical path for these beams. The magnitude of the chirp in the probe beam was much greater and was induced by placing a 30-cm rod of SF11 glass in the beam path. The spectrum of the nonresonant background four-wave mixing signal narrowed considerably with increasing pump and Stokes chirp, while the resonant CARS signal was relatively unaffected. Consequently, the interference of the nonresonant background with the resonant CARS signal in the frequency-spread dephasing region of the spectrum was minimized, and the increased rate of decay of the resonant CARS signal with increasing temperature was thus readily apparent. Our initial analysis of CPP fs CARS thermometry data indicates that temperature accuracy and precision are significantly improved by inducing moderate chirp in the pump and Stokes laser pulses. Inducing chirp in the pump and Stokes pulses will also help to eliminate interferences from hydrocarbon species in the N2 spectrum that were encountered in the spray flames.