Temperature measurements in flames at 1000 Hz using femtosecond coherent anti-Stokes Raman spectroscopy
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abstract
Single-laser-shot temperature measurements at a data rate of 1 kHz are demonstrated using femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy. The excitation of gas-phase Raman lines with spectral widths of 3 GHz by pump and Stokes beams with spectral widths of 3000 GHz is very efficient provided that the pump and Stokes beams are Fouriertransform- limited. The single-laser-shot measurements were performed by using a chirped probe pulse to map the time-dependent frequency-spread decay of the Raman coherence into the spectrum of the CARS signal pulse. Temperature is determined from the spectral shape of the chirped-probe femtosecond CARS signal for probe delays of approximately 2 picoseconds with respect to the impulsive pump-Stokes excitation of the Raman coherence. Fs CARS spectra with very high signal-to-noise ratios are acquired from laminar flames, forced unsteady flames, and turbulent flames. The fs CARS spectrum is not affected by collisional line shapes in contrast to ns CARS spectroscopy. However, the fs CARS spectrum is affected by the spectrum and phase of the pump, Stokes, and probe beams, and the effect of departures from the assumptions of Fourier-transform-limited pump and Stokes beam and a linearly chirped probe beam are discussed. Copyright 2010 by the American Institute of Aeronautics and Astronautics, Inc.
