Picosecond Diagnostics in Reaching Flows Using a Mode-Locked Ti:Sapphire Laser

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The PI has proposed a rather innovative approach to non-intrusive laser diagnostics of reacting flows based on the use of picosecond, mode-locked TI:Sapphire laser technology. The PI enumerates many advantages of using a mode-locked laser as compared to a pulsed laser. For example, since a mode-locked laser is essentially cw in nature, consisting of trains of very short picosecond pulses at MHz repetition rates and wide band-widths, improved signal to noise can be obtained by using phase-sensitive detection and can result in the recovery of pertinent frequency spectrum information such as power spectral densities and probability density functions. The very short pulse lengths make the measurements (absorption, fluorescence) less sensitive to collisional effects and is especially useful in pump/probe measurements. Mode-locked lasers also tend to be more stable, resulting in better spatial resolution. The peak power attainable, although much less than typical pulsed lasers, nevertheless, is sufficient to perform nonlinear optical techniques in particular degenerate four wave mixing (DFWM). The PI has outlined an ambitious program which hinges on the development of a reliable high power, Fourier-transform limited laser source with pulse length in the range of 30-50 psec. The PI plans to upgrade an existing TI:Sapphire laser for this purpose. An acoustically forced turbulent flame and a high pressure laminar flame will serve as test beds for the pump/probe and DFWM diagnostics.