Stimulated Raman Scattering in High Pressure Oxygen and its Role in the Relief (raman Excitation + Laser-Induced Fluorescence) Flow Tagging Technique.

Stimulated Raman scattering (SRS) in high pressure oxygen and nitrogen has been studied. The study includes the development of a theoretical analysis to describe the Raman cumulative gain factors for both spherical and cylindrical focusing schemes. An experimental study has also been conducted to examine the generation of efficient Stokes conversion and to characterize the spectral shift and narrowing of oxygen at high pressure. Coherent anti-Stokes Raman spectroscopy (CARS) was employed to examine the oxygen Q-branch spectral properties in both pure oxygen and oxygen -helium gas mixtures at various pressures and mole ratios. A number of useful approaches have been developed to optimize the performance of the oxygen Raman shifter including super -fluorescence seeding and thermally induced circulation in the Raman cell. With a 10-nanosecond laser operating at 10 pulses per second, super-fluorescence seeding almost triples the first-Stokes conversion efficiency to 22% and quadruples the second-Stokes conversion efficiency to 14% while thermally induced circulation in the cell improves the spatial mode quality dramatically. Moreover, by using a 100-picosecond laser, first-Stokes conversion efficiency as high as 27% has been achieved in a five-pass cylindrically focused oxygen cell with no fluorescence-seeding. High Stokes conversion efficiencies from other gases such as N_2, CO_2, and CH_4 have also been obtained in preliminary tests. The principal application of the high efficiency oxygen SRS cell is RELIEF (Raman excitation + laser induced electronic fluorescence) flow tagging of air. As a demonstration of the capability of the cell RELIEF measurements were performed in a Mach-3 high Reynolds number facility by using a double-pass Raman shifter. Velocities across the boundary layer of the 20 x 20-cm wind tunnel test section were measured with statistical errors below one part per thousand. The significance of this effort lies in that it is an important step in the implementation of RELIEF, a promising flow measurement technique, and the first demonstration of RELIEF in a large scale wind tunnel.