A single laser apparatus for writing patterns into unseeded air

In this paper we report a single laser apparatus for vibrationally exciting oxygen molecules in air, which is the first step in the RELIEF flow tagging method. This apparatus is based on new results for stimulated Raman scattering in high pressure O2/He mixtures. Use of stimulated Raman scattering rather than a dye laser to generate the Stokes beam for RELIEF flow tagging results in pump and Stokes beams which are automatically overlapped, both spatially and temporally. In addition, the stimulated Raman cell is a simple, passive device and the Nd:YAG pump laser is operated broadband, eliminating the need for injection seeding. The resulting short coherence length enhances the Raman conversion by suppressing the gain for stimulated Brillouin backscatter. Proper choice of He and O2 mole fractions and the total pressure is determined by the He and O2 pressure narrowing and pressure shifting of the O2 Q-branch spectrum. CARS measurements enable us to properly choose this operating point.

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