Coherent Raman spectroscopy for supersonic flow measurements

Abstract

Inverse Raman spectroscopy is used to measure non-intrusively the velocity, temperature, and density in a supersonic nitrogen gas flow. The present measurement uses two lasers, operating at different visible frequencies, to drive coherently the vibrational resonances in the flowing N2. A miniature wind tunnel produces the Mach 2 supersonic flow. Flow velocities are determined by measuring the flow induced Doppler shift of the Q-branch vibrational Raman transitions, while the rotational temperature is deduced from the relative strengths of two adjacent rotational lines in the Q-branch. Densities are obtained using the temperature measurement along with a measurement of the relative strength of a single rotational transition between the unknown flow density and a known reference density. Statistical measurement uncertainties are approximately 3% for velocities, 3% for temperatures, and 10% for densities. The measurements are in general agreement with the approximations of a one-dimensional supersonic flow model. In addition to flow parameter measurements, the properties of the coherent Raman process are also studied. Signal-to-noise ratios for two different coherent Raman processes (inverse Raman scattering and coherent Stokes Raman scattering) are about the same for the conditions in this experiment. The absolute signal strengths of both of these coherent Raman processes are found to be in rough agreement with those expected from theory. Finally, pressure broadening coefficients for three Q-branch rotational transitions (J = 8, 9, 10) in the v = 0 → 1 vibrational transition in N2 are measured and are in good agreement with other measurements recently reported.