Doppler-free saturated absorption velocimetry for low-density hypersonic flow diagnostics

Precise measurement of velocity for low-density hypersonic flow in high enthalpy shock tunnel requires the diagnostic technique to be non-intrusive to the flow field, have good signal-to-noise ratio, and have high spatial and temporal resolution, all preferably at low cost. We present the potential application of high-resolution Doppler-free saturated absorption spectroscopy as a point-measurement velocimetry technique by seeding the flow with rubidium chloride salt and exciting atomic rubidium in the hypersonic in a shock tunnel. Splitting the laser diode beam into counter-propagating pump and probe beams results in formation of narrow resonances on the Doppler-broadened profile, called Lamb dips, having a spectral half-width of approximately 6 MHz [1]. These narrow dips represent zero-velocity markers with respect to both beams within the distribution of Doppler width of approximately 600 MHz. The reduction in peak width is caused by only exciting those atoms which have zero velocity in the flow and which are excited by both of the oppositely oriented pump and probe beams where they cross one another. This makes for a much more precise zero-velocity reference point for absorption measurements, and the limitation of this signal to the region of beam overlap is designed to help overcome the drawback of standard path-integrated absorption techniques for velocity measurements. This paper presents a preliminary proof-of-concept of the experiment conducted in hypersonic freestream nozzle flow with this technique. The paper also presents some findings from numerical modelling for the potential use of saturated absorption spectroscopy of rubidium as a means of determining spatially resolved velocity within a small volume in a hypersonic nozzle-exit flow.