Abstract
Optical velocimetry is limited to measuring the component of the target velocity along the axis of the optical beam, thereby allowing a laterally moving tilted surface to approach a probe undetected. We are not discussing the detection of the lateral motion, but rather the detection of material approaching the probe due to lateral motion of a surface that is not perpendicular to the beam. This motion is not measured in optical velocimetry, and consequentially, integrating the velocity will in general give an incorrect position. We will present three approaches to overcome this limitation: Tilted wave-front interferometry, which maps time of flight into fringe displacement; pulse bursts for which we measure the change in the average arrival time of a burst, and amplitude modulation interferometry, in which a change in path length shows up as a change in the phase of the modulation. All three of these have the potential to be integrated with existing velocimetry probes for simultaneous velocity and displacement measurements. We will also report on initial tests of these approaches.
Highlights
Briggs et al, and Dolan et al, [1,2,3] showed in 2009 that Photon Doppler optical velocimetry (PDV) will not measure the full approach of material toward the probe, even though PDV is a displacement interferometer, but only measures the portion of the motion arising from the component of velocity along the beam
Since we must roughen the surface to get light in the non-specular direction, the phase information needed is scrambled by random additions of 2π
We propose three methods for achieving this performance, all of which share some variation on creating an effective wavelength greater than the surface roughness of a typical target, typically ~ 0.05 mm. used
Summary
Dolan et al, [1,2,3] showed in 2009 that Photon Doppler optical velocimetry (PDV) will not measure the full approach of material toward the probe, even though PDV is a displacement interferometer, but only measures the portion of the motion arising from the component of velocity along the beam. 2. To measure the full motion, we need an effective wavelength large compared to the surface roughness If one could measure light from a smooth surface from a non-specular direction (the dots in Fig. 5), the approach of the lateral motion shown would be measured.
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