The basic geometry of a velocimeter using the heterodyne technique uses fiber optics to transport light from the laser to a probe that launches light to the moving surface.
The heterodyne velocimeter is assembled from commercially available parts.
The four-channel heterodyne system is packaged into a portable case. From top to bottom are the fiber laser, the custom fiber/detector chassis, and the high-bandwidth digitizer.
(a) The beat wave form often shows amplitude fluctuations as the intensity of the light from the moving surface changes during the experiment. The individual beat cycles cannot be resolved with this time base. (b) Expanding the time base reveals the individual beat cycles. A measured period of corresponds to a velocity of .
(a) The beat wave form obtained from an explosively driven metal shows both amplitude and base line variations. (b) The spectrogram was calculated with 1024-point Fourier transform windows. (c) The Fourier transform code allows the velocity vs time data to be extracted from the spectrogram.
(a) The individual beat cycles can be seen in the early part of the beat wave forms on the cook-off experiment. (b) The velocity time histories span four decades.
(a) For the gas gun experiment, the light from the heterodyne probe passed through two LiF windows. (b) The resulting spectrogram shows two discrete frequencies. (c) The Fourier transform code allows each velocity profile to be extracted separately.
(a) The beat amplitude shows a slowly varying region as the elastic precursor arrives at the surface, and then a rapidly varying region once the actual shock arrives. (b) Fitting the shock arrival times to a sine wave yields residuals with a standard deviation among the six probes.
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