Heterodyne diffracted beam photonic Doppler velocimeter (DPDV) for measurement of transverse and normal particle velocities in pressure-shear plate impact experiments

Abstract

Pressure-shear plate impact (PSPI) experiments have traditionally relied on free space beam interferometers such as the transverse displacement interferometer (TDI) and normal displacement interferometer (NDI) or normal velocity interferometer (NVI), to measure transverse and normal velocities at the rear surface of the target plate [1]. Alternative interferometer schemes feature a dual beam VISAR arrangement [2] and a recently developed all fiber-optic TDI-NDI/PDV configuration [3]. Here, we present a heterodyne diffracted beam PDV (DPDV) which interferes a pair of symmetrically diffracted 1st order beams produced by a thin, specular, metallic grating deposited on the rear surface of the target plate. Each beam is collected by a fiber-optic probe and directed to interfere with a reference beam of a slightly increased wavelength to create an upshifted carrier signal frequency at zero particle velocity. Signal frequencies are extracted from the two fringe records using a moving-window DFT algorithm and then linearly combined in a post processing step to decouple the normal and transverse velocities. The 0th order beam can also be interfered in a heterodyne PDV to obtain an additional independent measurement of the normal particle velocity [4]. An overview of the DPDV configuration is presented along with a derivation of the interferometer sensitivities to transverse and normal particle velocities. Results from a normal impact experiment conducted on y-cut α-quartz are presented as experimental validation.