Heterodyne transverse velocimetry for pressure-shear plate impact experiments

Abstract

Pressure-shear plate impact experiments have traditionally relied on free space beam interferometers to measure transverse and normal particle velocities at the rear surface of the target plate. Here, we present two different interferometry schemes that leverage heterodyne techniques, which enable the simultaneous measurement of normal and transverse velocities using short-time Fourier transforms. Both techniques rely on diffracted 1st order beams that are generated by a specular, metallic grating deposited on the rear surface of the target plate. The diffracted beam photonic Doppler velocimetry technique interferes each 1st order beam with a reference of slightly higher wavelength to create a constant carrier frequency at zero particle velocity. The second technique interferes the 1st order beams with each other and employs an acousto-optic frequency shifter on the +1st order beam to create a heterodyne transverse velocimeter. For both interferometer techniques, the 0th order beam is interfered in a heterodyne photonic Doppler velocimetry arrangement to obtain a measurement of the normal particle velocity. An overview of both configurations is presented along with a derivation of the interferometer sensitivities to transverse and normal particle velocities as well as design guidelines for the optical system. Results from normal impact experiments conducted on Y-cut quartz are presented as the experimental validation of the two proposed techniques.