A Simple Dual-Beam Time-Multiplexed Photon Doppler Velocimeter for Pressure-Shear Plate Impact Experiments

Pressure-shear plate impact experiments generate normal and transverse particle velocities during high strain rate deformations. Traditionally, freespace lenscoupled tabletop laser interferometry techniques are used together with diffraction gratings to interrogate the evolving velocity vector at the back face of the target plate. Recently, fiberoptic velocimetry (photon Doppler velocimetry or PDV) has become commonplace for measuring normal particle velocities above 200m/sec. In this work, we demonstrate transverse velocity detection using a modified PDV system where we subtract the measured normal velocity history from a concurrent velocity history measured at a canted angle to the target surface to obtain the transverse velocity component. This modified system is time-multiplexed to reduce the number of components, and uses an erbium doped fiber amplifier (EDFA) to boost the angled signal intensity while maintaining low noise. The system operates as a heterodyne interferometer, but features a frequency upshifted reference leg to improve data analysis at the particle velocities expected in the experiment. We demonstrate by direct comparison that this inexpensive and simple approach is as effective as traditional grating methods.