10. Heterodyne Diffracted Beam Photonic Doppler Velocimeter (DPDV) for Pressure-Shear Shock Experiments

We present details on the design and validation of a heterodyne diffracted beam photonic Doppler velocimeter (DPDV) for pressure-shear plate impact (PSPI) shock experiments. The fiber optic interferometer collects symmetrically diffracted 1st order beams produced by a thin, specular, metallic grating deposited on the rear surface of the impacted target plate and separately interferes each of these beams with a reference beam of a slightly increased wavelength. The resulting interference signals contain an upshifted carrier signal with a constant frequency at zero particle velocity. Signal frequency content in recorded waveforms from PSPI experiments is extracted using a moving-window DFT algorithm and then linearly combined in a post- processing step to decouple and extract the normal and transverse velocity history of the rear target surface. The 0th order (normally reflected) beam can also be interfered in a separate heterodyne PDV configuration to obtain an additional, independent measurement of the normal particle velocity. An overview of the DPDV configuration is presented along with a discussion of the interferometer sensitivities to transverse and normal particle velocities. Results from a normal impact experiment conducted on Y-cut quartz are presented as experimental validation of the technique.