New heterodyne velocimeter and shock physics

Abstract
Heterodyne Velocimetry (or Photonic Doppler Velocimetry) has been used in detonics experiments for a few years now, mainly thanks to the recent evolution of the optoelectronics components. In its principle it is nothing else but a displacement interferometer, delivering beats versus time. Processing a sliding Fourier transform on the raw signal and deriving the resulted signal permit to obtain the velocity in function the time. The device requires a 1.55 μm Erbium fiber laser, single mode optical fibers, fast photodetectors and digitizers. We present a new heterodyne velocimeter setup with a second low-power frequency-tunable laser, acting as a local oscillator. The Doppler wave coming from the first laser and reflected by the moving target interferes with this shifted reference. This new device gives a convenient mean to match the static power level of the beats to the dynamics of the detector and is doubling the overall bandwidth (or velocity range) of the system. Two applications are presented: the first one deals with the classical measurement of free surface velocity on metallic shock loaded plates; in the second part, we present the velocity distribution of tin particles ejected under shock.