Heterodyne velocimetry and detonics experiments

Heterodyne Velocimetry (or Photonic Doppler Velocimetry) has been used in detonics experiments for a few years now, mainly thanks to the recent evolution of telecom components. In its principle it is nothing else but a displacement interferometer, delivering beats versus time. A sliding Fourier transform processing on the raw signal thus allows to derive velocity versus time. The device is made up of a 1.55 mu m Erbium laser delivering 2 W (split into 4 channels), single-mode optical fibers, fast photodetectors and digitizers (8 GHz bandwidth, 20 GS/s sampling). To begin with, we present a new heterodyne velocimeter setup embedding a second low-power frequency-tunable laser (50 mW) acting as a local oscillator. Its frequency can be shifted, to make it higher than the main laser, up to the bandwidth of the digitizer (13 GHz soon). The Doppler wave coming from the first laser and reflected by the moving target interferes with this shifted reference, therefore doubling the overall bandwidth of the system. On top of enhancing the measurable velocity range, the existence of beats at static gives a convenient means to tune the power levels of the laser and match the electric signal to the dynamics of the detector. Finally, three 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. The third application relates to direct measurement of the velocity of detonation wave into nitromethane, by using immersed optical fibers.