Velocimetry using heterodyne techniques

Abstract

At LLNL, we have been using heterodyne techniques for the past year and a half to measure velocities up to several kilometers-per-second on different types of experiments. We assembled this diagnostic, which we call the Heterodyne Velocimeter (HetV), using commercially available products developed for the communications industry. We use a 1550 nm fiber laser and single mode fibers to deliver light to and from the target. The return Doppler-shifted light is mixed with the original laser light to generate a beat frequency proportional to the velocity. At a velocity of 1000 m/s, the beat signal has a frequency of 1.29 GHz. We record the beat signals directly onto fast digitizers. The maximum velocity is limited by the bandwidth of the electronics and the sampling rate of the digitizers. The record length is limited by the amount of memory contained in the digitizers. This paper describes our approach to measuring velocities with this technique and presents recent data obtained with the HetV.

Velocity spectra from explosively driven powders and balls

“…We perform many experiments in which we drive metal with high explosives and attempt to observe the surface motion with Photon Doppler Velocimetry (PDV) [1,2]. Often we find that the motion begins with a single velocity but that after some time, the single velocity disappears and a broad distribution of velocities takes its place.…”

Interrupted pulse electromagnetic expanding ring test for sheet metal

“…The beam that is used to illuminate the specimen is reflected and captured by a sensor. If the specimen is in motion the frequency of the reflected wave will be different than the original beam and the difference in the frequencies, the beat frequency, is then used to obtain the velocity of the specimen [22,23]. The PDV outputs a voltage versus time signal with a frequency equal to the beat frequency.…”

A Generally Applicable Laser Doppler Velocimetry

“…For this reason, PDV is regarded as an exceptionally robust system. However, the light beam back reflected from the probe was used as the reference light in the original device [7]. Some types of probe available in our lab cannot meet the requirement of back reflection in the measurement.…”

“…In 2005, Strand and his colleagues at LLNL developed a laser based velocimetry named PDV (Photonic Doppler Velocimeter) which employs mainly off-the-shelf components developed for the telecommunications industry. Developments in telecom technologies have enabled PDV the advantages of compact, flexible arrangement, easy-to-use and relatively inexpensive compared to the traditional Doppler laser velocimetry [7].…”