Recent advances over the last five years in high-speed digitizing oscilloscopes and high-bandwidth photodiodes, driven
primarily by the telecommunications industry, have enabled the development of a new type of interferometer for
measuring high velocities, such as those found in detonics experiments.
The heterodyne velocimeter can be visualized as a fiber-based Michelson interferometer. The beam from a single-mode
fiber laser at 1550 nm is passed through a circulator, acting to separate bi-directional light. The beam is then reflected via
free-space optics from the surface of interest, and then focused back into the same fiber. This reflected light is mixed
with an approximately equal amount of non-reflected light, and the resulting interference is recorded using a high-bandwidth
photodiode and oscilloscope. In contrast to more traditional velocimetry techniques such as VISAR, only a
single data channel is required per probe.
The uses of heterodyne velocimetry have, to date, been primarily in the multi-microsecond time regime, i.e. explosively driven
metal plates. In this paper, we present a four-channel, ultra-high bandwidth system designed for use in the sub-microsecond
time regime, and present the results obtained from laser-driven flyer plates traveling in excess of 3 km s-1.
We have developed analysis software suited to use in this time regime, where a relatively small displacement is recorded.
The original heterodyne velocimeter relied on back-reflectance from the probe to obtain the non-reflected light. This
limits both the flexibility of the system and the efficiency of the probes. We have overcome this issue by introducing a
beam splitter into the system prior to the circulator. This allows the probing system to be designed for maximum
efficiency, and we are then able to tune the non-reflected light on a shot-to-shot basis.