The first part of the answer to “Why VISARs?” is that each alternative velocity-measuring technique has its own set of drawbacks and limitations. Moreover, VISARs can be applied to a very wide range of experimental conditions and specimen materials. Items that have been measured are peak shock pressures well into the Mbar range; specimens as diverse as metals, ceramics, plastics, composites, rocks, liquids, and explosives; motions due to plane dilatation shockwaves, shear waves, cylindrical waves, spherical waves, and combinations of such waves; specimen surfaces can be either specular or diffusely reflecting; there are no specimen temperature limitations with VISARs as long as light can be reflected from a defined specimen surface; and, velocity histories can be measured either at a specimen free surface or internally in the material if it is transparent, or at an interface between the specimen and a transparent window if it is not.
Another attribute of VISAR instrumentation is that the act of measuring does not change the quantity to be measured, at least where free surface (non-window) measurements are concerned. Also, VISARs provide very accurate measurements with 1 to 2 ns time resolution, and even much better with streak camera recording techniques. The mechanism of the instrumentation is based on very simple fundamental physics, requiring little or no calibration, and measurements are just as accurate during both shock loading and unloading. VISAR data containing two or more fringes can be reduced to an accuracy of better than 1% of the peak velocity. Finally, except for the Fixed-Cavity VISAR, the VISAR’s Velocity-Per-Fringe constant can be adjusted to optimize the VISAR‘s sensitivity for each measurement to be made.
This section discusses several VISAR instrumentation developments which are not considered to involve the design of the VISAR itself.
Windows: Even before the first VISAR in 1972, it was recognized that some equation-of-state experiments would benefit greatly from an ability to monitor the velocity history of an interface within the specimen, rather than the specimen free surface. For transparent materials the VISAR can do this if a reflective coating is deposited on one surface of a specimen, and that surface is then bonded to another piece of the same material, creating a reflective interface within the material. Even if the specimen material is opaque, an internal interface can be approximated by bonding a transparent “window” material of similar shock impedance onto the specimen.
The use of a window material in a VISAR shock experiment is not trivial, however, because the shockwave enters the window as the interface starts moving, changing its density and therefore its index of refraction as it moves. Because the laser beam is passing through a changing thickness of shocked window material, the changing index of refraction causes a shift in the laser wavelength which adds to the Doppler wavelength shift produced by the motion of the reflecting interface. In order to successfully use a window, one must know the amount of the wavelength shift to attribute to the index of refraction effect so it can be accounted for in the data reduction.
Several window materials have been calibrated for VISAR measurements, and their use is now common and easy. The calibrations are sufficiently precise to allow 1% accuracy of the final data. The most commonly used calibrated window materials include PMMA, fused silica, sapphire, and lithium fluoride. These materials span a large range of shock impedances to allow one to select a window with a shock impedance similar to the specimen of interest.
Fiber Optics: Until about 1991, VISAR laser light was guided to the specimen and then to the VISAR via mirrors and through transparent portholes in experiment chambers. Although extremely few if any laser-light related eye injuries resulted from this practice, the potential for such injuries cannot be disregarded. The resulting safety rules and regulations associated with laser beams passing through work areas can be detrimental to a laboratory’s efficiency.
The answer to this problem is to capture the laser beam into an optical fiber very close to the laser, and to transport the light all the way into the experiment chamber before allowing the light to emerge from the fiber. Similarly, after reflection from the VISAR specimen, some of the light is immediately collected into another fiber for transport to the VISAR interferometer. It has turned out that the use of fiber optics has not only increased safety, but also simplified the VISAR experiment setup, increased the usable depth-of-field over which the VISAR measures the specimen surface velocity, and eliminated depth-of-field errors which were formerly possible because of changing light paths through the interferometer during data collection.
VALYN VISAR Improvements: Several improvements to VISAR instrumentation by Valyn International should also be of interest. They include mechanisms for quickly and easily adjusting to the optimum circular Lissajous during shot setup; fiber optic probes providing an extra-small laser spot size on the specimen or extra-large depths-of-field; fiber optic beamsplitters which allow two VISARs to monitor the same specimen, thus eliminating ambiguities from shock jumps which cause lost fringes; convertible VISARs, with one Valyn model being convertible into another with the appropriate conversion kit; extended range options; Low-Noise Photomultiplier Units; Ultra-Narrow Bandpass Filters; and the Mini- and µ-Spot Size Flying Foil/VISAR Interface.
Another Valyn innovation is the use of an Alignment Monitoring Oscilloscope (AMO) to display the VISAR interferometer’s Lissajous alignment while the data-recording digitizing oscilloscope is simultaneously awaiting a trigger signal to record the shot data. The AMO prevents alignment drift from degrading the VISAR data by allowing the user to easily adjust the alignment right up to shot time. It also increases reliability by allowing all oscilloscope settings to be made, checked, and tested well before the hectic last-minute preparations which often precede a shot. The use of the AMO in this way is made possible by simple Valyn-produced signal splitters in the form of modified BNC “T”s, which allow both oscilloscopes to simultaneously observe the VISAR outputs without any signal degradation.
In the late 1990s, Valyn International developed and brought to market two remarkable new types of VISAR. One is the ORB, or optically Recorded Bull’s Eye VISAR. It allows researchers to measure extremely rapid events by using the blazing speed of streak cameras, which are up to 10 or more times faster than the photomultipliers traditionally used to record VISAR data. The second new VISAR type is the VALYN MULTI-BEAM VISAR. It can simultaneously measure the velocity histories of up to seven points on a specimen. The MULTI-BEAM VISAR provides the advantages of using several VISARs at once, but at a small fraction of the cost.
More recently, Valyn V.I.P. has perfected techniques for combining the features of the VALYN DOUBLE-DUTY ORB VISAR with those of the VALYN MULTI-BEAM VISAR, resulting in the VALYN MULTI-BEAM DOUBLE-DUTY ORB VISAR. This is an extremely powerful instrument for characterizing the flatness of very high-velocity flying foils, for example, by comparing the velocity histories of up to seven different points on the same foil.
Another saving is provided by Valyn’s Adjustable Multi-Beam Splitter and Fiber Coupler (AMBS/FC-2) which can be used to split the light from a single laser into as many as seven optical fibers. Therefore, Valyn’s Multi-Beam VISAR can simultaneously measure the velocity histories of up to seven points in a given experiment using only one laser and only one interferometer. What’s more, the amount of the laser light split off to each point is easily adjustable, so that more or less light can be assigned to each measured point as needed
Some of Valyn International’s innovations in VISAR instrumentatiion are covered by the following patents: No. 5,202,558, No. 5,481,359, No. 5,870,192, and No. 6,678, 447 B1.