Willard F. Hemsing

Velocity sensing interferometer (VISAR) modification.

The VISAR (velocity interferometer system for any reflector) has become a common tool used in experiments where high surface velocities must be measured. A modification that uses previously wasted interferometer light to more than double output signals and to cancel noise is described. Laser power is used more efficiently, VISAR performance in the presence of intense target self-light is improved, and only two data signals are required instead of the usual three or four. Effects of changing light intensity and fringe visibility are eliminated using a novel detection system with a simplified solution for velocity.

VISAR: Line-imaging interferometer

This paper describes a Velocity Interferometer System for Any Reflector (VISAR) technique that extends velocity measurements from single points to a line. Single-frequency argon laser light was focused through a cylindrical lens to illuminate a line on a surface. The initially stationary flat surface was accelerated unevenly during the experiment. Motion produced a Doppler-shift of light reflected from the surface that was proportional to the velocity at each point. The Dopplershifted image of the illuminated line was focused from the surface through a pushpull VISAR interferometer where the light was split into four quadrature-coded images. When the surface accelerated the Doppler-shift caused the interference for each point on each line image to oscillate sinusoidally. Coherent fiber optic bundles transmitted images from the interferometer to an electronic streak camera for sweeping in time and recording on film. Data reduction combined the images to yield a continuous velocity and displacement history for all points on the surface that reflected sufficient light. The technique was demonsirated in an experiment where most of the surface was rapidly driven to a saddle shape by an exploding foil. Computer graphics were used to display the measured velocity history and to aid visualization of the surface motion. 1.

VISAR: LINE-IMAGING INTERFEROMETER

A line-imaging Velocity Interferometer System for Any Reflector (VISAR) was applied to measure velocity across the diameter of a metal plate explosively accelerated to 5.5 km/s. Amplified, single-frequency laser light was focused to illuminate a line on the metal surface. The lines image was focused through the interferometer to a streak camera that swept in time and recorded directly on film. During the experiment, the Doppler-shift caused motion of the interference fringes. Analysis of the digitized film record yielded a continuum of time-resolved velocity histories. Velocity gradients across the plate that first swept radially inward, then reflected outward, were clearly measured. Increased power provided by the laser amplifier greatly improved the signal-to-noise ratio compared to our previous line VISAR experiments.

VISAR: displacement-mode data reduction

A Velocity Interferometer System for Any Reflector (VISAR) is a laboratory tool that measures high velocities by continuously measuring the Doppler shift of laser light reflected from a moving surface. It produces lower output frequencies than a displacement interferometer in which Doppler-shifted laser light from a moving target is mixed with unshifted laser light. To obtain lower frequencies a VISAR employs a wide-angle Michelson interferometer with a time delay in one leg. Undelayed and delayed light rays are thus mixed to detect the relatively small difference between two Doppler shifts produced by accelerating motion at two slightly different velocities. In most VISAR data reduction programs the velocity is assumed to be proportional to the interferometer fringe count at any instant. This yields velocity details that are inaccurate over the interferometer delay time. In the examples of this paper the signal time resolution was shorter than the interferometer delay. The subject of this paper is a data reduction method that uses the displacement information in suitable VISAR signals to recover velocity features that occur during the interferometer delay. 1.

FULL-FIELD FABRY-PEROT INTERFEROMETER

This paper describes the use of a Fabry-Perot interferometer for simultaneously measuring velocity at many points on the surface of a shock-loaded solid. The method is based upon work reported by S. Gidon and G. Behar in 1986, but the data analysis has been improved by the application of image-processing techniques. Light from a pulsed single-frequency laser is focused onto a moving target and the returned Doppler-shifted image passed through a Fabry-Perot interferometer. Output of the interferometer is a set of fringes that are formed for specific combinations of wavelength and light angle. These fringes are recorded on film for subsequent analysis. Fringe position determines the velocity for each point on the target that forms a fringe. A method for determining the velocity as a function of both position and time will also be discussed. 5 refs., 6 figs.

VISAR: 2½ Minutes for Data Reduction

Starting with a pair of signals obtained by electronically subtracting photodetector outputs from any VISAR, it is possible to plot velocity vs time, velocity vs distance, and distance vs time in 21/2 min using an LSI-11/23 computer. This includes time for six iterations of time shift with the resulting polar plots and operator interactions. The velocity calculating algorithm and a flow chart of the computer program are presented.

Velocity interferometry of miniature flyer plates with subnanosecond time resolution

Velocity interferomety has been used to determine velocities of miniature laser-driven flyer plates with sub-nanosecond (100 ps typical) time resolution. Since laser-driven flyer plates can have acceleration 101 0 m/s2 and attain 95 of terminal velocity within 20 nanoseconds the acceleration rate and terminal velocity cannot easily and accurately be resolved by current velocity interferometry techinques because of limitiations in temporal resolution of the interferometer and/or spatial resolution for conventional methods of recording raw data. By selecting an interferometer and recording system that is appropriate for the experiment sub-nanosecond time resolution is possible.

Line-imaging Fabry-Perot interferometer

A method for measuring the velocity history of a line element on a shock-loaded solid has been demonstrated. Light from a single-frequency laser is focused through a cylindrical lens to a line on a moving target. The return Doppler-shifted image is passed through a Fabry-Perot interferometer. Because only specific combinations of incident light angle and frequency can pass through the interferometer the output is an incomplete image of the moving target appearing as a set of fringes. This image is focused onto an electronic streak camera and swept in time. The fringe pattern changes with time as the target surface moves allowing determination of velocity for each point on the target that forms a fringe. Because the velocity can only be measured at the fringe positions it is necessary to use an interpolating polynomial to obtain a continuous function of time and velocity along the sampled line. 1.

Comment on ‘‘VISAR analysis in the presence of large intensity changes: Application to the expanding ring’’ [Rev. Sci. Instrum. 60, 754 (1989)]

A published method to salvage VISAR data that is imperfect, due to variations of light intensity, is necessary only when the instrument has not been properly set up. Normally corrections for changing intensity are made automatically.

VISAR: Some Things You Should Know

VISAR performance can be improved while reducing cost, set up time, and required operator skill. Simple features that increase laser light efficiency, improve signal-to-noise ratio, reduce the number of data channels, greatly simplify data reduction, ease VISAR adjustments, and reduce laser wear and tear are discussed. These features were collected, developed, tried, and proven over the last few years and several hundred VISAR shots.