Velocity versus Time Profile of Shockwave through Solid

Velocity interferometer system for any reflector (VISAR) is a time-resolved velocity measurement system that uses laser

interferometry Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...

to measure the surface velocity of solids moving at high speeds. For solids experiencing high velocity impact or explosive conditions, VISAR plots the free-surface velocity against time to show the

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...

profile of a material (See Figure). VISAR is a useful tool in determining the pressure-density relationship of a material known as the Rankine-Hugoniot conditions or simply the "Hugoniot". In recent years another time-resolved velocity measurement tool called laser Doppler velocimetry has achieved popularity in the shock physics community as an adjunct or replacement for VISAR. This device is essentially a displacement

interferometer Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...

of the normal

Michelson Michelson may refer to: * Michelson (surname), people with the given name or surname * 27758 Michelson discovered in 1991 * Michelson (crater) on the moon * Michelson-Gale-Pearson experiment, science * Michelson interferometer, most common config ...

variety. As such it requires extremely fast data acquisition devices (digital oscilloscopes with bandwidths of 10 GHz or higher) and is limited in the range of velocities it can cover. As the surface moves, the reflected light interferes with itself and sinusoidal 'fringes' in light intensity are produced and recorded. A cycle of light intensity or fringe count indicates a displacement of the surface corresponding to one wavelength of the light. The rate at which these fringes occur is thus proportional to the velocity of the surface. To derive a velocity history the fringe (displacement) data must be differentiated with respect to time, usually by

Fourier analysis In mathematics, Fourier analysis () is the study of the way general functions may be represented or approximated by sums of simpler trigonometric functions. Fourier analysis grew from the study of Fourier series, and is named after Josep ...

. This differentiation or FA step inevitably reduces the time resolution and accuracy of the velocity history. The VISAR on the other hand is configured to 'optically differentiate' so that the light intensity variation due to interference varies sinusoidally with the velocity of the surface not the displacement. Also called a 'Delay-Leg Interferometer', it remains the best and most accurate method for recording the velocity history of fast moving surfaces. The original VISARs were built at the National Laboratories and had free-space beams on optical tables with discrete optical components such as beam-splitting pellicles, mirrors, quarter wave delay plates, glass etalons, high voltage photo-multiplier tubes,

argon ion laser An ion laser is a gas laser that uses an ionized gas as its lasing medium. Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry–Pérot resonator. Unlike helium–neon lasers, the e ...

s and so on. Modern versions such as the Mark IV-3000 from Martin, Froeschner & Associates (mfaoptics.com) implement the same optical arrangement entirely in single-mode optical fibre with all solid state telecomm components such as InGaAs photodiodes, Er doped fibre amplifiers (EDFAs) and extremely high purity (<2 kHz linewidth) lasers. Velocity resolution down to 0.01 m/s has been demonstrated with time resolution <1ns.

References

Further reading