A variable sensitivity displacement interferometer with application to wave propagation experiments

The present paper introduces a variable sensitivity displacement interferometer (VSDI) used to monitor both normal and in-plane particle displacements in wave propagation experiments. The general system consists of two interferometers working in tandem. Normally reflected light is interfered with each of two symmetrically diffracted light beams generated by the specimen rear surface. In cases where the surface motion simultaneously exhibits both in-plane and normal displacements, the fringes represent a linear combination of the longitudinal and transverse components of motion. Decoupling of the normal and in-plane displacement histories may be achieved through a linear combination of the two VSD1 records. Alternatively, it is always possible to decouple the components' of motion by combining a VSDI record with an independent measurement of either component. Moreover, it is shown that in the case of pure normal motion, the VSDI system functions as a desensitized normal displacement interferometer (DNDI). Similarly, in situations involving purely in-plane motion, the VSDI is shown to function as a desensitized transverse displacement interferometer (DTDI). The DNDI and DTDl fringe sensitivities are in general shown to depend on the angle 0 or equivalently, the frequency cr of a grating manufactured at the observation point and the order n of the diffracted beams. The variable sensitivity feature of the VSDI greatly desensitizes normal displacement measurements and is particularly well suited for wave propagation studies in which normal particle velocities in excess of 100 m/s are generated. Experimental results are presented which demonstrate the application of this technique to monitoring particle motion histories in plate impact recovery experiments.