Two-dimensional imaging velocity interferometry: Data analysis techniques

We describe data analysis procedures for an emerging interferometric technique for measuring target motion across a two-dimensional image at a moment in time, i.e., a snapshot 2D-VISAR. Conventional VISARs (velocity interferometer system for any reflector) are commonly used in shock physics to measure velocity history at a single point or many points across a line on target. These however are not recorded in two-dimensions and cannot be used with ultrashort pulsed illumination because the coherence length is smaller than the interferometer delay, preventing fringe formation. In our scheme, dual matched interferometers allow use of low- or incoherent illumination such as ultrashort laser pulses to freeze motion of target, allowing use of slow CCD imaging detectors. Quadrature phase recording and push-pull data analysis simultaneously produces an ordinary nonfringing reflectivity image and a fringing image. The latter is converted into a 2D-phase map which is proportional to target velocity. Example data on shocked crystalline [111] Si shows incipient features of ∼50 μm scale.