Interferometry on diffuse surfaces in high‐velocity measurements

An interferometer is presented which is capable of measuring the free‐surface velocities and displacements of both specular and diffuse surfaces. The setup utilizes a previously used principle of producing a virtual image of one mirror at the same distance from the photodiode as the second mirror of the interferometer, albeit with considerable simplification. It is shown that use of a He–Ne laser of only 5‐mW power can produce high contrast displacement fringes from surfaces of materials with nonuniform microstructure, including composites. Substrates of carbon–carbon composites and polycrystalline alumina with nonuniform microstructure on the scale of 5–10 μm, and with peak velocities up to 150 m/s were considered. An experimental strategy which allows one to covert the optical setup to either a velocity or a displacement interferometer is also discussed. It is further shown that use of a fast photodiode and a high‐speed digitizer with a 5‐ps rise time provides a time resolution of 0.2 ns for recording the displacement fringes, and allows measuring free surface velocities up to 800 m/s. This is demonstrated by measuring such transient surface velocities with rise times of 1 ns on a specular Si surface. In all the experiments reported here, the surface velocities were produced by the reflection of a stress wave, which in turn was generated on the back surface of the substrate, using a Nd:YAG laser pulse.