Method and apparatus for studying surface vibrations by moving speckle interferometer

Abstract

A method of studying a surface using an interferometer, in which there is relative motion between the surface and the interferometer, the motion having a total velocity Vwhich includes a transversal or traversing component Vand a longitudinal component V. The method comprises: directing an object beam of coherent light to a measurement position at the surface, whereby there is relative motion between the surface and the measurement position; arranging an array of detectors on the interferometer in a line extending generally in the transversal direction, the detectors being arranged to detect light rays with different angular directions, representing different sensitivity directions; producing a reference beam of coherent light which is coherent with the abject beam; combining the reference beam with the reflected object beam from the surface to produce a cross interference in the speckle pattern providing information about the relative motion of the surface and the interferometer; detecting the speckle pattern and the cross interference pattern with the detectors; determining which detector in the array has zero or minimum sensitivity to the total velocity Vof the motion, thereby identifying the detector with a sensitivity direction line that is normal to V; monitoring the temporal change in which of the detectors has zero or minimum sensitivity, thereby ascertaining the change in direction of Vover time, brought about by changes in V: and determining temporal changes in V.