Measuring longitudinal displacements using laser beam diffraction changes near the focal point

Abstract

A nanometer displacement probe is created by using a knife edge, a two-part-divided photodiode, and high quality lenses. The laser beam, of wavelength 780 nm, is focused on the sample surface, where the image formed of the beam is read by the photodiode through the same optical path. The displacement signal is found to originate from the diffracted image change at the surface. A dynamic range of 2 μm and a minimal resolution of 1 nm for a lens of f=3.6 mm are obtained. The difference current, when normalized by the summation current, becomes a function of displacement and is independent of the sample surface reflectivity. As an application, a piezoceramic piece having a mean roughness of 1.03 μm was used. The frequency characteristics measured by this mechanical displacement method and the conventional electric oscillation measurement method were compared and found to be in good agreement.