High-sensitivity surface-profile measurements by heterodyne white-light interferometer

A tandem white-light interferometer has been developed, based on the heterodyne technique, for highly accurate and sensitive surface profiling of a three-dimensional (3-D) object. Two acousto-optic modulators (AOMs) and two spherical reflecting mirrors are used in a Michelson interferometer to provide the optical frequency shift for white light. By using this technique in the tandem interferometer, the profiles of mirrorlike and diffusing surfaces of 3-D objects are measured with heterodyne signals of 200 kHz. The optical path difference of the interferometer can be stabilized by simultaneously using coherent and incoherent light. The experimental results show a 27-fold improvement in the stability of the interferometer and the possibility of achieving high accuracy of several tens of nanometers.