Fiber optic interferometric sensor based on mechanical oscillation

Abstract

For geometry measurement of high precision machined mechanical or optical workpieces a resolution in the nanometer range is generally required. This can be reached by interferometric principles. In addition, measurement at steep flanks can be achieved by optical systems with high numerical apertures. Unfortunately, a high NA is always accompanied by a small depth of focus leading to a very limited measuring range. A possible solution in this context is a so-called depth scan. We realized a pointwise measuring interferometric sensor and use a piezo driven bending beam for the depth scan. A micro-optical fiber probe with an integrated reference surface is mounted at the top of this beam. By use of a piezoelectric actuator driven close to the resonant frequency of several hundred Hertz the beam deflects with a few micrometers of amplitude. By this oscillation the optical path length of the measuring rays of the interferometer is modulated, while the reference path remains unchanged. This leads to an interference signal which shows characteristic changes in phase as the average distance between optical probe and measuring object changes.