Sinusoidal optical path length modulation of the reference or the measurement arm of an interferometer is a technique which is a fast alternative to white light or phase shifting interferometry. In this paper three different sensors using this periodical modulation are presented. In addition, signal processing algorithms based on Discrete Fourier Transform, Hilbert Transform and parameter estimation are analyzed. These algorithms are used to obtain measurement results which demonstrate the capabilities of the presented interferometric sensors.
We established an interferometric sensor for optical precision measurement of distance changes. A fiber-coupled micro-optical
probe with an integrated reference surface is mounted on a bending beam. A piezoelectric actuator deflects the
beam. Besides focus scanning this deflection modulates the optical path length of the measuring arm of the
interferometer, while the reference path remains unchanged. If the distance between optical probe and measuring object
changes, characteristic phase shifts of the corresponding interference signals appear. This enables us to achieve an
interferometric resolution. The problem of λ/2 ambiguity is solved by using the signal envelope resulting from confocal
focus scanning.
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.
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