Phase-shift Fizeau interferometer in presence of vibration

In the context of this article we demonstrate a novel Fizeau interferometric system that copes with the presence of vibrations. Besides the conventional high spatial, but low temporal resolution detector system (the CCD camera) used in phase shifting interferometry, an additional high temporal, but low spatial resolution detector system was integrated, in order to measure the random phase shifts that are induced under the influence of the vibrations. The additional sensor consists of three photodiodes. The acquired analog signals enable the measurement of the occurring phase shifts at three non-collinear locations on the test surface. The resulting phase shifts at the three individual locations enable the determination of the random phase shifts over the entire image aperture. To avoid the smear phenomenon at very short exposure time, a beam shutter was integrated. Another alternative is to integrate a pulsed laser diode, for this purpose the concept of a wavelength meter is proposed. While the random oscillations of the test object are continuously measured, the CCD camera acquires several interferograms. In consequence, a phase shifting algorithm for random phase shifts was applied. In order to proof the validity of the new interferometer, a test surface of known topography was measured. The results of the measurements in presence of vibrations show very good concordance with the surface data given by the supplier. The analysis of the root mean square (RMS) over ten different measurement show a measurement repeatability of about 0.004 waves (approximately 2.5 nm for 632.8 nm laser wavelength).