White-light interferometers with polarizing optics for length measurements with an applicable zero-point detection

For absolute length and form measurements at a large working distance (>150 mm) two special interferometers, a tandem interferometer and a Michelson interferometer with achromatic polarizing optics are constructed. In our experiments, both consist of a combination of one low-coherence interferometer and one laser interferometer. For the low-coherence interferometer part, a simple white-light source with less than 100 µW optical power output is chosen. It bases upon a low-cost fiber-coupled near-infrared LED with a large spectral width (FWHM > 68 nm at 825 nm). The use of achromatic polarizing optics such as broadband polarizing beamsplitters and achromatic quarter-wave plates in the low-coherence interferometer parts increases the contrast level of the white-light signal fringe pattern to nearly 100%. Furthermore, the fringe pattern in a polarized interferometer has no subsignatures and is unique. Hence, different algorithms are tested for signal processing and automated zero-point detection of the white-light signature. The software for an automated measurement is tested in a standard room without thermal control and without damped oscillation. Therefore, in experiments with the tandem interferometer, it was possible to measure the zero-point position of a white-light signature with a peak-to-peak difference of 154 nm under uncontrolled environmental conditions without thermal stabilization. The white-light Michelson interferometer with polarizing achromatic optics allows zero-point detections with a standard deviation (mean value) of less than 15 nm. The drift is proved through measurement results.