The OpenRRI project is an open-source project offering a free and ready-to-use post-processing interferometer implementation based on the Range-Resolved Interferometry (RRI) technique. It is provided with convenient software functionalities as well as a guide for setting up and operating RRI interferometers. The main advantage of pseudo-heterodyne interferometers, such as those using the RRI approach, compared to classic homodyne or heterodyne signal processing, is their cost-effectiveness while maintaining the ability to perform relative displacement measurements with sub-nanometer precision. The interferometer uses a fiber-coupled laser diode with a wavelength of 1550 nm, which is sinusoidally modulated by laser injection current. At the fiber-coupled, collimated measurement head, the interference between the back reflection of the free-space beam reflected by the target mirror and the fiber tip reflection leads to an interferometric signal at the optical detector that can be evaluated. Additionally, the RRI concept, unlike most pseudo-heterodyne interferometric techniques, could simultaneously interrogate multiple interference sources within a single optical setup, for example if multiple semi-transparent glass surfaces are present. The interferometric signal acquired by the optical detector then can be demodulated using the software functionalities provided by the OpenRRI project. In the first part, this contribution focuses on the working principle of the RRI and the features provided by the OpenRRI system. The second part then presents a novel interferometric setup based on the RRI technique, enabling interferometric point-to-point measurement outside the optical axis.
This investigation proposes a method for absolute surface determination in a coordinate measuring machine (CMM) with a planar extension of 200 x 200 mm² which is based on the measurement of two spatial gradient fields. The gradient field data was obtained by measuring a test mirror in two equidistant shifted positions along two orthogonal axes while the reference mirror stayed in a steady position. The comparison of experimental data measured in an area of 192 × 192 mm² showed a small root-mean-square deviation of 5.3 nm between the reconstruction result and a regular measurement result. For an a priori estimation of the influence of experimental error sources on the reconstruction deviation, simulations of the measurement process were carried out. Alongside determining the optimal measurement strategy, the focus was investigating positional and orientational deviations of the test surface caused by the shifting motions. While the translational deviations have a subordinate effect, the simulated results show that small orientation deviations around the motion axes cause high reconstruction deviations. To eliminate the motion-induced share of the gradient fields orientation a separation from the topography intrinsic share, which has to remain part of the data, is necessary. This is achieved by the combination of the high-precision design of the mechanical shifting stage and the implementation of an additional boundary condition in the data processing using a least square algorithm.
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