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.
Nanometre accuracy and resolution metrology over large areas is becoming more and more a necessity for the progress of precision and especially for nano manufacturing. In recent years, the TU Ilmenau has succeeded in developing the scientific-technical basics of new ultra-high precision, so called nanopositioning and nanomeasuring machines. In further development of the first 25 mm machine, known as NMM-1 from SIOS Meßtechnik GmbH, we have developed and built new machines having measuring ranges of 200 mm x 200 mm x 25 mm at a resolution of 20 pm and enable measuring reproducibility of up to 80 pm. This means a relative resolution of 10 decades. The enormous accuracy is only made possible by the consistent application of error-minimum measurement principles, highly accurate interferometric measurement technology in combination with highly developed measurement signal processing and comprehensive error correction algorithms. The probing of the measurement objects can optionally be carried out with the aid of precision optical, interference-optical, tactile or atomic force sensors. A complex 3D measurement uncertainty model is used for error analysis. The high performance could be demonstrated as an example in step height measurements with a reproducibility of only 73 pm. The achieved resolution of 10-10 also presents new challenges for the frequency stability of the He-Ne lasers used. Here, the approach of direct coupling of the lasers to a phase-stabilized optical frequency comb synchronized with an atomic clock is pursued. The frequency stability is thus limited by the relative stability of the RFreference to better than 4•10-12 (1s).
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