OpenRRI: theory and application of open-source interferometry

Abstract

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.