Engineers and more specifically surveyors frequently choose optical interferometry techniques for high precision metrology applications, as they offer a very good resolution and traceability of the measured distances. Nowadays, multiple interferometric solutions and interferometer configurations are available on the market. Part of them, typically single wavelength ones, offer a relative displacement measurement capability, with the advantage of a very high (even picometre) resolution, but are impractical in applications where distance tracking is needed after the reboot of the interferometer unit. Other solutions are absolute interferometers, usually based on Frequency Sweeping Interferometry (FSI), which offer true distance measurement. For such solutions, the measurement accuracy is worse than for relative ones and is of the order of the micrometer, mainly due to vibrations causing an optical path length change during the laser scan. At CERN, a range of new alignment solutions using Fourier based FSI is under development and qualification. This FSI technique allows the simultaneous measurements of absolute distances to multiple targets and is less sensitive to variations of intensity from the reflected optical signal, hence predisposing it for the harsh environment of accelerators. One advantage of this measuring technique is its simplicity to distribute hundreds channels over large-scale alignment installations. Its main disadvantage comes from potential variations of distance during the laser sweep. Such variations are typically caused by vibration or displacements of the reflector.
Multiple tests were performed to characterize the impact of reflective target vibration on the measurement uncertainty of Fourier-based FSI solutions. This paper describes the results of these tests and the lessons learnt.
High radiation levels, ultra-high vacuum, cryogenic temperatures of the measured components and high electro-magnetic noise push accelerator surveyors to look for more robust and accurate solutions of alignment. In the framework of the High-Luminosity LHC project at CERN, a range of new and cost-optimized solutions using Fourier analysis based Frequency Sweeping Interferometry (FSI), are under development. The technique allows the measurement of absolute distances to multiple targets simultaneously and is less sensitive to reflected optical signal intensity variations. The advantage with respect to classical interferometers (based on the detection of interference signal phase-change and sensitive to light quality) is that even weak interference beat frequency peaks can be easily retrieved from the Fourier spectrum without significant degradation of measurement precision. Moreover, the detectability of light reflected from different types of surfaces (high and low reflectance ones) makes it possible to develop a new family of simple, universal and robust micrometric sensors for harsh environments, like particle accelerators. An application of this novel method is the monitoring of the position of magnet and crab cavity cold masses inside their cryostats. For this purpose, specially designed divergent beam FSI vacuum optics and low cost glass ball reflectors are being tested and will be used in the HLLHC project. A new family of simple and cost-optimized, single and multi-reflection sensors (levelling, inclinometer, distance) is under development in the same coordinated effort. This paper describes such a measurement system, the sensor design approach, the results obtained and their final use in the LHC accelerator.