Hydrostatic Levelling Sensors Based on Extrinsic Fibre Fabry-Perot Interferometer Technology

With the high luminosity upgrade of the LargeHadron Collider, the alignment monitoring systems inthe Long Straight Sections around ATLAS and CMSwill expect annual radiation doses of 100 kGy. Hydro-static levelling systems will be deployed for verticaland tilt monitoring. To resist to these high radiationdoses, the electronics have to be placed up to 200 mapart from the sensors. A candidate for this task isa hydrostatic levelling sensor based on extrinsic fibreFabry-Perot interferometer technology. Any variationin the level of the reference surface is detected as avariation in the optical path length between the fibreprobe situated in the sensor head and the mirror thatis submerged in the liquid. In laboratory conditionsthe prototype was successfully operated in a range of10 mm with a resolution of 5 nm. Thermal and atmo-spheric influences as well as the internal drift of the in-terferometer are compensated by the differential mea-surement carried out with a single laser source. Thesensing fibre can be several kilometres long and thesystem allows signal processing up to 10 kHz. Thesedesign parameters have been tested in two field tests,one at the Laboratoire Souterrain `a Bas Bruit for asensor validation test. The other one at CERN in com-parison with the currently used capacitive hydrostaticlevelling sensor type. The results show that the tech-nology can be applied for future hydrostatic levellingsensors. The approach to an absolute measurementsolution is still under investigation.