Richard Nickerson

Abstract:

Frequency Scanning Interferometry (FSI) has been developed for robust precise distance measurements. We present the reconstruction of two and three dimensional geodetic grids, using simultaneous FSI measurements between grid nodes.The shape of the particle physics tracker inside the ATLAS experiment (currently under construction for the Large Hadron Collider at CERN) will be monitored using a geodetic grid of 842 remotely measured, fibre-coupled interferometers. The challenge is to make precise distance measurements in the hostile, high radiation environment and combine them to reconstruct node coordinates and hence the grid shape in quasi real time to a three dimensional precision of around 10 mu m.Two and three dimensional prototype grids with adjustable geometries have been measured, demonstrating shape reconstruction to a precision of around 1 mu m. Error propagation through these grids was studied with different reconstruction models. Grid redundancy allowed the agreement between software models and node coordinate reconstruction to be verified. (c) 2004 Elsevier Ltd. All rights reserved.

Abstract:

ATLAS is the largest particle detector under construction at CERN Geneva. Frequency scanning interferometry (FSI), also known as absolute distance interferometry, will be used to monitor shape changes of the SCT (semiconductor tracker), a particle tracker in the inaccessible, high radiation environment at the centre of ATLAS. Geodetic grids with several hundred fibre-coupled interferometers (30 mm to 1.5 m long) will be measured simultaneously. These lengths will be measured by tuning two lasers and comparing the resulting phase shifts in grid line interferometers, (GLIs) with phase shifts in a reference interferometer. The novel inexpensive GLI design uses diverging beams to reduce sensitivity to misalignment, albeit with weaker signals. One micrometre precision length measurements of grid lines will allow 10 μm precision tracker shape corrections to be fed into ATLAS particle tracking analysis. The technique was demonstrated by measuring a 400 mm interferometer to better than 400 nm and a 1195 mm interferometer to better than 250 nm. Precise measurements were possible, even with poor quality signals, using numerical analysis of thousands of intensity samples. Errors due to drifts in interferometer length were substantially reduced using two lasers tuned in opposite directions and the precision was further improved by linking measurements made at widely separated laser frequencies. © 2004 IOP Publishing Ltd.