An investigation of frequency scanning interferometery for the alignment of the ATLAS semiconductor tracker

The relative alignment of the silicon detector modules of the ATLAS semiconductor tracker will need remote monitoring during operation, within a high radiation environment. A geodetic grid of distance measurement fibre-coupled interferometers will monitor changes in the shape of the support structure. Eight hundred fibre-coupled grid line interferometers (GLIs) will be compared simultaneously to a stable, evacuated reference interferometer using Frequency Scanning Interferometry (FSI). The GLIs, (from 70 mm to 1400mm long, with pW level return signals) must be measured to a precision of 1 micron, to reconstruct the grid shape, in three dimensions, to a precision of 10 microns. In this work two important limitations were overcome: 1. Inflated errors due to relative interferometer drift were significantly reduced using two lasers scanned in opposite directions. 2. The fine tuning range was effectively extended by linking the phase information in two 30 GHz fine tuning subscans, separated by a 3.5 THz coarse tuning interval. A demonstration system was built using tunable laser diodes operating at wavelengths close to 836 nm. Several different fibre coupled GLIs were built. Each was measured against an invar reference interferometer sharing the same laboratory air. The 400 mm GLI was measured to a (one standard deviation) precision of 120 nm and a 1195 nm GLI to a precision of 215 nm. Decreasing the GLI signal was not found to significantly degrade the measurement precision. Spurious reflections and vibrations were separately introduced to degrade the measurements. The errors were found to increase, with errors larger than 4 parts per million, observed for vibrations of 400 nm peak to peak amplitude. Suggestions are given for reducing remaining errors. Further investigations into the effects of vibrations and spurious reflections are recommended.