Monitoring the stability of the ALEPH Vertex Detector

The ALEPH Silicon Vertex Detector features an optical fibre laser system to monitor its mechanical stability. The operating principle and the general performance of the laser system are described. The experience obtained duri ng 1997 and 1998 operations confirms the important role that such a system can have with respect to the detector alignment requirements. In particular , the laser system has been used to monitor short-term temperature-related effects and long-term movements. These results and a description of the las er-based alignment correction applied to the 1998 data are presented.