Alignment of CMS structures using silicon photo-detectors

An important part of the muon alignment system in CMS relates the positions of barrel and endcap muon chambers to the tracker body in a common reference frame. This system, called the Link Alignment System, uses a distributed network of Amorphous Silicon Position Detector sensors (ASPDs) connected by laser lines. This optical network is complemented by additional displacement and inclination sensors. We describe the Link Alignment System and its performance, with emphasis on the characteristics and performance of the silicon photo-detectors.