The CMS ECAL Laser Monitoring System

The high precision lead tungstate electromagnetic calorimeter of the CMS detector has started to operate in 2008. To ensure the stability of the calorimetric response at a level of a few per mille, every channel of the detector is monitored with a laser monitoring system. This system allows to measure fluctuations in the detector response with high precision, in particular the expected radiation induced changes of the crystal transparency which are on the level of several per cent at nominal luminosity. We briefly review the layout of the system and give an overview of test beam results obtained during the design and construction phase of the CMS ECAL. To allow continuous monitoring of all the crystals the interplay between the CMS DAQ, the ECAL DAQ as well as the laser source has to be carefully tuned. The implementation and experience with this system during the 2008 commissioning and operation of CMS is described.