Laser monitoring for the CMS ECAL

The Compact Muon Solenoid (CMS) detector at the LHC is equipped with a high precision lead tungstate crystal electromagnetic calorimeter (ECAL). To ensure the stability of the calorimetric response at the level of a few per mille, every channel of the detector is monitored with a laser system. This system enables corrections for fluctuations in the detector response with high precision, in particular the expected radiation induced changes in the crystal transparency. We describe the implementation of the laser monitoring system and report results from tests on the fully equipped supermodules of the ECAL. Specifically, we discuss results concerning the dynamics of crystal transparency change from dedicated irradiation studies in test beams.