Performance of the Cms Ecal Laser Monitoring Source in the Test Beam

The CMS detector at LHC will be equipped with a high precision lead tungsten electromagnetic calorimeter, the CMS ECAL. In 2003, the laser source for the monitoring of the CMS ECAL was used in its final design for several months in a beam test of a CMS ECAL module at CERN. The laser source consists of two different laser systems providing laser pulses at four different wave lengths plus one back-up system. We report on the experience with this device and present results on the performance and stability of the system over a time period of several months. The achieved performance is discussed in view of the design goals for the CMS ECAL.