Performance and Measurements of the Light Monitoring System for CMS-ECAL from 2002 Test Beam Data

The electromagnetic calorimeter (ECAL) of CMS is constituted of nearly 75000 PbWO_4 crystals. Small variations in the light transmission of these crystals are expected in the harsh high luminosity conditions at LHC. This feature requires frequent optical transmission measurements over the full ECAL in order to obtain short-term corrections of the measured energies of physics events. Therefore an optical monitoring system was developed consisting of a laser source and an optical fibre distribution system. The validation of the light injection system with its final laser and read out electronics and the feasibility of determining the short-term corrections were the main goals of a dedicated test beam during summer 2002. Data were collected with a 400 crystal final production module, partially equipped with electronics (100 channels) for these tests. In this note we will present the monitor ing system and its performance obtained during this test beam period. The results on the stability of the system and results on irradiation tests with electrons and pions are given.