Laser monitoring system for the CMS lead tungstate crystal calorimeter

We report on the multiple wavelength laser monitoring system designed for the compact muon solenoid (CMS) lead tungstate crystal calorimeter. Results are presented for the test-beam performance of the system designed to achieve ⩽0.2% relative optical transmittance inter-calibration for 75 848 lead tungstate crystals. The system cycles continuously over the calorimeter to follow each crystal's evolution under the irradiation and recovery periods foreseen during operation at the LHC.