A Diode-Pumped DP2-447 Blue Laser for Monitoring CMS Lead Tungstate Crystal Calorimeter at the LHC

Monitoring the transparency of the lead tungstate crystals of the CMS electromagnetic calorimeter (ECAL) plays a crucial role in maintaining the ECAL energy resolution. To meet the stringent requirements on the light monitoring precision and stability a new commercial diode-pumped blue laser ("DP2-447") has been commissioned and installed at CERN for the 2012 operation of the CMS ECAL. The laser unit has a simple structure and is expected to be more reliable than the existing lamp-pumped lasers used by the monitoring system. The stability of critical quantities such as the intensity, width and timing, is better than that of the lamp-pumped lasers. The characteristics of the new blue laser will be elaborated. Its performance in-situ in CMS will be described and the prospects for improving the light monitoring precision will be discussed.