Implementation of a Software Feedback Control for the CMS Monitoring Lasers

Light monitoring will play a crucial role in maintaining energy resolution for the CMS lead tungstate crystal calorimeter in situ at LHC. Since 2003, a laser based monitoring system in its final design has been installed and used in beam tests at CERN. While the stability of the laser pulse energy and FWHM width, measured in 24 hours, is at 3% level, a long term degradation and a drift of the laser pulse center timing at 2 ns/day were observed. The degradation and drift were caused by the aging of the DC Kr lamp used to pump the Nd:YLF laser, and would affect the monitoring precision. This paper presents the design and implementation of a software feedback control which stabilizes laser pulse energy, width and timing by trimming the Nd:YLF laser pumping current. For laser runs lasted for more than 650 hours a stability of pulse energy and FWHM width at 3% level and a pulse timing jitter at 2 ns have been achieved when the laser pulse center timing is used as the feedback parameter.