The development of a laser system for use in the timing performance measurements of CMS HGCAL silicon modules

For optimal operations in the high radiation and pileup environment of the HL-LHC, the CMS-HGCAL requires precise timing information at the level of 30 ps (RMS) for a particle shower. The time measurement in silicon detector modules is performed using a per -channel time -of -arrival discriminator coupled with charge measurement to correct for the time -walk. The module design includes access holes in the PCB and in the sensor passivation to enable infrared laser light to be injected directly into the sensor cells. We present the calibration and timing -in of the system used to perform measurements.