The upgraded laser calibration system of the ICARUS experiment at FNAL

The calibration of the ICARUS photo-detection system employs a low power laser diode at 405 nm. The laser pulses are delivered in front of the 360 large area PMTs with a custom optical system, that has been recently upgraded to calibrate all of them in a single run, by replacing an optical switch with a custom optical splitter. The distribution of the PMTs’ signal arrival time has a distribution with a resolution better than 1 ns, thus allowing a good determination of the absolute event timing.