  The calibration of the ICARUS photo-detection system is based on a low power
laser diode at 405 nm. Laser pulses arrive to one optical switch and then are
sent to 36 UHV flanges, by 20 meters long optical patches. Light is then
delivered to the ten PMTs connected to a single flange, by 7m long injection
optical patches. Extensive tests of the used components and care in the design
of the optical system have guaranteed to each PMT a sizeable calibration signal
with minimal distortion, with respect to the original one. Gain equalization of
PMTs has reached a 1 per cent resolution. In this procedure data from
background photons were also used. The distribution of the PMTs signal arrival
time has a distribution with a resolution less than 1 ns, thus allowing a good
determination of the absolute event timing. The status of the laser calibration
system with its possible upgrades will be reported.
