Construction of a fast laser-based calibration system for the Harp TOF counters wall

A calibration and monitoring system for the Harp experiment scintillator-based time of flight system has been developed, by using an Nd-YAG laser with passive Q-switch and active/passive mode-locking and a custom made laser light injection system based on it bundle of IR monomode optical fibers. For the laser pulse timing a novel ultrafast InGaAs MSM photodiode, with 30 ps risetime, has been used. Experience over a several months data taking period in 2001 and 2002 shows that drifts in timing down to about 70 ps can be traced.