  We have developed a new laser-based time calibration system for highly
segmented scintillator counters like the MEG II pixelated Timing Counter (pTC),
consisting of 512-centimeter scale scintillator counters read out by silicon
photomultipliers (SiPMs). It is difficult to apply previous laser-based
calibration methods for conventional meter-scale Time-Of-Flight detectors to
the MEG II pTC from the implementation and the accuracy points of view. This
paper presents a new laser-based time calibration system which can overcome
such difficulties. A laser pulse is split into each scintillator counter via
several optical components so that we can directly measure the time offset of
each counter relative to the laser-emitted time. We carefully tested all the
components and procedures prior to the actual operation. The laser system was
installed into the pTC and thoroughly tested under the real experimental
condition. The system showed good stability and being sensitive to any change
of timing larger than ~10 ps. Moreover, it showed an uncertainty of 48 ps in
the determination of the time offsets, which meets our requirements. The new
method provides an example of the implementation of a precise timing alignment
for the new type of detectors enabled by the advance of SiPM technology.
