  A recent report on the over saturation in SiPMs is puzzling. The
measurements, using a variety of SiPMs, show an excess in signal far beyond the
physical limit of the number of SiPM microcells without indication of an
ultimate saturation. In this work I propose a solution to this problem.
Different measurements and theoretical models of avalanche propagation indicate
that multiple simultaneous primary avalanches produce an ever narrower and
faster signal. This is because of a speed-up of effective avalanche propagation
processes. It means that SiPMs, operated at their saturation regime, should
become faster the more light they detect. Therefore, signal extraction methods
that use the amplitude of the signal should see an over saturation effect.
Measurements with a commercial SiPM illuminated with bright picosecond pulses
in the saturation regime demonstrate that indeed the rising edge of the SiPM
signal gets faster as the light pulses get brighter. A signal extractor based
on the amplitude shows a nonlinear behavior in comparison to an integrating
charge extractor. This supports the proposed solution for the over saturation
effect. Furthermore I show that this effect can already be seen with a
bandwidth of 300MHz, which means that it should be taken into account for fast
sampling experiments.
