KETEK's SiPM data sheets detail key performance parameters of their silicon photomultipliers - a key parameter is linearity which is explained in more detail below.
SiPMs exhibit linear response to incident light over a limited signal range. The two main parameters causing saturation are the number of available microcells and the recovery time of those microcells.
The plots below show number of detected photons versus the number of impinging photons over a range of signal levels for a short (70ps) and a relatively long (1µs) pulse.
In the case of the 70ps duration the pulse is significantly shorter than the recovery time and so the only limit will be the number of microcells - as the light level exceeds the available detection sites (microcells) so a non-linearity emerges.
For the 1µs pulse length, it is also important that the cells recover as fast as possible again. Therefore in terms of linearity, PM3315-WB (left) shows linear behaviour over a wider range compared to PM3325-WB, as it has both a higher number of cells (38800 vs. 13920) and also a shorter cell recovery time (13 ns vs. 40 ns).
As the number of incident photons approaches ~70% of the saturation level the SiPM begins to demonstrate non-linearity as photons are detected by partially recharged microcells. These partially charged microcells will therefore release a lower charge per photon.
It should also be noted that larger microcells achieve a higher microcell fill factor and therefore higher PDE. Higher PDE affects the linear range as more of the incident photons will be detected. This is demonstrated by the PM3325-WB exhibiting a lower roll-off than the PM3315-WB.