  Optical parametric down-conversion (PDC) is a central tool in quantum optics
experiments. The number of collected down-converted modes greatly affects the
quality of the produced photon state. We use Silicon Photomultiplier (SiPM)
number-resolving detectors in order to directly observe the photon-number
distribution of a PDC source, and show its dependence on the number of
collected modes. Additionally, we show how the stimulated emission of photons
and the partition of photons into several modes determine the overall photon
number. We present a novel analytical model for the optical crosstalk effect in
SiPM detectors, and use it to analyze the results.
