We discuss the statistics of Poissonian and thermal
sources, and verify that as an approximation to a single-photon
source, both Poissonian and thermal sources can be used, when
the average number of photons per pulse is of the order of
0.1. An experimental characterization of the photon statistics
of a single-photon source obtained from the four-wave mixing
(FWM) process in optical fibers is presented. We employ an
on/off avalanche photodetector operating in the Geiger mode,
and use the maximum-likelihood estimation (MLE) method and
the expectation-maximization (EM) algorithm to reconstruct the
data. We find that the statistics of the source goes from thermal,
at a low signal power regime, to Poissonian, in a high signal
power regime.