  We report the efficient generation of high-gain parametric down-conversion,
including pump depletion, with pump powers as low as 100 $\mu$W (energies
$0.1$~$\mu$J/pulse) and conversion efficiencies up to 33\%. In our simple
configuration, the pump beam is tightly focused into a bulk periodically poled
lithium niobate crystal placed in free space. We also observe a change in the
photon number statistics for both the pump and down-converted beams as the pump
power increases to reach the depleted pump regime. The experimental results are
a clear signature of the interplay between the pump and the down-converted
beams in highly efficient parametric down-conversion sources.
