  Light beams with orbital angular momentum (OAM) are convenient carriers of
quantum information. They can be also used for imparting rotational motion to
particles and provide high resolution in imaging. Due to the conservation of
OAM in parametric down-conversion (PDC), signal and idler photons generated at
low gain have perfectly anti-correlated OAM values. It is interesting to study
the OAM properties of high-gain PDC, where the same OAM modes can be populated
with large, but correlated, numbers of photons. Here we investigate the OAM
spectrum of high-gain PDC and show that the OAM mode content can be controlled
by varying the pump power and the configuration of the source. In our
experiment, we use a source consisting of two nonlinear crystals separated by
an air gap. We discuss the OAM properties of PDC radiation emitted by this
source and suggest possible modifications.
