  We present a new method for the spectral characterization of pulsed twin beam
sources in the high gain regime, using cascaded stimulated emission. We show an
implementation of this method for a ppKTP spontaneous parametric
down-conversion source generating up to 60 photon pairs per pulse, and
demonstrate excellent agreement between our experiments and our theory. This
work enables the complete and accurate experimental characterization of high
gain effects in parametric down conversion, including self and cross-phase
modulation. Moreover, our theory allows the exploration of designs with the
goal of improving the specifications of twin beam sources for application in
quantum information, computation, sampling, and metrology.
