  We present a comprehensive model for predicting the full performance of a
second harmonic generationoptical parametric amplification system that aims at
enhancing the temporal contrast of laser pulses. The model simultaneously takes
into account all the main parameters at play in the system such as the group
velocity mismatch, the beam divergence, the spectral content, the pump
depletion, and the length of the nonlinear crystals. We monitor the influence
of the initial parameters of the input pulse and the interdependence of the two
related non-linear processes on the performance of the system and show its
optimum configuration. The influence of the initial beam divergence on the
spectral and the temporal characteristics of the generated pulse is discussed.
In addition, we show that using a crystal slightly longer than the optimum
length and introducing small delay between the seed and the pump ensures
maximum efficiency and compensates for the spectral shift in the optical
parametric amplification stage in case of chirped input pulse. As an example,
calculations for bandwidth transform limited and chirped pulses of
sub-picosecond duration in beta barium borate crystal are presented.
