  High gain parametric amplifier with a single-pass pulsed pump is known to
generate broadband twin photon fields that are entangled in amplitude and phase
but have complicated spectral correlation. Fortunately, they can be decomposed
into independent temporal modes. But the common treatment of parametric
interaction Hamiltonian does not consider the issue of time ordering problem of
interaction Hamiltonian and thus leads to incorrect conclusion that the mode
structure and the temporal mode functions do not change as the gain increases.
In this paper, we use an approach that is usually employed for treating
nonlinear interferometers and avoids the time ordering issue. This allows us to
derive an evolution equation in differential-integral form. Numerical solutions
for high gain situation indicate a gain-dependent mode structure that has its
mode distributions changed and mode functions broadened as the gain increases.
