  As four-wave-mixing-based photon-pair sources mature, accurate modelling of
the photon-pair properties becomes important. Unlike spontaneous parametric
down-conversion, four-wave mixing is accompanied by a number of parasitic
effects such as nonlinear phase modulation. Currently, most modelling of
photon-pair states are analytic in nature, which limits the number and type of
effects that can be taken into account. In this work, we derive a complete,
dual-pump evolution equation for the joint amplitude of photon pairs, wherein
any desired effects can be included. We describe how to efficiently obtain
numerical solution to this equation using a split-step approach. Lastly, we
cover a few analytical solutions and compare two schemes for pure-photon
generation under three different parasitic effects. We show how one scheme is
highly sensitive to parasitic effects, while the other is very robust.
