  We study the joint spectral properties of photon pairs generated by
spontaneous parametric down-conversion in a one-dimensional nonlinear photonic
crystal in a collinear, degenerate, type-II geometry. We show that the photonic
crystal properties may be exploited to compensate for material dispersion and
obtain photon pairs that are nearly factorable, in principle, for arbitrary
materials and spectral regions, limited by the ability to fabricate the
nonlinear crystal with the required periodic variation in the refractive
indices for the ordinary and extraordinary waves.
