  Linear discrete diffraction of light within a waveguide array allows control
over the longitudinal spatial profile of light. We propose a method of using
this control to effectively modulate the nonlinearity in a spontaneous four
wave mixing system. This allows the removal spurious frequency correlations
between the generated signal and idler photon pairs. Weaving our method into a
recipe for waveguide design, we demonstrate a spectral purity improvement from
P = 0.78 to near saturation at P = 0.97, for a simple silicon-on-insulator
geometry.
