The effects of an intracavity photonic crystal in a multimode optical
parametric oscillator are studied, with a special focus on quantum fluctuations.
The capability to either stimulate or inhibit the spatial instability, lowering or increasing the
parametric threshold, allows to control the intensity fluctuations and correlations.
A significative quantum noise reduction and an
increase of the range of squeezed quadratures are found above threshold where spatial
Einstein-Podolsky-Rosen entanglement and inseparability are found.