We study experimentally the spatial distribution of quantum noise in the twin beams produced by a type-II optical parametric oscillator operating in a confocal cavity above threshold. The measured intensity correlations are at the same time below the standard quantum limit and not uniformly distributed inside the beams. We show that this feature is an unambiguous evidence for the multimode and nonclassical character of the quantum state generated by the device.

• Received 16 September 2002

DOI:https://doi.org/10.1103/PhysRevA.67.023808