We have developed a full multimode theory of a synchronously pumped type-I optical parametric oscillator. We calculate the output quantum fluctuations of the device and find that, in the degenerate case (coincident signal and idler set of frequencies), significant squeezing is obtained when one approaches threshold from below for a set of well-defined “supermodes,” or frequency combs, consisting of a coherent linear superposition of signal modes of different frequencies which are resonant in the cavity.

• Received 3 May 2006

DOI:https://doi.org/10.1103/PhysRevA.74.061801