Subthreshold optical parametric oscillator with nonorthogonal polarization eigenmodes

We study the behavior of a type-II degenerate parametric amplifier in a cavity with nonorthogonal polarization eigenmodes. The mode nonorthogonality is achieved by introducing circular birefringence and linear dichroism. We use a scattering matrix formalism to investigate the role of excess quantum noise in such a device. Since only two modes are involved we are able to derive an analytical expression for the twin-photon generation rate measured outside the cavity as a function of the degree of mode nonorthogonality. Contrary to recent claims we conclude that there is no evidence of excess quantum noise for a parametric amplifier working so far below threshold that spontaneous processes dominate. Using the same scattering matrix formalism we also investigate the output spectrum of the amplifier near the threshold of parametric oscillation. We find optical band structures very similar to those known for passive ring cavities. These optical band structures are studied as a function of mode nonorthogonality and mirror reflectivity.