  We investigate the role of excess quantum noise in type-II degenerate
parametric down conversion in a cavity with non-orthogonal polarization
eigenmodes. 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.
