We calculate the influence of excess noise on parametric down conversion in
an unstable optical parametric oscillator (OPO), using a quantum quasimode
description. We find a strongly enhanced pair photon generation rate below
threshold as compared to a conventional stable cavity setup of comparable
gain and loss. In addition, the oscillation threshold is lowered due to the
influence of the excess noise and the squeezing properties of the emitted
light are significantly changed. In general, the maximal quantum-noise supp
ression in one quadrature component is reduced, which poses strong limitati
ons for the practical usefulness of a geometrically unstable OPO source. Th
e analytical results from our quasimode description are in good agreement w
ith numerical simulations using a positive-P representation of the field in
mode space and in position space.