  We compute the output of multimode cavity-enhanced spontaneous parametric
down-conversion (SPDC) for sub-threshold, but otherwise arbitrary, gain. We
find analytic Bogoliubov transformations that allow us to calculate arbitrary
field correlation functions, including the second-order intensity correlation
function $G^{(2)}(T)$. The results show evidence of increased coherence due to
stimulated SPDC. We extend an earlier model [Lu and Ou, Phys. Rev. A, 62,
033804 (2000)] to arbitrary gain and finesse, and show the extension gives
accurate results in most scenarios. The results will allow simple, analytic
description of cavity-based nonclassical light sources for quantum networking,
quantum-enhanced sensing of atoms and generation of highly non-classical field
states
