  We study a dispersion-compensated high-finesse optical Fabry-Perot
microcavity under high-intensity cw pumping. The Kerr non-linearity in the
optical coatings causes a spontaneous four-wave mixing process, which leads to
the emission of time-correlated photon pairs. The photon frequencies are
shifted by $\pm 1$ free spectral range relative to the pump frequency. This
setup allows for constructing a photon-pair source with precisely adjustable
frequency difference between the emitted photons, which may have applications
in quantum communication.
