  The threshold of triply resonant optical parametric oscillation in a
semiconductor microcavity in the strong coupling regime is investigated.
Because of the third-order nature of the excitonic nonlinearity, a variety of
different behaviours is observed thanks to the interplay of parametric
oscillation and optical bistability effects. The behaviour of the signal
amplitude and of the quantum fluctuations in approaching the threshold has been
characterized as a function of the pump, signal and idler frequencies.
