  We describe an experiment showing a spontaneous symmetry breaking phenomenon
between the intensities of the ordinary and extraordinary components of the
fundamental field in intracavity type-II harmonic generation. It is based on a
triply resonant dual cavity containing a type II phase matched $\chi^{(2)}$
crystal pumped at the fundamental frequency $\omega_0$. The pump beam generates
in the cavity a second harmonic mode at frequency $2\omega_0$ which acts as a
pump for frequency degenerate type II parametric down conversion. Under
operating conditions which are precisely symmetric with respect to the ordinary
and extraordinary components of the fundamental wave, we have observed a
breaking of the symmetry on the intensities of these two waves in agreement
with the theoretical predictions.
