  We report a strong experimental evidence of the optical anisotropy in a
CdTe-based microcavity: the polarization of light is pinned to one of the
crystallographic axes independently on the polarization of the excitation. The
polarization degree depends strongly on the excitation power, reaching almost
100 % in the stimulated regime. The relaxation time of the polarization is
about 1 ns. We argue that all this is an effect of a splitting of the polariton
doublet at k=0. We consider different sources for the splitting and conclude
that the most likely one is optical birefringence in the mirrors and/or the
cavity.
