  We study experimentally the Bloch-wave instabilities in optically-induced
lattices. We reveal two different instability scenarios associated with either
the transverse modulational instability of a single Bloch wave, or the
nonlinear inter-band coupling between different Bloch waves. These effects are
compared with the transverse instability in homogeneous media, and it is shown
that the periodic modulation of the refractive index greatly enhances the
transverse instability.
