  We address nonlinear signal switching between two low-index defect channels
induced in periodic optical lattices. In contrast to conventional directional
couplers, where the guiding mechanism is total internal reflection or
refraction, in such Bragg-type coupler, the guidance is of a photonic-bandgap
origin. The coupling length in the low-index coupler is controlled by the
lattice parameters and by the channel spacing. In the nonlinear regime the
Bragg-type coupler behaves as an all-optical switch, exhibiting a remarkable
difference of switching power for focusing versus defocusing nonlinearity.
