Phononic band-gap guidance of acoustic modes in photonic crystal fibers

The elastic modes guided along the axis of an optical fiber are obtained for an arbitrary finite cross section using waveguide finite element analysis. The band structure of acoustic phonons is obtained from this full-vector computation. The analysis is applied to the case of a photonic crystal fiber possessing a honeycomb lattice. It is shown that this fiber exhibits band gaps for elastic modes propagating along the longitudinal fiber axis. For frequencies within a band gap, the external boundary of the fiber becomes a defect of the phononic crystal that supports the propagation of guided elastic modes. Such boundary modes are very sensitive to the boundary conditions. The further introduction of a defect within the two-dimensional phononic crystal leads to the formation of highly confined elastic waveguide modes that copropagate in the same core volume as the guided optical mode. We consider the application of these properties to the suppression of stimulated Brillouin scattering and to enhanced collinear acousto-optical interactions. In particular, we obtain the optimum elastic modal shape that maximizes the acousto-optical scattering coefficient for given optical modes.