Electromagnetic Wave Propagation in a Quasi-1D Rhombic Linear Optical   Waveguide Array
  The quasi-one-dimensional rhombic array of the waveguides is considered. System of equations describing coupled waves in the waveguide in the linear limit is solved exactly. The electric field distribution was found both for the diffractionless (or dispersionless) flat band modes and for the dispersive modes. 
Andrey I. Maimistov, Viktor A. Patrikeev
physics.optics