In this paper

we present an optical add-drop filter design on the basis of the coupling properties between waveguide and the ring resonators in photonic crystal.We investigate the normalized transmission spectra for varied ring configurations by using a two-dimensional finite element model in a square lattice dielectric-rod photonic-crystal structure and achieve both backward-and forward-dropping by controlling the size of the ring resonators and the refractive index of the dielectric-rod in the ring resonators.On the other hand

by changing the arrangement of the column boundary scattering medium and the size of column radius of the ring cavity boundary medium

we can reduce the scattering loss between the waveguide and the ring cavity so as to achieve the optimum filtering effect.