A directional coupler is composed of multiple photonic crystal monomode waveguides placed in parallel and adjacent to each other.When the input optical field is symmetric to the incidence

its propagations in this system are numerically analyzed in accordance with the self-imaging principle.On the basis of this structure

an ultracompact multichannel

say 3-channel

beam splitter is designed and the optical field is transversely redistributed simply by symmetrically changing the refractive indexes of two dielectric rods in the coupling region

realizing the uniform or free distribution of the output energy.This kind of modulation is simple and more efficient.