Improved coupling efficiency by the gradual change the air holes dimensions in the photonic crystal waveguide

Graded photonic crystals (GPCs) control effectively the propagation of light. Their compact size makes them very attractive in integrated optics. In this work, we´ll use the graded photonic crystal as a coupler between the dielectric waveguide and photonic crystal waveguide (PCW) obtained by omitting one row of air holes in the ΓK direction of a photonic crystal triangular lattice, to improve the coupling efficiency between the two waveguides. The insertion of a graded photonic crystal between the dielectric waveguide and the photonic crystal waveguide has increased the coefficient of transmission from 5.6%, relative to the direct coupling, to 43% [1]. The purpose of this work is to improve of the advantage the coupling efficiency by introducing some changes in the coupling structure. This modification concerns the gradual change of the radii of air holes of PCW. The numerical code CrystalWave is used to model our structures. This one is based on the FDTD method. The results of our optimization show that the coefficient of transmission passes from 43% to 54%.