A Novel Design of Photonic Crystal Directional Coupler with Ultra Short Coupling Length Using Core /Shell Rods Defect

In this paper, a novel method to diminish the coupling length and enhance the coupling coefficient in a two-dimensional (2D) photonic crystal (PC) directional coupler (DC)is investigated. The DC comprises two closely spaced linear waveguides separated by one row of silicon columns. In this new approach, we used core/shell (C/S) rods between the waveguides to reduce the coupling length. The C/S rod is obtained by etching a hole in the middle of the silicon rod. This modification leads to a very short coupling length of 5 a , where a is the lattice constant of the structure. Simulation results show that increasing the inner core radius increases the coupling coefficient. The coupling properties have been numerically simulated and analyzed using the finite difference time domain (FDTD) and the plane-wave expansion (PWE) methods. Based on the modified structure, we analyzed the output spectrum of a very compact channel drop filter. The proposed modified design would offer significant potential for novel ultrasmall devices based on photonic crystal couplers.