Design of matching stage for directional coupler-based slow-light photonic crystal waveguide

In this paper, two slow-light photonic crystal waveguides are designed based on a directional coupler. For the coupling of the waveguides to the conventional photonic crystal W1 waveguide, matching structures are proposed. For simulating the proposed structures, finite difference time domain and plane wave expansion methods are used. For the first structure, a group-index of 18.05 and a GBP equal to 0.278 and for the second structure, by modifying a section of photonic crystal lattice structure from hexagonal to square, a group-index of 11.37 and a GBP equal to 0.255 is obtained. For matching these waveguides to the W1 waveguide, a matching stage based on a Y-splitter is designed and proposed. Gradual change technique at the intersection of slow-light waveguide and coupling stage are used in order to maximize the waveguides coupling within the photonic crystal waveguide frequency range. For this purpose, a light pulse is inserted to the structure and the transmitted and reflected powers are calculated for each case. The bandwidth obtained for the first structure is equal to 28 nm and for the second structure it is equal to 7 nm. For the first structure, the maximum simulated transmittance is equal to 97.3% and for the second structure it is equal to 98.5%. Due to the 28 nm bandwidth obtained for the first structure, the proposed slow-light waveguide is well suited for the 1550 nm wavelength which is used in optical communications.