Investigation for ultra-shorten coupling length in woodpile structure

An off-plane directional coupler in woodpile structure is investigated theoretically by finite-difference time-domain method. Our study shows that the coupling length can be adjusted effectively by controlling the refractive index in central region between input and coupling waveguides. And if the distance between rods is more than 1.5a, a coupling length less 2a can be also obtained, where a is the lattice constant. Also, we investigate the effect of the width of rods on the coupling length. The results show that the coupling length can be shortened further by decreasing the width of rods. All results we simulated are very important for the high-density integrated circuit.