Generalized scheme for coupling length reduction in photonic crystal based directional coupler designed with square lattice of dielectric rods in air background

Abstract

A general technique is proposed to reduce coupling length of optical couplers embedded in square latticed photonic crystals. In this approach, reduction in coupling length is achieved by engineering dispersion characteristics of coupled waveguide system with the introduction of an additional mode associated with special type of line defect. Two such line defects, namely type-I and type-II, are investigated and both the co-directional and contra-directional propagations are realized. The smallest coupling length achieved using the type-I is 2.05a, while that obtained using the type-II is 2.47a in contrast with the existing value of 28a for square lattice and 3.83a in the case of triangular lattice. Greater control over the resulting coupling length of a system is also attained. Frequency responses of the proposed couplers are finally verified using FDTD method.