Modified Maxwell fish-eye approach for efficient coupler design by graded photonic crystals.

We present a novel design of two dimensional graded index medium that provides coupling of light with high coupling efficiency between two planar dielectric waveguides of different widths (15.46 μm vs. 2.21 μm). Poor light coupling performance of butt-coupler can be mitigated by implementing tapered coupler at the expense of long coupler section. In order to reduce coupling losses, a new coupling device approach based on graded index (GRIN) concept is proposed. The refractive index distribution is in the form of modified version of the Maxwell fish-eye lens. The inhomogeneous refractive index distribution is approximated by photonic crystals (PCs) such that the positions of each PC rods are appropriately arranged. Strong electric field focusing ability of the designed GRIN PC medium provides relatively high coupling efficiency that is around 90%. Spectral region corresponding to coupling efficiency over 75% has a bandwidth of Δω = 18.56% (284 nm). Finally, we discuss the durability of the proposed coupler against the lateral displacement and angular misalignment of output waveguides.