Analysis of couplers between photonic nanowires and subwavelength grating waveguides

Abstract

Subwavelength grating (SWG) waveguides offer the freedom of (effective) refractive index variation in the design of integrated optical components and devices in silicon-on-insulator waveguides without significantly increasing fabrication complexity. An SWG waveguide is formed by a subwavelength (quasi)-periodic structure consisting of short segments of silicon embedded into a lower-index superstrate. As a result, to the first approximation, the SWG waveguide behaves as a channel waveguide with its core refractive index determined by the filling factor of silicon in the superstrate. By changing the filling factor, i.e., the duty-cycle of the SWG structure, its (effective) refractive index can be varied essentially between that of the superstrate and that of silicon. Here we present a numerical analysis of light coupling between a conventional silicon nanowire waveguide and a periodic SWG waveguide by means of a tapered SWG coupler. The coupler function is to facilitate the smooth and low-loss transition from a conventional mode of a photonic nanowire to a Bloch mode of a periodic SWG waveguide, both propagating with different group velocities. To increase the reliability of numerical simulations, two independent 3D numerical codes based on different formulations of a Fourier modal method (FMM) are used for the analysis. Results of modeling of tapered SWG couplers of different lengths confirm excellent optical properties of these couplers, including very low coupling and return losses.