Global design rules for silicon microphotonic waveguides: sensitivity, polarization and resonance tunability

Abstract

Abstract: In a rigorous design study of silicon-in-silica waveguides and resonators we address critical parameters for tunable filters. 6:1 aspect-ratio TE and 2:1 TM waveguide designs optimize resonance-frequency dimensional tolerances, proximate metal-electrode loss and other constraints. High-index-contrast (HIC) microphotonic circuits employ strong index confinement to provide high Q's in small resonators with a free spectral range (FSR) in the 10's of nm. They enable widely tunable integrated add-drop filters for transparent optical networks In this paper, we investigate optimal designs of silica-clad silicon-core (Si) waveguides in terms of waveguide cross-section and field polarization, with respect to an extensive set of practically relevant criteria: sufficiently large feature sizes; low sensitivity of resonance frequencies and waveguide-cavity couplings to dimensional variations; high Q and large FSR; small propagation loss due to waveguide roughness; and efficient thermo-optic tuning. With a view toward thermally tunable high-order microring resonators, we find that dimensional sensitivity of the resonance frequency, and proximity of metallic heaters (causing optical absorption) ultimately determine the choice of design. The results give two very different optimal designs for the choice of TE or TM device operation (about 700x120nm and 480x260nm, respectively). In comparison, the Si waveguides typically employed for TE excitation (~450x200nm) are much more sensitive to dimensional error, rendering high-order filters difficult to realize. We parameterize our study throughout by waveguide aspect ratio (A R ), for designs using TE and TM excitation