Conditions for Single-Mode and Birefringence-Free Ultrasmall SOI Rib Waveguides at 1310 nm

The design of silicon-on-insulator (SOI) rib waveguide structures requires careful selection of dimensions for single-mode and birefringence-free operation, which is often necessary in optical devices. At large SOI heights, the single-mode condition (SMC) is well understood. Here, we present a SMC study extending to ultrasmall waveguides, from slab heights of the order of a wavelength down to 220 nm at the 1310 nm wavelength. In addition, the condition for achieving zero-birefringence is presented alongside the SMC for a complete design guideline when both conditions must be met simultaneously. Simulation results show that at small SOI heights (800 nm), previous SMCs no longer apply, and birefringence-free operation becomes difficult to achieve. New expressions are proposed that define the single-mode cut-off condition at these dimensions.