Single-mode condition for a rib waveguide on the SOI platform: a revisit with new perspective

Rib waveguides are one of the most widely used optical waveguide structures in integrated photonics, primarily owing to their low propagation loss and ease of control. A unique and often highly desirable feature of the rib waveguides is that even when the overall cross-sectional dimension of the waveguide is large, single-mode conditions can still be achieved with specific structural configurations. In this work, we revisit and analyze the single-mode conditions for the rib waveguides using an insightful mode-matching method. Based on the theoretically exact single-mode condition, a set of highly accurate empirical single-mode conditions are established for silicon-on-insulator (SOI) rib waveguides with different cross-sections: a true single-mode condition, ensuring exclusive propagation of the HE 11 mode, and a quasi-single-mode condition, which also allows the EH 11 mode. These conditions are verified using both Si and silicon nitride waveguides in SOI platforms. The proposed single-mode condition is of great significance for achieving high-density, low loss, and highly reliable silicon photonic integrated circuits.