Re-Analysis of Single-Mode Conditions for Silicon Rib Waveguides at 1550 nm Wavelength

Single-mode waveguiding devices are desired for photonic integrated circuits. Previous studies on the single-mode conditions (SMCs) of a silicon-on-insulator (SOI) rib waveguide lack a comprehensive discussion to include a large to small range of cross-section cases, thus revealing the discrepancies of various determination methods. Here, we reanalyze the SMCs for large and small SOI rib waveguides by utilizing a finite-element method mode solver. The results are compared with both classical SMCs of Soref's expression and Pogossian's effective-index method, indicating that the constant value α shown in the Soref's analytical expression depends on the rib heights and polarizations. For transverse-electric polarized light and the waveguide height exceeding 5 μm, α is approaching that of Soref's equation; if the height is ≤5 μm, then α = 0.35, 0.4, 0.45, 0.7, and 1.0 are determined for the cases of 5, 3, 2, 0.8, and 0.4 μm heights, respectively. However, for transverse-magnetic polarized light, if the height ≥ 2 μm, α is constant (0.25) and independent on the rib heights, and α = 0.18 and 0.12 are attained for the cases of 0.8 and 0.4 heights, respectively. This detailed analysis indicates that a wider rib width can be used while preserving a SMC leading to devices with improved coupling efficiencies and reduced insertion losses for on-chip photonic circuits.