Directional coupler and multimode interference devices based on SiN: An approach to investigating fabrication tolerances

Directional coupler (DC) and multimode interference (MMI) devices are widely used components in photonic integrated circuits (PICs). In this study, the structures of DC and MMI devices based on silicon nitride (Si 3 N 4 ) are introduced and simulated, followed by a comprehensive comparison of these two beam splitters using the finite element method (FEM). Their performance and fabrication tolerances are evaluated, with the effects of manufacturing limitations, wavelength and polarization dependency being considered. Through numerical simulations, the impact of fabrication constraints on the robustness, efficiency, and practicality of these components is assessed. The results highlight the advantages and limitations of both devices under realistic fabrication conditions, providing valuable insights into their applicability for a range of optical applications. • Emphasized the critical role of geometrical structure and refractive index contrast in determining the performance of photonic devices. • Conducted a step-by-step analysis of DC and MMI designs and simulations, supported by rigorous single-mode, even-, and odd-mode analyses. • Compared full-field and approximate analyses using the FEM, assessing their accuracy and applicability. • Evaluated the robustness of DC and MMI structures against fabrication-induced variations through detailed comparative studies.