Optimization of Edge Couplers for a SiN Platform for Visible and NIR Applications

Efficient, broadband optical coupling between optical fibers and photonic integrated circuits (PICs) poses a fundamental challenge in photonic system design. Among various coupling strategies, edge couplers offer low insertion loss, polarization-independent operation, and broad bandwidth. This thesis presents the design, optimization, and performance evaluation of edge couplers for a silicon nitride (SiN) photonic platform operating in the visible and near-infrared (NIR) spectral ranges. A fabrication-aware design is emphasized. The work is conducted within the framework of the IMEC SiN platform, and a comprehensive, simulation-based design methodology was developed using Ansys Lumerical tools. A detailed investigation and development of the simulation model yields more realistic results, including factors such as the substrate and the oxide interfaces. A systematic and modular design methodology is adopted to create a relatively simple and quick optimization procedure that can be repeated for multiple wavelengths. To assess trade-offs between performance and manufacturability, three geometrical configurations of increasing complexity (single-tip, two-tips, and four-tips designs) were optimized for four spectral bands: Blue, Green, Red, and NIR. The results show that optimized SiN edge couplers can achieve insertion losses below 1 dB in the range between Green and NIR, and below 4 dB in the whole visible range. The most complex four-tips design shows the best performance in terms of the nominal values of the considered figures of merit (FOM) and stability against fabrication uncertainties. Conversely, the simplest single-tip design performs the worst. Adding one tip to the same SiN layer provides considerable advantages at the expense of minimal additional effort in the design process. The developed methodology offers a robust framework for designing integrated, high-performance, fabrication-tolerant edge couplers for use in standard photonic design kits (PDKs). This enables the scalable production of broadband SiN-based photonic integrated circuits.