Silicon nitride （Si₃N₄） photonic integrated circuits are promising for next-generation optical communications due to their low loss and wide transparency window. However， efficient coupling between submicron Si₃N₄ waveguides and standard optical fibers remains challenging due to mode-field mismatch. Existing solutions like inverse tapers or polymer claddings involve trade-offs among bandwidth， loss， and manufacturability. This work proposes a taper-straight-taper edge coupler， where the straight section acts as a mode stabilizer and reflection suppressor. The optimized device achieves sub-decibel coupling loss across the S+C+L bands with high alignment tolerance， offering a compact and fabrication-friendly solution for Si₃N₄ photonic integration.

To overcome these limitations， this work proposes and demonstrates an innovative edge coupler architecture that combines two inverse tapers with an intermediate straight waveguide section. The straight section is introduced as a mode stabilizer and reflection suppressor， effectively filtering residual higher-order modes and eliminating parasitic reflections through phase-matching interference. This configuration allows the optical mode to evolve smoothly from the tightly confined on-chip waveguide to the expanded fiber mode， while maintaining high fundamental-mode purity and minimal spectral ripple. The design principle ensures adiabatic mode conversion without relying on extreme geometries or high-aspect-ratio structures， thus preserving compatibility with standard photolithography and etching processes.

Comprehensive electromagnetic simulations based on eigenmode expansion were employed to optimize the key design parameters， including the taper lengths， transition widths， and the straight section geometry. The optimized configuration exhibits an excellent balance between low coupling loss， wide operational bandwidth， and compact device size. The inclusion of the straight section significantly enhances the stability of the mode conversion process and mitigates the oscillations typically observed in conventional two-taper couplers. The simulation results predict broadband， flat coupling performance with high tolerance to fabrication deviations and fiber alignment errors， confirming the robustness of the proposed design.

Experimental characterization was performed on fabricated devices covering the S， C， and L communication bands. The measured results show broadband low-loss performance with minimal wavelength dependence and excellent alignment tolerance. Specifically， the coupler achieves sub-decibel coupling loss per facet over a wide spectral range， with the wavelength-dependent variation remaining below one decibel across the entire operational window. The measured results agree well with the theoretical predictions， confirming that the phase-matching mechanism in the straight section effectively suppresses reflection-induced interference and improves modal stability. Furthermore， the design maintains low loss under several micrometers of lateral or vertical fiber misalignment， demonstrating strong packaging robustness suitable for large-scale manufacturing.

The proposed edge coupler achieves these results without the need for complex processes such as electron-beam lithography or high-aspect-ratio etching. Its fully planar structure and moderate feature sizes ensure high yield and compatibility with commercial foundry processes. The broadband and low-loss characteristics make it particularly advantageous for integrated systems that require efficient chip-to-fiber interfaces， such as coherent transceivers， frequency comb sources， and integrated photonic sensors.

In summary， this study introduces a broadband， low-loss， and fabrication-tolerant silicon nitride edge coupler based on a taper-straight-taper configuration. By integrating a straight single-mode section between two inverse tapers， the design simultaneously achieves mode stabilization， reflection suppression， and efficient field expansion. Both simulation and experimental results demonstrate superior performance across a wide wavelength range， with low insertion loss， minimal wavelength sensitivity， and high alignment tolerance. The proposed structure provides a compact， manufacturable， and scalable coupling solution for silicon nitride photonic integrated circuits， enabling their deployment in next-generation high-speed optical communication and broadband photonic applications.