High-bandwidth and low-loss thin-film lithium niobate trident edge coupler

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This work reports a pioneering design of high-bandwidth, ultra-low-loss, and process-robust edge coupler optimized for 9-μm single-mode fibers. The architecture employs a bilayer waveguide structure comprising a silicon nitride top layer and a lithium niobate bottom layer. Through systematic geometric parameter optimization, the vertical mode matching efficiency is substantially enhanced, while a trident-shaped lithium niobate waveguide configuration is innovatively incorporated to fine-tune the lateral mode field distribution. Leveraging a two-stage inverse-tapered waveguide design, optical mode energy is efficiently transferred from the silica cladding to the lithium niobate ridge waveguide. Simulation results demonstrate that at 1550 nm, the coupler achieves an edge mode overlap loss of 0.05 dB and a TE-mode coupling loss of 0.16 dB per facet. Compared with current state-of-the-art processes, the achieved coupling efficiency exhibits exceptional performance benchmarks. Notably, it maintains exceptional transmission stability across a broad 300-nm(1400 nm-1700 nm) spectral bandwidth (Coupling loss < 0.3 dB/facet). In alignment tolerance characterization, vertical/lateral offsets of ± 2 μm result in insertion losses of 0.67 dB and 0.79 dB, respectively, underscoring its superior process robustness and engineering viability. Compared to current state-of-the-art designs, this work represents a notable advancement in reconciling process robustness with low-loss performance. This design provides a viable option for broadband optical communication devices.