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In the past decade, photonic integrated circuits (PICs) based on thin-filmlithium niobate (TFLN) have advanced in various fields, including opticalcommunication, nonlinear photonics, and quantum optics. A critical component isan efficient edge coupler connecting PICs to light sources or detectors. Here,we propose an innovative edge coupler design with a wedge-shaped TFLN waveguideand a silicon oxynitride (SiON) cladding. Experimental results show that thecoupling loss between the TFLN PIC and a 3-{\mu}m mode field diameter (MFD)lensed fiber is low at 1.52 dB/facet, with the potential for improvement to0.43 dB/facet theoretically. The coupling loss between the edge coupler and aUHNA7 fiber with an MFD of 3.2 {\mu}m is reduced to 0.92 dB/facet. This designmaintains robust fabrication and alignment tolerance. Importantly, the minimumlinewidth of the TFLN waveguide of the coupler (600 nm) can be easily achievedusing foundry-available i-line stepper lithography. This work benefits thedevelopment of TFLN integrated platforms, such as on-chip electro-opticmodulators, frequency comb generation, and quantum sensors.

High-efficiency edge couplers enabled by vertically tapering on lithium-niobate photonic chips