High-efficiency and easy-processing thin-film lithium niobate edge coupler

Fiber-to-chip coupling with ultralow loss and broadband operation wavelength range is essential in the practical applications of thin-film lithium niobate (TFLN) integrated photonic devices. However, the existing edge couplers often require electron beam lithography overlaying and multiple etching processes, which are expensive and complex. In this Letter, we demonstrate an edge coupler that includes only a vertically tapered TFLN waveguide and silicon dioxide cladding. The coupling efficiency between a lensed fiber and an on-chip LN waveguide is down to 1.43 dB/facet, while the 3-dB bandwidth exceeds the range from 1510 to 1630 nm. These edge couplers also show reliable fiber misalignment tolerance. Furthermore, the fabrication complexity is greatly reduced since only a single etching step for the TFLN waveguide is needed. The minimum waveguide width of 1.3 μm guarantees compatibility with i-line photolithography, providing a potential for massive production of TFLN devices.