Sub-wavelength Gratings Assisted Ridge Waveguide Surface Couplers on Lithium Niobate Thin Film

A thin-film lithium niobate ridge-waveguide grating coupler integrated with waveguide subwavelength gratings is experimentally demonstrated. An ultra-high coupling efficiency of -5.1 dB/coupler for TE polarization and over 90 nm 3-dB optical bandwidth are achieved.