One-dimensional grating coupler on lithium-niobate-on-insulator for high-efficiency and polarization-independent coupling

A metal-based one-dimensional grating coupler on an x-cut lithium-niobate-on-insulator wafer structure for a polarization-independent fiber interface is designed and demonstrated. By using a metal-based plasmonic mode, the diffractive angle for the two polarized modes in the lithium niobate ridge waveguide can be tuned to be the same. The polarization dependence of the grating coupler therefore can be effectively reduced. The fabricated device exhibits -3.56-dB and -4.08-dB peak coupling losses per coupler at 1573 nm for the TE and TM modes, respectively. The polarization-dependent losses are less than 0.69 dB in a 44-nm wavelength range. The demonstrated grating coupler can serve as a polarization-independent optical fiber interface on lithium-niobate-on-insulator and facilitate on-chip polarization diversity applications.