Efficient self-imaging grating couplers on a lithium-niobate-on-insulator platform at near-visible and telecom wavelengths

Lithium-Niobate-On-Insulator (LNOI) has emerged as a promising platform in the field of integrated photonics. Nonlinear optical processes and fast electro-optic modulation have been reported with outstanding performance ultra-low loss waveguides. In order to harness advantages offered by LNOI technology, suitable fiber-to-chip interconnects operating at different wavelength ranges are demanded. Here we present easily manufacturable, self-imaging apodized grating couplers, featuring coupling efficiency TE0 mode high $\simeq 47.1\%$ $\lambda$=1550 nm 44.9\%$ $\lambda$=775 nm. Our approach avoids use any metal back-reflector for an improved directivity or multi-layer structures enhanced strength.