High-efficiency nano-integrated input/output based on shallow-etched lithium niobate gratings

Diffractive waveguide integrates both the light transmission, optical modulation and the signal input/output functions based on a waveguide structure etched with gratings. Such compact structures realized by micro-nano fabrication technologies offer a flexible solution for achieving efficiently optical coupling at selectable locations on the surfaces of waveguides, while the grating etching process will inevitably affect the original waveguide efficiency. Deep-etched gratings can achieve high coupling efficiency but may cause nonnegligible damage to the waveguide device. In this study, we develop a shallow-etched grating coupler with high efficiency for diffractive waveguides based on the lithium niobate on insulator platform. The key parameter of the etching depth of grating is precisely designed to balance the coupling efficiency and minimize the damage to the waveguide. By controlling the etching depth at the level of hundred nanometers, a coupling efficiency of 33.9 % is achieved in experiment. Moreover, the diffractive waveguide demonstrates excellent input/output functions and the angle-dependent spectral modulation effect. This work provides valuable insights for the design and optimization of grating couplers for augmented reality glasses.