Mode-selective edge coupler with cladding grating for a lithium-niobate-on-insulator waveguide

Lithium-niobate-on-insulator（LNOI） chips have shown outstanding performance in various photonic devices including modulators, lasers, nonlinear converters, and quantum sources. LNOI-based edge couplers are quite important for further promotion of the above devices in practical applications, especially for large-scale multiport photonic uses, where efficient and mode-selective coupling between chips and fibers is of necessity. Previously, several LNOI edge couplers have been demonstrated, but they mainly focus on achieving high coupling efficiency of the fundamental mode, and sub-wavelength etched lithium-niobate（LN） structures are normally needed, which increases fabrication complexity. Here we propose a new type of edge coupler with direct mode-selective excitation ability, using only Si ON cladding grating structures without additional etching of LN. By introducing a cladding waveguide with periodic structures on the uniform LNOI waveguide, highefficiency excitation of multiple modes can be realized directly with easier fabrication. For a specific simulation here, TE00,TM00, and TE10 core modes can be excited, respectively, at optimized periods and grating lengths with a tunable central wavelength, at the launch of the TM cladding mode. The periods of the needed Si ON gratings are all over 2 μm, which is feasible with i-line UV lithography. Our results provide a low-cost edge coupler for LNOI photonic circuits with the ability of flexible spatial mode selectivity, which may promote LNOI devices in large-scale multiport photonic integrated circuits in the future.