Highly-Efficient thin Film LiNbO3 Surface Couplers Connected by Ridge-Waveguide Subwavelength Gratings

The ridge waveguide integrated grating couplers (GCs) in lithium niobate on insulator (LiNbO3, LNOI) were designed, fabricated and characterized. Two ends of the tapered GCs were connected by the subwavelength gratings (SWG) waveguide of a sub-micrometric-diameter, the photonic-wire SWG structure was featured with the profile of side-walls corrugations, and the effect of geometrical dimensions on the output optical response was investigated. All the devices structure patterns for the integrated LNOI GCs could be simultaneously defined by one step of electron-beam lithography, and then easily fabricated by the optimized dry-etching processes, followed by samples surface cleaning. After the fabrication, a low coupling loss of − 5.1 dB/coupler at the telecommunication wavelength of 1561 nm was measured in the best thin-film LiNbO3 (TFLN) surface grating coupler for quasi-transverse-electric (quasi-TE) polarized signals, and a broad 3-dB optical bandwidth of wider than 95 nm was also obtained. The compact components exhibited magnificent performance, and might show the potential functionalities for the TFLN-based integrated optical waveguide devices.