Low Loss Ridge-Waveguide Grating Couplers In Lithium Niobate On Insulator

The conventional ridge waveguides and grating-couplers in x-cut single-crystal lithium niobate on insulator (LNOI), have been designed, fabricated and characterized. All the device structures patterned on the sample were monolithically defined by one step of the electron-beam lithography process, followed by dry-etching. A low insertion loss (IL) of & minus;6.3 dB/coupler for transverse-electric (TE) polarization inputs at the wavelength of 1543 nm was measured in the fabricated best device with the tapered structures, and exhibited a broad 3-dB optical bandwidth of more than 90 nm. This work may pave the way towards the future research of high-efficiency photonic waveguide components in thin-film LNOI.The successful commercialization of lithium niobate on insulator (LNOI) wafer fabricated by the smart-cut technique and wafer-bonding process [1,2], has increasingly aroused a great enthusiasm in the research of high-performance photonic waveguide components, due to the preeminent electro-optic (EO) property in single crystal thin film lithium niobate (TF-LiNbO3, TFLN), and the high refractive-index contrast between LN optical waveguide core and its surrounding cladding [3-6]. There are two common approaches for all the off-chip light-coupling schemes between optical fiber and waveguide, namely, the end coupling and grating coupling methods. It was reported that the high coupling efficiency (CE) can be realized in end-face couplers with thick cladding layer, but it is sensitive to end alignment-error, and complicated to be fabricated with end-polishing process [7,8]. On the contrary, surface grating couplers (GCs) device is ease of fabrication due to its simple structure, and can be placed at arbitrary locations on the sample. Moreover, there even exists a high alignment-tolerance in GCs for the convenient