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

Automated summary

This study presents the design, fabrication, and characterization of conventional ridge waveguides and grating-couplers in x-cut single-crystal lithium niobate on insulator, showing low insertion loss and broad optical bandwidth in the best device. The successful commercialization of LNOI wafers has sparked great enthusiasm in the research of high-performance photonic waveguide components, due to the excellent electro-optic properties and refractive-index contrast.