Thin-film lithium niobate ridge-waveguide grating couplers integrated with anti-reflection slots interface

Abstract Based on the X-cut single crystal lithium niobate thin film (TFLN), the deeply etched ridge-waveguide tapered grating couplers (GCs) embedded with additional modified slots, had been firstly theoretically and experimentally demonstrated. The entire device structures were simultaneously defined in the chip, which were patterned by a single step of electron-beam lithography, and subsequent monolithically manufactured by dry-etching process. For the fabricated GCs when operated at the telecom bands for transverse-electric (TE) polarization, the minimum insertion loss (IL) of −6.6 dB/coupler was measured at 1541 nm in the common GCs interfacing with positive scattering angle fiber, while for the backscattering design of GCs coupled with negative scattering angle fiber, a relatively much lower loss of −5.4 dB/coupler was even obtained at 1567 nm owing to the anti-reflection slots interface between optical waveguide and gratings, the evaluated 3-dB optical bandwidth was greater than 80 nm, and there was a good agreement between the modeling and experimental results. Moreover, the improved integrated TFLN GCs structure, which not only could be used for effective light-coupling between negative angle tilted fiber and waveguide gratings, but also could be simultaneously applied for the bidirectional transmission of optical beams. And this work might boost the developments of large scale photonic integrated components.