Broadband and easily fabricated double-tip edge coupler based on thin-film lithium niobate platform

Abstract

Due to the significant difference in spot size between lithium-niobate-on-insulator (LNOI) waveguide and fiber, there is a substantial coupling loss between them, which limits the interconnection and application of LN photonic chips. Although some LNOI edge couplers have been designed and fabricated with low coupling loss between chip and fiber, they require high etching accuracy. In this paper, a low-loss, broadband and easily fabricated edge coupler is designed and fabricated on the LNOI platform. To reduce the etching requirement for tip width, a design has been adopted with symmetrical double tips at the edge. Additionally, a multimode interferometer (MMI) is introduced to ensure large fabrication tolerance. The proposed design only requires a single full etching step, significantly reducing stringent requirements for process with high precise. Experimental results demonstrate that the device exhibits a coupling loss less than 3.5 dB/facet in the wavelength range of 1500–1640 nm, while achieving a low coupling loss of 2.71 dB/facet at 1550 nm.