Design and fabrication of cantilevered fiber-to-waveguide mode size converter for thin-film lithium niobate photonic integrated circuits

摘要

The recently emerged photonic integration technology based on thin-film lithium niobate (LN) have been regarded as a very promising candidate for advanced photonic integrated circuits (PICs) due to its attractive nonlinear properties, wide-spread use in electro-optic applications, and etc. Generally, the thin-film LN optical waveguide used in PICs is sub-micrometer scale. Mode mismatch between fiber and sub-micrometer LN waveguide in chip is the main factor of increasing the fiber-to-chip coupling loss and the total insertion loss of LN PICs. Therefore, for practical applications, low-loss mode size converter for coupling between fiber and sub-micrometer LN waveguide is essential. In this paper, an efficient and novel fiber-to-chip mode size converter for thin-film LN PICs was designed and fabricated. The converter consists of a LN nano-taper and a cantilevered SiO_2 waveguide. The nano-taper is embedded in the center of SiO_2 waveguide. Laterally connected SiO_2 cantilever beams are fabricated to provide structural support for the cantilevered SiO_2 waveguide. Our work provides an efficient way to realize low-loss fiber-to-chip interface for thin-film LN PICs.