Integrated low-cost and low-loss spot size converter for thin-film lithium niobate

An integrated low-cost and low-loss spot size converter (SSC) for thin film lithium niobate (TFLN) is important to support various high-performance hetero-integrated devices, such as electro-optic modulators, optical frequency combs, and optical phased array. The high performance and power consumption restrict the development of hetero-integrated devices consisting of silicon on insulator and lithium niobate, because of a large difference in mode field sizes between TFLN waveguides and standard single mode fibers or semiconductor lasers. Up to now, most low-loss SSCs whose feature sizes range from tens to hundreds of nanometers, are fabricated by high-cost electron beam lithography. Here we theoretically propose an integrated low-cost and low-loss SSC that reduces the end-face coupling losses in integrated system and improve the coupling efficiency, meanwhile especially its feature size of 1 μm can be fabricated by standard lithography. Based on evanescent wave coupling theory, a Si₃N₄ tapered waveguide surrounded by an amorphous silicon thin layer is used to enhance evanescent wave coupling strength, thereby achieving a high coupling efficiency of 88% between the TFLN and semiconductor lasers. The low-cost and low-loss SSC provides a potential method for hetero-integrated TFLN devices.