Spot-Size Converter of Thin-film Lithium Niobate on Quartz Substrate.

Thin-film lithium niobate on insulator (LNOI) has gained significant popularity due to its outstanding optical properties. However, a highly efficient fiber-chip optical coupler is still necessary for LNOI-based devices for practical applications. This paper presents a proposed thin-film lithium niobate spot-size converter (SSC) fabricated on a quartz substrate. The SSC enables highly efficient coupling between fibers and waveguides, with a coupling efficiency of 93% at 1550 nm. The SSC exhibits a large mode spot size of up to 3 μm., provides relatively high optical deviation tolerance and manufacturing tolerance, and enables the connection between optical equipment on quartz substrate and optical fiber.