Demonstration of a scalable mode converter in lithium niobate on insulator

Lithium niobate on insulator (LNOI) attracts extensive attention from academic and industrial communities due to LN's excellent electro-optic (EO) properties, which also brings bright prospects for achieving high-speed and large-capacity photonic integrated circuits (PICs) including active and passive components such as modulator, filter, and multiplexer. A mode converter can implement optical mode conversion from a fundamental mode to a high-order mode, which plays a key role in an on-chip mode-division multiplexing (MDM) system. In this Letter, we propose high-performance and scalable mode converters in LNOI, and based on our proposed scalable principle, the mode converter can be designed to implement the optical mode conversion from the fundamental mode to arbitrary high-order mode. As a proof of concept, we design and fabricate TE₀-TE_i (i = 1,2,3) mode converters. The experimental results show the insertion loss (IL) and cross talk (CT) are less than 2.5 dB and -10.3 dB in the wavelength range of 1525-1565 nm for all fabricated devices, which are basically consistent with the simulation results. This work brings great promise for achieving high-performance PICs on the LNOI platform in the future.