Ultra-broadband lithium niobate optical mode (de)multiplexer enabled by SWG-based modal-field redistribution.

Mode-division multiplexing is a common technology in photonic integrated circuits, which can combine various optical mode signals into a single channel, such as a multimode waveguide, to improve the communication system's capacity. The optical mode (de)multiplexer plays a critical role in mode-division multiplexing by converting multiple single-mode signals from different channels into corresponding multimode signals, respectively, and then combining them into the same physical channel and vice versa. Here, we propose an ultra-broadband optical mode (de)multiplexer based on modal-field redistribution assisted by subwavelength grating (SWG) structures in lithium niobate on insulator. As a proof of concept, we design and fabricate a three-channel mode (de)multiplexer. The simulation results show an operating bandwidth of 800 nm (from 1.2 to 2.0 μm) with an insertion loss of less than 1 dB, and the experimental results show that the insertion loss and crosstalk of all devices are less than 1.0 dB and -15.9 dB in the wavelength range of 1500-1600 nm, which is basically consistent with the simulation results. This work holds significant promise for the future implementation of high-performance photonic integrated circuits (PICs) on the LNOI platform.