Broadband adiabatic mode division (de)multiplexer in thin film lithium niobate

We report a broadband adiabatic mode division (de)multiplexer in a thin film lithium niobate (TFLN) waveguide based on the stimulated Raman adiabatic passage scheme. This particular adiabatic tunneling process is implemented in a three-waveguide structure, in which the intermediate waveguide is tilted relative to the outer waveguides. The excitation of the intermediate waveguide is negligible during the mode (de)multiplexing. A taper structure is included in the intermediate waveguide to ensure efficient mode conversion. The proposed (de)multiplexer can realize multiplexing or demultiplexing of the modes TE0, TE1, and TE2. The structural parameters are determined by simulation, and the results show that conversion efficiencies of 96.3% and 94.6% can be obtained for TE2 and TE1, while the 1-dB operation bandwidth is around 420 nm. The cross talks of TE2, TE1, and TE0 are lower than −13.4dB, −16.4dB, and −34.2dB. The designed (de)multiplexer has relatively good fabrication tolerance. Therefore, it is suitable in TFLN integrated chips.