Compact and broadband 3D mode (de)multiplexer via displacement of an asymmetric slot-strip coupler

A compact and broadband three-dimensional (3D) quasi-TE0 and quasi-TE1 mode (de)multiplexer [(De)MUX] is proposed via an asymmetric slot-strip coupler, in which the optimally lateral displacement between the bottom silicon slot waveguide and the upper poly-silicon strip-waveguide can enable the vertical mode-coupling efficiently. The design of the proposed 3D mode (De)MUX is based on the full-vectorial finite-element method, 3D full-vectorial finite-difference time-domain method, and mode expansion method. The optimized 3D mode (De)MUX is with a compact coupling length of only 5.65 µm, a broad 3 dB bandwidth of 231.5 nm, a low insertion loss of 0.3 dB, and a mode cross talk of −27.54dB. The fabrication tolerance is also studied with a width error of ±20nm for both the upper and lower layers.