Figure 2: Design of spatial mode beamsplitter.

(a) Si₃N₄ dispersion for a multi-mode waveguide with 190 nm height. Horizontal red line shows phase matching for waveguides with different widths for spatial mode multiplexing. Vertical red line shows phase matching between modes in a single waveguide for the spatial mode beamsplitter. (b) Symmetric grating structure for coupling the TE0 and TE2 modes. The period is defined by the difference in effective index between the modes in a particular waveguide. The period (Λ) is 6.675 μm and the grating depth, d, is 24 nm. The width, w, is 1,600 nm and the height is 190 nm. Inset: SEM of fabricated grating structure. Scale bar, 200 nm. (c) Simulation of mode conversion in a 50:50 splitter for η=0.5, where N=20 periods and all other dimensions are the same as in b.