We present a new design for beam splitting components employing a silicon-on-insulator rib waveguide structures. In the new design, a high-index thin-film layer is deposited in the rib section to reduce the wave field dispersive tails in the slab section and accordingly render the mode field a confined spot. This in turn improves the beam splitting performance of some conventional waveguide components such as y branches and multimode interference couplers (MMICs), in terms of the excess loss, fiber coupling loss, and compactness of these components. For a y-branch beam splitter, the excess loss can be as small as in the new design, which is much lower than that for a conventional rib waveguide structure (which is ). For a MMIC in our example, the new rib waveguide structure presents an excess loss of for the TE mode and for the TM mode, with negligible nonuniformity in dimensions of , whereas its counterpart (i.e., the one with the same dimensions but without a thin-film layer) presents an excess loss of for both modes. A conventional MMIC must have dimensions larger than to maintain almost the same low excess loss.
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