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High Efficiency Multimode Waveguide Grating Coupler for Few-Mode Fibers

We describe a novel high coupling efficiency multimode waveguide grating coupler which uses a polysilicon overlay numerically optimized to improve both the directivity and mode-matching of the grating coupler with different modes. The device was realized experimentally and designed with a large minimum feature size of 181 nm for fabrication by deep ultraviolet photolithography in the commercial foundry. Simulations predicted the coupling efficiencies to be −1.1 dB for LP01 mode and −1.56 dB for the LP11 mode. The experimentally measured coupling efficiencies were −1.36 dB and −2.21 dB for LP01 mode and LP11 modes, respectively. These results demonstrate a significant advance over previous designs, and have the potential for use in silicon photonic transceivers for mode division multiplexed communications in multimode fibers.