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We propose a novel and highly efficient multimode waveguide grating coupler which can simultaneously launch four channels including two polarizations of the linear polarized (LP) modes, LP 01 and LP 11 of a step-index few-mode fiber (FMF). The waveguide grating coupler on the silicon-on-insulator (SOI) platform is based on a subwavelength structure which was optimized using the genetic optimization approach with 2-dimensional finite-difference time-domain (2-D FDTD) simulations combined with the second - order effective medium theory (EMT). Simulations predicted the coupling efficiencies to be -4.3 dB for LP 01 mode and -5.0 dB for the LP 11 mode. The design was fabricated in a multi-project wafer (MPW) run for silicon photonics. Coupling efficiency of -4.9 dB and -6.1 dB was experimentally demonstrated for LP 01 mode and LP 11 mode, respectively. The proposed mode multiplexer is entirely passive and suitable for future applications with FMFs in the space-division-multiplexing (SDM) networks.
Tong et al. (Tue,) studied this question.