Efficient Mode Multiplexer for Few-Mode Fibers Using Integrated Silicon-on-Insulator Waveguide Grating Coupler

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, LP₀₁ and LP₁₁ 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 LP₀₁ mode and -5.0 dB for the LP₁₁ 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 LP₀₁ mode and LP₁₁ mode, respectively. The proposed mode multiplexer is entirely passive and suitable for future applications with FMFs in the space-division-multiplexing (SDM) networks.