Scalable and Low Crosstalk Silicon Mode Exchanger for Mode Division Multiplexing System Enabled by Inverse Design

In this paper, we design and experimentally demonstrate a two-mode and three-mode mode exchanger (ME) using inverse design method. The designed MEs provide more flexibility for mode division multiplexing (MDM) system links. The optimized designs are compact, 16 m2 and 24 m2 for the two-mode and three-mode ME, respectively. During the optimization process, the fabrication imperfection tolerance, insertion loss (IL), and crosstalk performance are optimized. Considering the symmetry of these devices, only N simulations per iteration are required for N mode ME. The fabricated two-mode ME exhibits IL less than 0.52 dB within the wavelength range from 1500 nm to 1600 nm. The corresponding crosstalk is at most -18.5 dB. The 210Gbps non-return to zero (NRZ) PRBS-31 payload transmission shows clear and open eye diagrams for all output modes. A three-mode ME is also experimentally demonstrated validating the scalability. The fabricated three-mode ME exhibits an IL less than 0.85 dB and 0.9 dB for the conversions from TE0 to TE1 and from TE1 to TE0, while the TE2 to TE2 transmission has an IL of 1.7 dB within the same wavelength range. The crosstalk is less than -16.5 for all three output modes with 310Gbps NRZ open eye diagrams.