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 an inverse design method. The designed MEs provide more flexibility for mode division multiplexing (MDM) system links. The optimized designs are compact, 16 mu m(2) and 24 mu m(2) 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, some forward and adjoint simulations are reused. Thus, only N simulations per iteration are required for N mode ME even considering crosstalk in the figure of merit (FOM). The fabricated two-mode ME exhibits IL less than 0.52 dB within the wavelength range from 1.5 mu m to 1.6 mu m. The corresponding crosstalk is at most -18.5 dB within the same wavelength range. The 2 x 10 Gbps 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 using inverse design for adaptable ME devices. 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 3 x 10 Gbps NRZ open eye diagrams.