Ultracompact and Polarization-Independent On-Chip Mode Exchangers Enabled by Subwavelength Grating Metamaterials

Different from conventional mode-order converters, the mode exchangers (EXC ij ), enabling flexible data exchanging between i - th -mode and j - th -mode, are essential building blocks for passive signal switching, signal routing, and high-capacity mode division multiplexing (MDM) networks. However, mode changers are limited by the strong polarization-dependence, especially on the 220-nm-thick silicon-on-insulator (SOI) platform. In this paper, an ultracompact and polarization-independent EXC 01 and EXC 02 are proposed to overcome this limitation by using MZI-like models. Here, subwavelength grating (SWG) metamaterials are introduced to the input/output Y-branches for mode splitting/combining and one arm for phase shifting. As such, a π phase difference is formed by the SWG-manipulated phase shifter and then the modes guided in branches are combined into the required output modes. The characterizations show low insertion losses (<1.6 dB) and crosstalk (<−15 dB) over bandwidths of 31 nm and 49 nm, for EXC 01 and EXC 02 , respectively. For footprints, EXC 01 /EXC 02 has ultracompact size of 13.42 × 1.704/2.044 × 8.434 μm 2 . For the first time, a polarization-independent mode change between 0- th -mode (0- th -mode) and 1- th -mode (2- th -mode) are achieved and can be applied in in flexible MDM systems, signal routing, passive signal switching and beyond.