Ultracompact and Polarization-Independent On-Chip Mode Exchangers Enabled by Subwavelength Grating Metamaterials
Zhenzhao Guo, Shengbao Wu, Yunfeng Lai, Shuying Cheng
Abstract
Different from conventional mode-order converters, the mode exchangers (EXC <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><sub>ij</sub></i> ), enabling flexible data exchanging between <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</i> -mode and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</i> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</i> -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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and EXC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and EXC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> , respectively. For footprints, EXC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> /EXC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> has ultracompact size of 13.42 × 1.704/2.044 × 8.434 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . For the first time, a polarization-independent mode change between 0- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</i> -mode (0- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</i> -mode) and 1- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</i> -mode (2- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</i> -mode) are achieved and can be applied in in flexible MDM systems, signal routing, passive signal switching and beyond.