Compact and Flexible Mode-Order Converter Based on Mode Transitions Composed of Asymmetric Tapers and Subwavelength Gratings

Abstract

An ultracompact and highly flexible mode-order conversion scheme by exploiting reciprocal mode transitions is proposed and investigated in detail, where an inverse/asymmetric input taper and an asymmetric output taper are introduced at both ends of a subwavelength gratings (SWGs) waveguide, respectively, to form two mode transition configurations. For given input TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> mode, it is gradually transited into a single-mode-like (SML) component from the input taper to the SWGs waveguide and then this SML component is inversely transited into the desired TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</sub> mode from the SWGs waveguide to the output taper, where p<; qand effective indices of given and desired modes are matched. Thus, such a transited SML component can serve as an exchanging light between given TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> and target TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</sub> modes with matched effective indices so that arbitrary and flexible mode conversions can be realized by only choosing appropriate dimension parameters for specific mode transitions, significantly simplifying the device design. Results show that TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> -to-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</sub> (p= 0, 1, 2, 3&q= 1, 2, 3, 4) converters can achieve a high conversion efficiency of 83.2% in the worst-case at 1.55 μm, where total device lengths are ranged from 4.1 to 14.9 μm. Moreover, conversion efficiency of > 80% and modal crosstalk of <; -10 dB can be achieved over a broad bandwidth of 80 nm in the worst-case. Additionally, fabrication tolerances are analyzed and light propagation profiles are also demonstrated.