Ultra‐Sharp Multi‐Mode Waveguide Bending Assisted with Metamaterial‐Based Mode Converters

Abstract

On-chip multi-mode communication has attracted much attention for its potential to greatly expand the bandwidth of data transmission. However, the application of multi-mode transmission is still restricted since the multi-mode waveguide can't be compactly routed, especially in the sharp bending region. The challenge lies in decreasing the significant inter-mode coupling in the multi-mode bending with a small radius. Few works have been reported to achieve multi-mode bending, but all suffer from the complex fabrication. In this paper, an ultra-sharp multi-mode waveguide is proposed, which is assisted with metamaterial-based mode converters. The sub-wavelength structures are directly written onto the polymer cladding. By engineering the effective medium index of the metamaterial cladding in the mode converters, the inter-mode coupling is dramatically inhibited. The fabricated sharp bending with a 30-μm radius shows that the crosstalk is <−20 dB and the loss is <1 dB over a >80 nm wavelength band.

Metamaterial‐Based Maxwell's Fisheye Lens for Multimode Waveguide Crossing

“…The mode division multiplexing (MDM) has been proposed to scale up the capacity with a single wavelength carrier, where independent signals are carried by different eigenmodes in parallel . Over the recent years, numerous key elements in the multimode transmission system have been reported, including the mode (de)multiplexers, multimode switches, and multimode bent waveguides . The waveguide crossing is essential in the on‐chip photonic circuit to obtain the complex topology of interconnects .…”

Ultra‐Sharp Multimode Waveguide Bends with Subwavelength Gratings

“…The present SMWB also enables much smaller bending radius than that given in ref. []. As shown in Table , for the case with four modes, the theoretical crosstalk for our 90° arc‐bend is as low as less than −26 dB in the broadband even when R = 20 µm.…”

“…However, for a multimode SOI nanophotonic waveguide, the situation becomes very complicated because all modal fields become highly asymmetric with respect to the center of the waveguide when the bending radius is small . As a consequence, there is significant mode mismatch between a straight waveguide (SWG) and a sharp bent waveguide (BWG), which introduces not only notable excess losses but also high inter‐mode crosstalk . As a result, one needs to choose very large bending radii when using regular arc‐bends.…”

“…Unfortunately, it is still difficult to be extended for more than two modes. Recently, a novel mode converter with a metamaterial assistant PMMA upper‐cladding was introduced to enable a sharp multimode waveguide bend for four TM mode‐channels . One should notice that a SiO 2 upper‐cladding and TE polarization modes are more preferred to be used as a mainstream option in the silicon photonic integrated circuits .…”

“…For example, the length of the mode converter presented in ref. [] is L tp = 15.8 µm. When the bending radius R = 30 µm, the effective radius for the demonstrated 90° arc‐bend is given as R ′ = 45.8 µm ( = R + L tp ) by including the length of the mode converter.…”