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

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-m radius shows that the crosstalk is <-20dB and the loss is <1 dB over a >80 nm wavelength band.