Ultra-compact waveguide crossing for a mode-division multiplexing optical network

We propose and experimentally demonstrate an ultra-compact multimode waveguide crossing that can process two modes simultaneously. The symmetric Y-junction is introduced to split the high-order modes into fundamental ones, easing the subsequent processing. The footprint of the proposed crossing is as compact as 21 μm×21 μm. The measured results show an insertion loss of ∼1.82 dB for the TE mode and ∼0.46 dB for the TE mode at 1550 nm, as well as a crosstalk of