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 TE0 mode and ∼0.46  dB for the TE1 mode at 1550 nm, as well as a crosstalk of <-18  dB from 1510 to 1600 nm.