Multimode waveguide crossing with ultralow loss and low imbalance.

The mode-division-multiplexing (MDM) technology has become an alternative solution to further increase the link capacity in optical communication systems. Ultralow loss waveguide crossings for multimode waveguides are requisite in on-chip MDM systems. We propose and demonstrate an ultralow loss silicon multimode waveguide crossing using a combination of fully etched and shallowly etched waveguides in the multimode-interference coupler region to reduce the imbalance for two transverse electric polarized (TE) modes. By engineering the geometries and the proportion of the two waveguides, the self-imaging positions for different modes can coincide exactly. Simulated results show that the insertion losses are 0.043 and 0.084 dB for the fundamental TE (TE0) mode and the first-order TE (TE1) mode at 1550 nm, while the experimental values are 0.1 and 0.12 dB, respectively. The measured crosstalk is less than -30 dB for both modes within a 75 nm wavelength span.