Ultra-broadband mode multiplexers based on three-dimensional asymmetric waveguide branches.

We propose a three-dimensional waveguide mode multiplexer structure that operates on the principle of adiabatic mode transition along multilayer asymmetric waveguide branches. Using this structure, we designed and fabricated a three-mode and a four-mode multiplexer with polymer material that can operate over the C+L band and beyond with small modal crosstalk (<-10  dB) and negligible polarization dependence and be directly connected to fibers with low loss. These multiplexers could be used as mode-selective devices for ultra-broadband mode-division multiplexing based on few-mode fibers.