Abstract A potentially high throughput waveguide mode multiplexer/demultiplexer (MUX/DEMUX) exemplified by two cascaded Y-junctions is shown to be capable of efficiently transforming modes from single mode waveguides into a two-dimensional set of the orthogonal ones in a multi-mode waveguide, and back. The design parameters of the cascade and the mode transformations are found by accurate beam-propagation modeling for possible realization with optical polymers. The proposed MUX/DEMUX can enable multifold increase of the data rates of multimode links used in datacom and optical computer communications.