  Mode-division multiplexing (MDM) is becoming an enabling technique for
large-capacity data communications via encoding the information on orthogonal
guiding modes. However, the on-chip routing of a multimode waveguide occupies
too large chip area due to the constraints on inter-mode cross talk and mode
leakage. Very recently, many efforts have been made to shrink the footprint of
individual element like bending and crossing, but the devices still occupy
>10x10 um2 footprint for three-mode multiplexed signals and the high-speed
signal transmission has not been demonstrated yet. In this work, we demonstrate
the first MDM circuits based on digitized meta-structures which have extremely
compact footprints. The radius for a three-mode bending is only 3.9 {\mu}m and
the footprint of a crossing is only 8x8um2. The 3x100 Gbit/s mode-multiplexed
signals are arbitrarily routed through the circuits consists of many sharp
bends and compact crossing with a bit error rate under forward error correction
limit. This work is a significant step towards the large-scale and dense
integration of MDM photonic integrated circuits.
