  Mode-division multiplexing has shown its ability to significantly increase
the capacity of free-space optical communications. An accurate alignment is
crucial to enable such links due to possible performance degradation induced by
mode crosstalk and narrow beam divergence. Conventionally, a beacon beam is
necessary for system alignment due to multiple local maximums in the
mode-division multiplexed beam profile. However, the beacon beam introduces
excess system complexity, power consumption, and alignment errors. Here we
demonstrate a beaconless system with significantly higher alignment accuracy
and faster acquisition. This system also excludes excess complexity, power
consumption, and alignment errors, facilitating simplified system calibration
and supporting a record-high 5.14 Tbit/s line rate in a single-wavelength
free-space optical link. We anticipate our paper to be a starting point for
more sophisticated alignment scenarios in future multi-Terabit mode-division
multiplexing free-space optical communications for long-distance applications
with a generalised mode basis.
