Beaconless Auto-Alignment for Single-Wavelength 4.18 Tbit/s Mode-Division Multiplexing Free-Space Optical Communications

Mode-division multiplexing has shown its ability to significantly increase the capacity of free-space optical communications. Accurate alignment of transmitter and receiver is crucial in maintaining performance, because a sub-optimal beam alignment will induce modal crosstalk. 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. Besides improving energy efficiency, the beaconless method also simplifies the implementation and calibration of pointing, acquisition, and tracking systems, enabling a record-high achievable information rate of 4.187 Tbit/s in a single-wavelength free-space optical link.