System calibration for single-wavelength multi-terabit mode-division multiplexing free-space optical communication systems

In this paper, we propose a optical calibration strategy to support a single-wavelength mode-division multiplexing multiple-input multiple-output free-space optical communication system utilising 6 Hermite-Gaussian modes. To minimise the inter-mode crosstalk, higher-order modes at the receiver were exploited to calibrate the optical setup. The signal distortions were minimised for 80 GBaud signals by employing Volterra pre-distortion. Probabilistically-shaped 256-QAM signals were transmitted to maximise the achievable information rate. As a result, this system obtained a record-high achievable information rate of 4.49 Tbit/s/<i>λ</i> in free-space optical communications.