Dual Line Coherent Detection

We experimentally demonstrate dual-line coherent detection using an optical frequency comb local oscillator, enabling large frequency offset tolerance with minimal additional signal processing. The proposed method achieves 200 GHz offset tolerance for 400 Gbit/s signals with low penalty, supporting uncooled, low-cost coherent transceivers.

Beaconless Auto-Alignment for Single-Wavelength 5 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. 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.