Demonstration of Coherent Beam Combining for Atmospheric Free Space Optical Communication over 10 km

We report the demonstration of a novel free space optical (FSO) communication scheme utilizing transmitter-side coherent beam combining (CBC) based on an optical phased array (OPA) for adaptive atmospheric turbulence compensation. Coherent combining of 32 amplified laser elements, spatially arranged in a 2D rectangular array, was performed in free space by actuating 32 phase modulators using a high-speed embedded controller. The phase of each element was adjusted in a real-time closed loop to maximize the received beam intensity, thus mitigating turbulence-induced impairments such as beam wander and scintillations. The CBC system was experimentally evaluated over retroreflected FSO links of 2 km and 10 km distance. Alongside a considerable increase in on-axis irradiance, beam steering and tracking abilities were also demonstrated, allowing the main lobe to steer within a spot with a diameter five times wider. In 10 km, CBC pre-compensation proved crucial for detecting the reflected signal, indicating for a considerable gain for atmospheric effects mitigation. A CBC-aided FSO link over 10 km was successfully established, achieving single-channel and polarization data rates of 64 Gbit/s and 100 Gbit/s, with availability rates of 96% and 77%, respectively.