Bertrand Denolle, Gauthier Trunet, David Allioux, Pu Jian, Olivier Pinel, and Guillaume Labroille
Free space optical links offer secure and mobile high-data-rate communication. However, the availability and reliability of long-range links are restricted by turbulence-induced fading. One way of mitigating turbulence is to use spatial diversity. However, classically-used multi-aperture systems often present a large footprint that is detrimental to their implementation. In this work, we show that using a spatial mode demultiplexer enables a significant increase in signal collection at the receiver in the presence of strong atmospheric turbulence while requiring only one telescope and being compatible with standard single-mode fiber (SMF) based telecom components.
A Gaussian beam propagating through the atmosphere suffers from perturbations which modify the spatial mode of the beam and therefore causes severe fading of the signal when coupling into an SMF. These beam perturbations can be decomposed on a limited number of orthogonal spatial modes: we show that 3 to 15 Hermite-Gaussian modes are sufficient to cover most high order turbulence effects.
The Multi-Plane Light Conversion technique enables to efficiently implement a spatial mode demultiplexer designed to take the selected perturbation modes in input (coupled in free-space or in a multi-mode fiber) and convert them into SMFs at the output. By detecting then combining these output signals, one can passively collect the full energy of the perturbed beam, therefore strongly increasing the coupling efficiency under turbulent conditions. We demonstrate that this approach offers a similar level of fading mitigation compared to a multi-aperture approach, using uncorrelated spatial mode channels instead of uncorrelated single-mode paths.