Spatial Diversity for Optical Ground-to-Satellite Uplinks and for Laser Line of Sight Links based on Incoherent Spatial Mode Multiplexing

Authors: Thibault Michel, Pu Jian, Olivier Pinel, Guillaume Labroille

Cailabs, Rennes, FRANCE

Abstract

Free-space optical communications offers high throughput, licence-free communication in use cases where optical fiber links are not available. However, laser communications performance suffers greatly from the effects of atmospheric turbulence, inducing scintillation, beam wandering, and ultimately link fading and bit error rate deterioration.

One technique to mitigate these effects is to use multiple uncorrelated laser transmitters, resulting in signals transmitted through different fading channels. Conventionally, spatial diversity schemes are based on the emitters being transmitted through multiple telescopes or multiple sub-pupils of the same telescope. However, these schemes present challenges in the implementation, as it may be difficult to co-align a large number of emitters.

In this paper, we present a new technique for implementing spatial diversity, based on multiple colinear orthogonal spatial modes, such as Hermite-Gaussian modes, instead of parallel Gaussian beams. These colinear spatial modes are generated by an incoherent beam combiner based on Multi-Plane Light Conversion (MPLC), thus a single device with no co-alignment to perform for the user. We show simulation results of the fading improvement obtained by this spatial mode diversity scheme in various configurations, such as ground-to-LEO, ground-to-GEO and ground-to-ground links.