Turbulence Mitigation via Multi-Plane Light Conversion and Coherent Optical Combination on a 200 m and a 10 km Link

Abstract

Current LEO satellites are generating more and more data which needs to be brought back to Earth for processing and analysis. Robust optical communications links are compulsory to be able to follow this growing demand for high-speed downlink with targets ranging from 10Gbps to 1Tbps. Atmospheric turbulences compensation is a key element to achieve such throughput. Here we investigate the capacity of turbulence mitigation via the use of a MPLC followed by an active optical recombiner, also called Tilba-Atmo. The MPLC demultiplexes the incoming turbulent beam into a set of gaussians beams whose relative energy distribution and relative phase evolves according to turbulence fluctuations. These gaussians are then sent into an active system based on a photonic integrated chip where the channels are optically recombined two-by-two in separate Mach-Zehnder interferometers. An 8-HGmode MPLC was placed at the reception end of a free space optical link in C or L-band. Different configurations where tried such as different levels of turbulence. Two different link lengths were tested, a 200m link with a 20cm telescope and a 10 km link at the DLR with a 10cm telescope which is normally use for testing and developing adaptive optics solutions. The first link showed high level of phase degradation on the received beam whilst on the second link the main effect of atmospheric turbulence was scintillation inside the pupil. In both cases tip-tilt was compensated via an auxiliary system and was not implemented inside the Tilba-Atmo component. The results are focused on comparison of fading in CW condition and in OOK communication between an SMF channel and the Tilba-Atmo channel. As this first version Tilba-Atmo is contributing to optical losses and optical recombining might be limited by the control electronics, a numerical sum of the demultiplexed modes is also performed to determine the upper limits of such a system.