Atmospheric turbulence mitigation in Free Space Optical communication systems with retro-modulators

Abstract

After a brief description of the Free Space Optical communication systems that has been developed for both direct mode and retro mode and which are used by the Naval Research Laboratory for their research on modulating retro-reflectors, this paper presents the implementation of an efficient cone-tracking algorithm for the optimization of the beam pointing in retro-mode links. It is shown how the algorithm can be augmented in order to support the same function in direct-mode links. Furthermore it is shown how this cone-tracking technique, implemented initially to compensate for the system misalignment and the slow drifts related to temperature or remote distance changes, can be used to characterize the beam wander in the far field. The results of a beam wander measurement experiment are then discussed. These results led to developing a multi-dithering adaptive optics algorithm to mitigate the effect of atmospheric turbulence in a retro-mode link. This algorithm works by optimizing the power coupled into the MRR and therefore reducing the fluctuations of the power reflected back to the Interrogator. This algorithm can be thought of as an extension of the cone-tracking technique to higher order aberrations. Finally, results are presented of a proof of concept laboratory experiment of this single-metric multi-dithering modal AO algorithm applied to the coupling into a single mode fiber. The results show the successful convergence of the simultaneous compensation of six low order aberrations. The conclusion is that using faster hardware these encouraging results could eventually lead to a significant reduction of the power fluctuation induced by the atmospheric turbulence in a FSO retro-mode link.