Modelling the effects of adaptive optics in optical feeder links for a time-efficient power vector generator

Abstract

Free-space optical links performed between ground and space are impaired by the presence of the atmospheric turbulence. Stochastic processes such as scintillation and beam wander cause highly dynamic power outages, also referred to as fades, which can have a detrimental impact on the receiver performance. Furthermore, especially in case of coherent communications, losses due to the coupling of the received light into an optical fibre constitute an additional source of link performance degradation. In order to correct wave-front distortions and achieve an enhanced fibre coupling loss, state-of-the-art solutions for optical ground stations include the presence of an adaptive optics system. It is hence straightforward that an end-to-end ground-to-space channel model, used to assess the overall performance of the designed optical link, must include the effects of an adaptive optics system and its impact on fibre coupling. This work proposes to model the mitigative effects of the adaptive optics system as a process completely characterized by its statistics and frequency response. The statistical nature and dynamics of the corrected fibre coupling process in downlink and of the pre-distortion process in uplink are derived from a sufficiently large set of simulations performed with a validated phase screen propagator. The motivation behind this endeavour is that despite the extraordinary advantages of phase screen simulations, computational time and effort for a single link are significant. This is a limitation when it is required to carry out link performance assessments of large-scale space- and ground-based optical networks. Finally, by integrating the derived model in a previously developed and validated power vector generator, the resultant tool is presented as a holistic and modular solution to assess the performance of optical feeder links in an efficient manner. This ultimately enables fast iterations in the design of communications systems as well as in physical layer coding strategies.