Requirements and Solutions for Robust Beam Alignment in Fiber-Coupled Free-Space Optical Systems

Abstract

The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous advantages, FSO also entails several particular challenges regarding the mitigation of the stochastic impairments induced by turbulence and the strict alignment requirements. One of the main issues of FSO communication systems is the mitigation of pointing errors and angle-of-arrival (AoA) fluctuations, which arise from misalignments induced by atmospheric turbulence and vibrations at the transmitting and receiving stations. A common approach to mitigate the impact of pointing errors is the use of an acquisition, tracking and pointing (ATP) system on one or both ends of the FSO link. In this paper, we present a characterization of the pointing errors and the AoA impact on the power budget of the FSO link to quantify the misalignment impairments. Afterwards, we experimentally demonstrate an FSO link with an ATP mechanism at both ends, managed by a control plane that enables the continuous and accurate alignment of the FSO link. To increase the misalignment tolerance, the ATP mechanism comprises two stages: the first one is based on a spatial diversity method provided by a quadrant detector, while the second stage maximizes the optical received power. Lastly, the impact of the beam misalignment on the achievable information rate of a coherent optical wireless system is theoretically addressed and characterized.