The future of wireless communication requires the unique capabilities of free-space optics (FSO). Therefore, it is crucial to develop methods for achieving reliable long-distance FSO communications. This study presents a field trial of a 1.8 km FSO link that can achieve 4 Tbps+ using coherent optics, wavelength division multiplexing (WDM), atmospheric turbulence mitigation, and optimized forward error correction (FEC) coding. Our study focuses on the impact of atmospheric turbulence on FSO communication and the methods used to mitigate its effects. We compensate for the turbulence-induced power fluctuations by using an optical pre-amplification technique with automatic power control (APC) to stabilize the received power. This technique reduces the perceived Rytov variance by a factor of ten, making the FSO communication more reliable and efficient. Furthermore, we explore the maximization of net bit-rate by optimising the transmitted channels' FEC overhead, considering two different architectures: individual and joint wavelength processing. The proposed optimization techniques are shown to provide significant gains both in terms of capacity and reliability, making FSO technology a more practical solution for long-range wireless communication.

The range of applications enabled by the low-Earth orbit (LEO) constellations continues to increase, going from providing worldwide broadband internet access, to the transmission of enormous quantities of data gathered from space sensing and exploration. Therefore, it is imperative to find a technology for Earth-satellite and inter-satellite links that is able to provide ultra-high capacity and reliability. The alliance of coherent optical systems with free-space optics (FSO) can be considered a solution for these links, bringing the ultra-high capacity of fiber optic transmission systems to the wireless domain. However, the free-space channel brings some challenges to the telecommunication system, namely the atmospheric turbulence that can hinder data transmission, mainly when associated with typical satellite communication features such as the Doppler effect. Driven by this challenge, in this work, we propose a technique for mitigation of the impact of Doppler effect by dynamically adapting the transmitted signal symbol-rate while keeping the system bit-rate fixed through probabilistic constellation shaping (PCS). This methodology significantly increases the range of supported frequency shifts imposed by the Doppler effect in an experimental 600 Gbps transmission. Moreover, we address the reliability problem imposed by atmospheric turbulence, considering two different scenarios: i) a continuous transmission system with guaranteed reliability and ii) a bursty transmission without reliability constraints that solely aims at the maximization of the overall throughput of the system. In both scenarios, we show that the proposed methodology of adapting the system symbol-rate together with the transmitted constellation shaping can increase the supported bit-rate by more than 70 Gbps.

The evolving requirements of next-generation mobile communications networks can be met by leveraging vertically deployed Unmanned Aerial Vehicle (UAV) platforms integrated with Free Space Optical communications (FSO). This integration offers a flexible and scalable architecture capable of delivering high-rate communication without requiring licenses while aligning with the multi-gigabit paradigm. In recent times, the increasing availability of commercial aerial platforms has facilitated experimental demonstrations of UAV-enabled FSO systems, which play a crucial role in proposed backhaul networks and point-to-point communications by overcoming Line-of-Sight (LOS) challenges. These systems can be rapidly deployed to meet sudden demand scenarios. This document provides a comprehensive review of relevant field demonstrations of UAV-enabled FSO relay systems, with a particular focus on commercially available, free-flying platforms that are driving advancements in this domain. It categorizes the different platforms by considering the operational altitudes of these systems and their payload actuation capacity, which determines their adaptability to variables. The analysis aims to distill the design considerations that lead to optimal performance regarding communications throughput and other relevant metrics. Moreover, it also attempts to highlight areas where design choices have fallen short, indicating gaps in current research efforts toward the widespread adoption of UAV-enabled FSO relay systems. Finally, this work endeavors to outline effective design considerations, guidelines, and recommendations to bridge these identified gaps. It serves as a valuable reference guide for researchers involved in developing UAV-enabled FSO relay systems, enabling them to make informed decisions and pave the way for the successful implementation of such systems.

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.

Fiber-coupled free-space optics (FSO) communications have emerged as a promising wireless technology due to their flexibility, speed, security, and seamless integration with fiber optic systems. However, these systems are very sensitive to atmospheric conditions, particularly turbulence, necessitating robust and tailored solutions according to the application scenarios. However, the intrinsic random nature of atmospheric turbulence, together with the strong dependence on multiple external factors (e.g. varying weather conditions), makes it extremely challenging to adequately compare the merit of different turbulence mitigation techniques. In this work, resorting to the use of a custom-made atmospheric chamber, we tackle the issue of turbulence-induced power fading employing two complementary optical mitigation techniques: i) through enhanced fiber coupling efficiency using either standard single-mode fiber (SSMF) or multi-mode fiber (MMF) and ii) through tailored optical pre-amplification using either Erbium-doped fiber amplifiers (EDFAs) or semiconductor fiber amplifiers (SOAs). By carrying out a comprehensive set of repeatable and inter-comparable turbulence emulation tests, we demonstrate that MMF-based fiber coupling can be highly advantageous for less demanding FSO links, achieving 100% reliability after a real-time BER assessment at 4.5Gbps over weak-to-moderate turbulence, which represents a major improvement over the ~10-50% reliability obtained under the same circumstances for the SSMF-coupled system. In turn, for FSO links requiring high-capacity SSMF-coupled receivers, we demonstrate that EDFA-based pre-amplification operating either in saturation or automatic power control (APC) enabling 10 Gbps connectivity with 99% reliability in weak turbulence and ~96-98% in moderate turbulence regimes, in contrast with the baseline reliability of unamplified systems in the same turbulence conditions, which have shown <50% and <10% reliability with weak and moderate turbulence, respectively. Although also highly effective in mitigating turbulence-induced power fades, SOA-based pre-amplification was found to fall short of its EDFA counterpart, mainly owing to its degraded noise figure and nonlinear response, leading to reliability performance in the range of ~70-90%, over the same turbulent conditions.

We propose an in-line wavelength selective core switch for multicore fiber (MCF) transmission systems, based on the acousto-optic effect. A theoretical model addressing the interaction between flexural acoustic waves and the optical signal in MCFs is developed. We show that an optical signal propagating in a particular core can be switched to any other core or distributed over all the cores. By tuning the acoustic wave amplitude, we can adjust the amount of optical power transferred between the cores.