On the Impact and Mitigation of Turbulence in Fiber-Coupled FSO Systems

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.