Fast and reliable beam steering technologies for free-space optics systems

• Autor(a) principal: Freitas, Manuel José Matias de
• Data de Publicação: 2022
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/39007
• topic: Free-space optics; Optical beam steering; Acquisition tracking and pointing mechanisms; Pointing errors; Multi-mode fiber; 5G

With the ever-increasing demand for higher data rates and link capacity, signal distribution through high-capacity links becomes a crucial requirement. On top of that, the radio frequency spectrum congestion allied with the wireless connections’ intrinsic limitations requires alternatives. Free-Space Optics (FSO) communications appear as a very attractive solution for ultra-high-capacity fiber-wireless transmission in indoor and outdoor networks, for applications going from 5G and beyond, mainly due to its high optical bandwidth availability, high-security levels, and for being low-cost, and ease of deployment. If the FSO beam is directly collimated in an optical fiber, the received signal can be transmitted and routed under the conventional fiber-based network taking advantage of currently available optical communications systems and subsystems. However, this Line-of-sight (LOS) technology poses some constraints associated with atmospheric turbulence, beam divergence and pointing errors. Pointing errors is an adverse effect that mainly results from the misalignment between the transmitter and receiver, degrading the FSO link. Therefore, highly accurate acquisition, tracking and pointing (ATP) mechanisms will play an important role in FSO systems performance. This thesis aims to develop a multi-stage ATP system capable of providing bidirectional alignment. To accomplish that, gimbal-based ATP mechanisms are explored, starting with the characterization of technologies that can enable its full potential: i) quadrant-detector sensor, used for tracking and alignment of the FSO optical beam; and ii) power-based characterization of the light coupled in the optical fiber. The tolerance to pointing errors when using multi-mode fibers (MMFs) instead of single-mode fibers (SMFs) in seamless FSO-fiber links is also an object of study, with MMFs showing greater resilience mostly due to their higher core and numerical aperture. In that regard, algorithms for coarse and fine-tuning alignment are a cornerstone of this thesis. The last section demonstrates the advantages of using a cost-effective fiber-coupled multi-mode receiver over SMF coupling, registering a 2-fold bit-rate increase for a given 0.25 mrad FSO pointing error. The system capacity is enhanced through the use of an entropy loading technique based on probabilistic constellation shaping for multi-carrier signals, permitting to reach a bit-rate of ∼40 Gbps over 3m of free space distance. Finally, is analysed the interplay between beam alignment and multi-modal dispersion in the receiver-side MMF, by testing MMF lengths up to 1 km.