0733-8724 (c) 2021 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JLT.2021.3133070, Journal of Lightwave Technology JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. XX, NO. X, OCTOBER 2020 1 Free-Space Terabit Optical Interconnects Marco A. Fernandes , Paulo P. Monteiro , and Fernando P. Guiomar (Invited Paper) Abstract—The continuous growth of Internet data traffic is progressively increasing the pressure over wireless access technologies. To overcome the imminent bandwidth bottleneck, free-space optics (FSO) is currently deemed as a key breakthrough towards next-generation ultra-high-capacity wireless links. As this technology matures, it is starting to find its place not only in future mobile access networks, but also in applications such as datacenter interconnections, satellite communications and high-frequency trading. To meet the ever- increasing demand for high-capacity links, future-proof FSO systems should seek maximum compatibility with state-of-the-art optical fiber systems, which nowadays can already provide data rates per channel in the Terabit/s range. However, the ultra-wideband potential of FSO systems comes at the expense of very tight requirements in terms of optical beam alignment, so that reliable end-to-end communications can be ensured. Following this research challenge, in this work we demonstrate a dynamic single-wavelength 1 Tbps FSO link with high pointing error tolerance, boosted by adaptive probabilistic constellation shaping (PCS) and an active gimbal-based acquisition, tracking and pointing (ATP) mechanism. Our results demonstrate successful indoor FSO transmission over 3 m with enhanced resilience towards pointing errors, enabling to adapt the data rate in the range of 0.8–1 Tbps. Index Terms—Coherent optical interconnects, seamless-Free- Space-Optics, Terabit, probabilistic constellation shaping. I. I NTRODUCTION W IRELESS access has become a fundamental technology in today’s communication networks, delivering the portability and ease-of-access that wired links by definition cannot provide. Whilst typical wireless communications are hand-in-hand with radio frequency (RF), due to the congestion of the RF spectrum and the ever-increasing demand for capacity, future wireless communications are progressively shifting to the optical domain. In that sense, optical wireless communications (OWC) or free-space optics (FSO), have been emerging as a solution for a wide variety of scenarios, such as datacenter interconnects (DCI) [1], fifth-generation (5G) and beyond-5G (B5G) networks [2], campus-interconnection [3], satellite communications [4], quantum key distribution [5] or high-frequency trading [6]. The adoption of FSO This work was partially supported by FEDER, through the CENTRO 2020 programme, project ORCIP (CENTRO-01-0145-FEDER-022141), and by FCT/MCTES through project FreeComm-B5G (UIDB/EEA/50008/2020). Fernando P. Guiomar acknowledges a fellowship from “la Caixa” Foundation (ID 100010434). The fellowship code is LCF/BQ/PR20/11770015. Marco Fernandes acknowledges a PhD fellowship from FCT. The fellowship code is 2020.07521.BD. Marco A. Fernandes, Paulo P. Monteiro and Fernando P. Guiomar are with Instituto de Telecomunicações, University of Aveiro, 3810-193, Aveiro, Portugal. (e-mail:marcofernandes@av.it.pt) Manuscript received October XX, 2020; revised November XX, 2020. communications brings the potential of having ultra-high capacity wireless links in an unlicensed spectrum without bandwidth limitations. In addition, FSO provides improved security due to its high directivity and narrow beam, immunity to electromagnetic interference (EMI), easy and quick deployment, and the capability of quick recovery in disaster scenarios [7]. With the increase of bandwidth requirements, coherent optical transceivers, which were typically used solely for long- haul fiber links, are now being proposed for other applications [8], [9]. With the progressive appearance of commercial-off- the-shelf (COTS) optical coherent transceivers in the market, following the OIF standardization of 400G and ongoing 800G developments [10], it is more common to see this technology being applied in several short-reach scenarios. The mass production of these COTS transceivers will inherently drive the development of high-capacity optical coherent plugabbles with progressively smaller complexity, cost and footprint. The agreement between FSO and high-capacity coherent communications emerge when using seamless fiber-FSO links, where the free-space receiver consists of a collimator, capable of gathering the FSO signal and directly collimate it into the fiber-core. This architecture removes all optical-to-electrical (O/E) conversion at the FSO receiver, thus allowing to keep intact the full bandwidth inherent to optical fibers [11]. These characteristics potentiate the integration of seamless fiber- FSO links in any type of systems supporting high-capacity transmission, which might be required to transport ultra- wide bandwidth signals or signals multiplexed in polarization, wavelength and/or amplitude [12]. One notable example of the symbiosis between coherent links and FSO transmission is provided by NASA’s Terabyte Infrared Delivery (TBIRD) Program, which tackles low-earth-orbit (LEO) satellite-to- earth communications, through a 200 Gbps FSO-link powered by COTS coherent optical transceivers [13]. Nonetheless, the high-capacity of seamless fiber-FSO links, comes at the expense of increased sensitivity to pointing errors and angle-of-arrival (AoA), with these impairments being the ultimate bottleneck for indoor FSO-links. While for more power-relaxed scenarios active ATP systems can be used to mitigate for pointing errors [14], in other scenarios such as datacenter (DC) networks, the power consumption of an active ATP system is unbearable. In those scenarios, other techniques need to be implemented to compensate for the impact of pointing errors. Regarding outdoors scenarios, atmospheric turbulence and time-varying weather conditions, become the main bottleneck for FSO systems. In that sense, multiple techniques were already proposed to compensate for these impairments [15]–[17]. However, when addressing Authorized licensed use limited to: b-on: UNIVERSIDADE DE AVEIRO. Downloaded on January 21,2022 at 15:35:14 UTC from IEEE Xplore. Restrictions apply.