# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends  
[PERSON]  
1 Department of Electrical and Computer Engineering, University of Patras, 26504 Patras, Greece;  
[EMAIL_ADDRESS] (D.U. ); [EMAIL_ADDRESS] (I.T.) Department of Science & Technology, Hellenic Open University, 26335 Patras, Greece;  
[EMAIL_ADDRESS]  
the anticipated capacity crunch of the coming decades. It makes use of multiple spatial channels which can be physically placed in bundles of standard single-mode fibers (Bu-SMFs), as well as in multi-core fibers (MCFs) and multi-mode fibers (MMFs) [4]. MCFs and MMFs are currently under investigation for future use as they not only multiply the capacity of optical networks but also achieve lower costs and power consumption (per transmitted bit) [3]. From this point of view, SDM can provide modular capacity scalability, energy reduction, and improved reliability in submarine cable systems [5]. One major problem in long-haul submarine networks is power feeding, as power can be provided only from coasts and in general is an expensive resource. SDM is a promising candidate to efficiently solve the critical power-feeding problem. The first generation of SDM-based optical submarine systems utilizes space parallelism (parallel fibers in a cable) and specific high-power voltage (in the range of 18 kV end-to-end) [3] in order to achieve an increased system capacity and power resource optimization. SDM in principle is a system that incorporates at least one subsystem, being a transmission fiber, an amplifier, and a switching node, or terminal equipment, which implements the concept of \"spatial integration of network elements\" [6]. For example, for a Bu-SMF to be considered as an SDM, it needs to incorporate at least a pump-sharing scheme in the optical amplifiers/repeaters. Therefore, the first SDM generation is mainly based on novel pump-sharing technology and a high count of FPs (>8) in the cable [3,4]. The FPs use optimized fiber characteristics such as reduced coating fibers (RCF), (200 \(\mu\)m) [4], that allows for higher fiber density within the cable. The use of multiple spatial channels and pump-sharing schemes results in reduction of cost/bit and power/bit, while providing the obvious modular capacity scaling. In the future, the expected second generation of SDM will use multiple spatial channels physically placed in MCFs and/or MMFs, constituting Bu-SMFs, and taking advantage of the formers' impressive capabilities as already demonstrated in experimental studies [3,4]. The contribution of this work is twofold. First, the time for SDM in submarine environments has come; therefore we review in detail all recent developments and announcements about the forthcoming deployments and discuss the ways that SDM will make inroads into the submarine networks' space. More specifically, we focus on the most important submarine networks' issues: systems' capacity, amplification, architecture, power, economics, and security. In addition, we analytically explore all SDMs' special features that will enhance submarine networks' transmission capacity and efficiency. Second, we focus on the pros and cons of different possible alternatives and discover the exact capabilities of all recently presented SDM-based technologies especially designed for the submarine environment. We explain why these new capabilities have led to the first SDM deployment in submarine networks. Finally, based on all these findings and on our own study findings, we present all possible future directions mainly in the fields of fiber types, fiber counts per cable, fiber-coating variants, modulation formats, and the type and layout structure of optical amplifiers that are expected to lead to the second generation of SDM submarine deployments. The article is organized as follows: Section 2 discusses the basics of submarine optical networks and reports some of the most important achievements. Section 3 overviews all recently announced submarine cable systems.