# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 9 Can We Predict the Future in Submarine Networking? As subsea traffic demands have been growing rapidly, SDM is becoming a de-facto subsea technology promising to solve bandwidth-demand issues, avert the capacity crunch, and reduce the cost/bit. In this review, we presented and extensively analyzed all the latest technologies and innovations in the sector of submarine communications focusing on SDM technology. We both presented and analyzed the majority of the latest submarine cable systems (already existed or planned), examined their fundamental components' technology and combined this with up-to-date long-haul experiments (at transoceanic distances). As for the new submarine era presented in Figure 36, (e.g., for the evolution of undersea amplifiers), the history of which was full of innovations and milestones, we are expecting a promising future speculating that we can benefit from SDM technology which focuses on integrated elements with small undersea housing [22] and smaller output power, allowing us to share pumps across multiple amplifiers through techniques such as pump-farming amplification schemes. Figure 35: Dangers of submarine cable systems (in percentages). As for submarine cables, higher fiber counts of up to 50 parallel fiber pairs per cable [77], combined with cost-effective aluminum conductors and smart environmental sensors [87], will be a near-future concept achieving lower cost/bit compared to already existing systems. The trend is that SDM systems with thinner coating variants (200 \(\upmu\)m) will enable up to 50% higher fiber density [10] within an existing cable design. Ultra-low attenuations will play a crucial role, as every improvement (i.e., 0.001 dB/km) will lead to cost/bit reduction. An alternative ultimate future solution will be based on very high fiber count cables (enabled by the 200-\(\upmu\)m coating fiber presented in Figure 37) in combination with C + L band technology and ultra-low attenuation, but to our knowledge with lower possibilities than the SDM. As for multicore fibers, additional losses from fan in-out devices and likely higher typical attenuation compared to SC fibers, will reduce the achievable cable capacity as it is likely that the cost/per core of an n-core MCF will be higher than the cost of \"n\" individual fibers, but as new manufacturing techniques continue to evolve, MCFs can \"cut the ribbon\" and win its SC counterparts. On the other hand, ROADMs and eBUs supporting optical switching and flexible traffic routing of many trunks, combined with greater than 800 G transponders, high order modulation formats (\(\geq\)256 QAM) achieving high baud rates (200 Gbaud) will be a future aspect too. Moreover, the optimum spectral efficiency range (2-3 b/s/Hz) compared with traditional technology of (5 b/s/Hz) will be a future trend  
Figure 36: Evolution of submarine system’s technology. Figure 37: Comparison of recent and future fiber coating technology. too. Furthermore, future demands dictate that more electrical power should be launched into the submarine cables, by either increasing the voltage (more than 20 kV) or by using lower resistivity cable because recent research show that a 20% improvement of power output translates into over a 50% increase in system capacity by increasing the number of fiber pairs. These steps will be mandatory to achieve 500-terabit/s cables or even 1-petabit/s cables in the near future. Finally, security is a crucial parameter as vulnerabilities of submarine cables could make the entire global telecommunication to break down so new optical techniques are needed to solve present and future security issues [88, 89]. Our study concludes that SDM technology will be the most important candidate as it meets all key requirements to support the new submarine era. Conceptualization, C.P. ; writing--original draft preparation, C.P. and K.P. ; data curation, K.P. and D.U. ; writing--review and editing, I.T. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.