# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 2 Submarine Optical Networks Basics and Demonstrations Technologies on Submarine Cable Systems
### Important Milestones at Submarine Systems Evolution
\begin{table}
\begin{tabular}{c c c c c} \hline
**Year** & **Cable System** & **Key Technology** & **Capacity/Channels** & **Length** \\ \hline
1996–2008 & **TAT-12/13** & **EDFA technology** implemented & **2 \(\times\) 5 Gb/s** & **12,307 \(\mathrm{km}\)** \\ \hline
2001 & **TAT-14** & **Employs 4 fiber-pair (2 FPs active + 2 FPs backup) WDM technology with direct detection** & **3.2 Tb/s** & **15,428 \(\mathrm{km}\)** \\ \hline
2008 & **TPE** & **Employs 10 G DWDM technology** & **5.12 Tb/s** & **17,968 \(\mathrm{km}\)** \\ \hline
2013 & **PC-1** & **Employs 100 G coherent technology** & **8.4 Tb/s** & **21,000 \(\mathrm{km}\)** \\ \hline
2018 & **PLCN** & **First submarine cable employs C + L band optical technology** & **144 Tb/s** & **12,971 \(\mathrm{km}\)** \\ \hline
2021 & **Dunant** & **First submarine cable employs 12 FPs introduces first-generation SDM technology, pump lasers, pump-sharing technology** & **250 Tb/s** & **6600 \(\mathrm{km}\)** \\ \hline
2021 & **H2 HE** & **The world’s first 16-fiber-pair repeated submarine cable system. A milestone in technological innovation** & **300 Tb/s** & **675 \(\mathrm{km}\)** \\ \hline
2022 & **Peace** & **WSS ROADM BU, 200 G technology, SDM repeater** & **90 Tb/s** & **15,000 \(\mathrm{km}\)** \\ \hline
2022 & **Equiano** & **First submarine cable employs optical switching at the fiber-pair level (instead of wavelength-level switching)** & **200 Tb/s** & **15,000 \(\mathrm{km}\)** \\ \hline
2023 & **Confluence-1** & **First submarine cable employs 24 fiber-pair SDM technology and it will be the largest to be recently installed** & **>500 Tb/s** & **2571 \(\mathrm{km}\)** \\ \hline
2023 & **Ellalink** & **First submarine system to incorporate state-of-the-art ICE6 800 G coherent technology** & **100 Tb/s** & **6200 \(\mathrm{km}\)** \\ \hline
2023 & **Firmina** & **To be the world’s longest undersea cable capable of maintaining operations with single-end feed power by using 18-kV power technology** & **N.A** & **2500 \(\mathrm{km}\)** \\ \hline
2023–2024 & **2 Africa** & **2 Africa aims to be one of the largest submarine cable systems (46 landing stations, 45,000 km)** & **180 Tb/s** & **45,000 \(\mathrm{km}\)** \\ \hline
2025 & **Arctic. Connect** & **To be the first transarctic cable system with new innovative cable type and will connect three continents (85\% of total world population)** & **200 Tb/s** & **14,000 \(\mathrm{km}\)** \\ \hline
2024–2025 & **Apricot** & **To incorporate 400 G technology, all new submersible ROADM, flexible bandwidth management based on SDM-based design.** & **190 Tb/s** & **12,000 \(\mathrm{km}\)** \\ \hline
2025 & **SEA-ME-WE-6** & **Utilizes SDM cable, supporting up to 24 FPs and incorporates enhanced branching units (eBUs) providing flexible electrical power and optical fiber routing with shore-based telemetry control** & **126 Tb/s** & **19,200 \(\mathrm{km}\)** \\ \hline \end{tabular}  
* Characterized as IEEE milestone.  
\end{table} 
Table 1: _Cont._