# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 4 Submarine Amplifiers  
### Overview and History Evolution of Submarine Amplifiers  
Signal amplification is one of the most important key parameters in telecommunication networks. In submarine networks, power stations and power amount are limited. Distances are in the order of thousands of km and so amplification plays the most crucial role in signal transmission. There has been a lot of progress on submarine amplifiers' evolution from the first generation (TAT-1 electric repeater which used only electric parts, most of them presented and pictorially described in Figures 15 and 16), through the fifth generation 32 FP pretabit-level optical amplifier (repeater, Figure 17), which is almost ready to release to service [30]. The submarine amplifiers evolution history starts with the first generation of submerged repeater as a main part of the cable TAT-1, the first transatlantic telephone cable (already characterized as an IEEE milestone).  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Reference** & **C (Tb/s)** & **L (Km)** & **SE (b/s/Hz)** & **Technology** \\ \hline
[26] & 71.65 & 6970 & **7.36** & non-SDM \\ \hline
[26] & 70.46 & 7600 & **7.23** & non-SDM \\ \hline
[16] * & 24.6 & 6664 & **6.21** & non-SDM \\ \hline
[27] & 30.58 & 6630 & **6.10** & non-SDM \\ \hline
[28] & 105.1 & 14,350 & **3.20** & SDM \\ \hline
[15] & 8.12 & 9750 & **3.20** & SDM \\ \hline
[14] & 12 & 17,680 & **2.91** & SDM \\ \hline
[29] & 9.0 & 15,050 & **2.00** & SDM \\ \hline \end{tabular}  
* Review the deployment of MAREA cable system.  
\end{table}
Table 6: Comparison of transmission experiments at transoceanic distances.  
Nowadays, for undersea operations there are two main types of submarine amplifiers. The first type is named the remote optical pump amplifier (ROPA) and is presented in Figure 17, and it is applied in unrepeated systems.  
ROPA is capable of amplifying the optical signal efficiently, increasing both transmission distance and cable system capacity and also compensating the optical signal. This type of amplifier can support signal amplification for different bands (C-Band, etc.) with up to 32 fiber pairs, which is the commercial systems' limit up until the present. Finally, it features a pressure-resistant, anti-corrosion, high-strength housing, and it is able to support underwater operations under big depths.  
A second type of fifth-generation submarine amplifier is presented in Figure 18. It is capable of supporting up to a 32 FPs system (not commercially existing at present), and was first applied in a 16 FPs system presented in Figure 19, aiming to satisfy the latest huge transmission demands.  
This type of amplifier embedded new innovative features such as a new electronic design and an innovative pump sharing scheme aiming to reduce voltage drop by 30%.  
Figure 16: First-generation sectioned submerged repeater for TAT-1 the first transatlantic telephone cable [1].  
Figure 17: Fifth-generation remote optical pumping amplifier [12].  
Figure 15: All element parts of the first-generation sectioned submerged repeater for TAT-1 the first transatlantic telephone cable [1].  
Pump sharing is a key concept for signal transmission over long transoceanic distances. It has the capability of providing 39.5 nm bandwidth, and so can enhance the maximum achievable capacity of existing systems. It can support up to a 160-km transmission of each segment in repeated submarine cable systems and up to a 16,000-km transmission in transcontinental systems [30]. Finally, this type of amplifier has an innovative design, ensuring stable operation at high water depths with an approximately 25-year lifetime.