# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 4 Submarine Amplifiers
### Experimental Demonstrations of Submarine SDM-Based Amplifiers at Transoceanic Distances  
Most long-haul transmission experiments that adopt optical amplifiers can be divided into three main categories: circulating loops, test beds, and finally free trial or specific measurements performed on deployed systems. The first category's evaluation was held in 1991 to examine the feasibility of optical amplifier transmission systems. On the other hand, loop transmission experiments are a useful tool for optical amplifier feasibility demonstrations, measuring the BER of long, pseudo-random data patterns introduced by [PERSON] et al. in [60, 61]. Later (1992 to 1993), long amplifier chains were developed for laboratory use as a test bed for establishing design parameters and feasibility of monitoring systems concepts for submarine components. \begin{table}
\begin{tabular}{c c c c c} \hline
**Fumping Scheme** & **Acronym** & **Gore Fumping** & **Cid Fumping** & **Comments** \\ \hline Individual Core Pumping & ICP & Yes & No & _Reference_ \\
**Shared Core Pumping** & **SCP** & Yes & No & _With 3 dB coupler_ \\ \hline Variable Shared Core Pumping & VSCP & Yes & No & _With tunable coupler_ \\ \hline
**Common Cladding Pumping** & **CCP** & **No** & **Yes** & _Need of core attenuation_ \\ Hybrid with Individual & **HICP** & **Yes** & **Yes** & **-** \\
**Core Pumping** & **HSCP** & **Yes** & **Yes** & _With 3 dB coupler_ \\ \hline
**Hybrid with Variable Shared** & **HVSCP** & **Yes** & **Yes** & _With tunable coupler_ \\ Core Pumping & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \end{tabular}
\end{table} 
Table 8: Available Pumping Schemes [13]. Lastly, specific measurements or today's named free trials occurred on the first installed amplifier systems (1994 to 1995) in order to determine the feasibility of upgrading a system before or after its installation. In 1995, [PERSON] et al. [62] performed a 100-Gb/s transmission experiment in which 20 \(\times\) 5-Gb/s NRZ data channels were transmitted over 6300 km in 11.4 nm of optical bandwidth by using a gain-flattened EDFA chain. In this experiment, the usable EDFA bandwidth was increased by a factor of 3 by using long-period fiber grating filters ([PERSON], [PERSON], [PERSON], et al. in 1995) as gain equalizers. Afterward, the 100-Gb/s WDM experiment was eventually extended to 9300 km, adopting a low noise, 980-nm pumped amplifier chain. NRZ was the transmission format selected, combined with a synchronous polarization modulation and amplitude modulation ([PERSON] et al. in 1996). Authors in [38] demonstrated a 140.7-Tbit/s transmission consists of 7 \(\times\) 201-channel super-Nyquist 100-Gbit/s WDM signals over 7326 km by using 7-core EDFAs, achieving a record capacity distance product of 1 Exabit/s \(\times\) km. [PERSON] in [63] overviewed past and present similarities and differences in the requirements of submarine and terrestrial amplifiers. He studied the impact of these requirements in the design of submarine amplifiers and finally discussed future possibilities. In [64], [PERSON] et al. experimentally found the optimal SE for a 12-core fiber, SDM-based transmission using EDFAs. Their findings showed the optimal SNR and SE values that can maximize the power efficiency of a power limited SDM-based transmission system. Also, their experimental results showed that the optimum SNR and SE do not depend on system parameters (i.e., the transmission distance) and a system operating at the Shannon limit is most power efficient at SNR \(\approx\) 1.72 and SE \(\approx\) 2.89 b/s/Hz. A long-haul MCF transmission of 2520 km using cladding-pumped 7-core EDFAs was experimentally demonstrated in [65]. Authors confirmed that 73 \(\times\) 128-Gbit/s Nyquist-pulse-shaped dual-polarization QPSK signals were successfully transmitted in the above distance. [PERSON] et al. in [43] demonstrated the potential of high core-count EDFAs to support high-capacity transmission over transoceanic distances with SDM integration.