# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 3 Recently Announced Submarine Cable Systems
### Experimental Demonstrations That Show a Glimpse to Possible Future Evolution  
Many important submarine cable experiments have been performed so far. In [14], [PERSON], [PERSON] et al. demonstrated a transmission of 200-G carriers with 60\({}^{+}\) Gbaud each over 17,680 km in distance. Such a cable aims to operate in the linear regime of an SDM-optimized span length. The spectral efficiency (SE) was in the order of 2.91 b/s/Hz resulting at a rate of 2.67 b/s/Hz with adequate safety margins. This demonstration seems to win the gold medal for the longest baud-rate (>60 Gbaud) channel transmission. An 8.12 Tb/s long-haul transmission over a transoceanic distance of 9750 km using 100 Gb/s 8D coded modulation, minimizing nonlinear transmission penalty, and resulting in a high-power efficiency of 54.8 Pb/s \(\times\) km/W was demonstrated in [15]. Also, we have another gold medal for reaching a capacity of 24 Tb/s over 6644 km transatlantic cable system. The system used 16 QAM synthesized subcarriers, forward error correction (FEC) gain sharing and multi-carrier wave locking [16]. The work in [17] surveyed the evolution and the future of submarine technology. The authors concluded that the maximum number of fiber pairs (FPs) that can be efficiently positioned in a submarine cable is approximately 30. As current WDM systems may face the capacity crunch soon, it is rather obvious that new multiplexing technologies must be adopted. Enriching the previous findings, authors in [18] reported a 38% capacity increment by using 24 FPs placed in a 4-core MCF. The work in [19] spotted the design issues and construction processes of submarine cable systems. Authors discussed all steps for construction of a submarine optical network: proper route selection, branching units' selection, topology design, type of cables, defining undersea housing and proper spacing for the repeaters selecting the proper PFE' architecture, organizing ship operations, and finally selection of the proper submarine terminal equipment (SLTE). The latest advances include specialized WSSs for submarine applications (recently announced for commercial use). These WSSs can distinguish every wavelength of each incoming fiber and then switch it to the appropriate output port of add-drop multiplexer (OADM) subchannels at a granularity of 3.125 GHz [20]. The use of WSSs in terrestrial ROADMs [21] and submarine OADMs [22] solves several design issues. Other basic components of submarine OADMs are the branch unit (BU) and the wavelength management unit [13]. Optical amplifiers are of extreme importance in the submarine environment. Topics that are under investigation are the efficiency of cladding pumping EDFAs and the ability  
Figure 14: Submarine cable system capacity \(\times\) cable length through time for 70 different cable systems. of PFEs to provide power to the submerged equipment over ultra-long distances (in the order of thousands of kilometers). Amplification efficiency of cladding pumping depends on core density (thus the ratio of the number of cores by the cladding area) whereas core pumping will improve by using the pump light to support multiple optical fiber cores. All recently announced pumping schemes for SDM submarine networks were described in [13, 23]. A new and promising technique is the pump recycling [24], explained below (in Section 4.7). A detailed view of amplifiers technology follows in Section 4. Benchmarking of submarine architectures are of a great interest during the design phase of a submarine network. The layout of submarine architectures and topologies can be designed considering three categories: point-to-point, trunk and branch, and connecting ring [13]. The basic design decision, about which option to adopt, is the distance between land stations. The use of BUs may be rather limited if the distances between candidate land stations are relatively short (i.e., closed seas, such as the Mediterranean Sea).