# 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
### SDM Transmission Technologies in Long Haul Transoceanic Systems  
#### 2.4.1 Features of SDM-Based Submarine Cable Systems  
The evolution of SDM in submarine networks started with Suboptic'16 (powered by Alcatel-Lucent Submarine Networks (ASN) Ltd., founded in 1983) which was the first 16-FPs submarine cable system [8]. Until recently, in all networks, the initial goal was twofold: to increase the total cable capacity (up to 70% with regard to traditional cable) and to decrease the required cost and power per transmitted bit. The innovative features that characterize the first generation of SDM submarine networks are [8] as follows. * A relatively high count of FPs (in the same cable) in order to increase the transported capacity. * The deployment of lower effective area fibers in order to optimize cost through the use of a smaller number of regenerators. * The implementation of the novel \"pump farming\" repeaters' technology. Pump farming means that a set of pump lasers issued to amplify a set of FPs. Reliability, redundancy, and better power management are the main advantages. In particular, reliability can be a cost-reduction factor as submarine cables' failures and repairs (bringing downtime in provided services) are very costly. * SDM aims to achieve higher capacities by using the same amount of used power through a more efficient power management. The key concept is to reduce the optical power provided to each FP as a way to decrease the nonlinearities as implementing high count of FPs in the same cable. However, in the event we want to cover small-distance undersea links (e.g., unrepeated-festoon networks) or if we want to increase the capacity of an existing traditional submarine cable system (consisting of a limited number of FPs), multiband transmission is a more effective solution compared with SDM, which is mostly preferable for long-haul distance links. More specifically, in multiband transmission there is no need to change the existing wet plant infrastructure during the upgrade process and this can result in both an increase (by double) of the capacity/FPs and in cost savings. Table 2 presents the pros and cons of using SDM over a single band as opposed to multiple bands. The options presented are doubling the number of fibers at C band only and using the same number of FPs over the C + L bands. Note however that the C + L transmission is less efficient because C + L has to be separated and recombined (mux/demux) in the repeater for each span. This extra multiplexing/de-multiplexing leads to an extra loss per span of about a few dBs, which is contrary to the \"optimizing efficiency\" basic SDM concept. Figure 5: Summation of existing and new cable performance metrics and indication of the exact location where each one of them is measured [7]. Figure 6 shows the different types of submarine cables and the various types of fibers, respectively. The selection of the optimal cable type depends on the depth at which each cable is sinked. For example, double-armored (DA) submarine cable is used at the shore end, terminated at the beach manhole of the cable landing site, and interconnected with a much lighter land cable (LWA) moving toward the cable landing station [3].