# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 6 Submarine Power Feeding  
Submarine cable systems extend to ranges from hundreds to thousands of kilometers. Power feeding in short extended systems is straightforward, but for long-haul systems it is complicated. Long-haul systems incorporate large numbers of repeaters (one every 75-100 km [3]) and must ensure that all of them obtain power in a certain range of volts. Unfortunately, power experiences losses over long distances. The use of very high voltage power systems (to compensate for the losses) is not an option as such voltage may exceed the maximum voltage specifications of the equipment (cables, repeaters/amplifiers) and damage it. Typical voltages of PFE are in the range of 12-18 kVs and more recently 20 kVs [3]. [PERSON] in [71] presented design methods that maximize the submarine systems' capacity considering PFE limitations. Optimization of the optical submarine system based on the Gaussian noise (GN) model is accomplished either by seeking for the optimum span loss that minimizes the PFE voltage or by seeking the span loss that maximizes the capacity for a given PFE and optical to signal-to-noise-ratio (OSNR). [PERSON] et al. in [22] and [PERSON] et al. in [72] initially reported some experiments made for long-haul SMF experiments. These experiments showed that certain amount of power can be spread over a wider range of bandwidth (or over a wider range of space dimension), and can increase a system's capacity. Moreover, to optimize power efficiency an appropriate modulation format must be selected. Moreover, the authors noted that the optimal system capacity is not achieved at the optimal efficient power utilization region. Later experiments were based on a 12-core MCF, reaching 105.1 TB/s over a 14,350-km, 22-nm bandwidth EDFA, and 8D-APSK modulation operating at SE of 32 bit/s/Hz were also incorporated. Results revealed that SDM-based MCFs managed to achieve higher capacities than those achieved by SMFs without the need for extra power. So, as SDM seems promising, the next dilemma is how to allocate the PFEs' power to several SDM dimensions and find the range of SE values that can lead to maximum capacities. Experiments showed that maximum capacities can be achieved in specific values of SE. The authors concluded that SDM can provide the increased capacities and at the same time manage the power efficiently. These two SDM characteristics are especially crucial in long-haul submarine systems. SDM in submarine networks is evolving not only because of the upcoming deployment of MCFs/MMFs but also because of its ability to handle power more efficiently. The Shannon formula dictates that cables' capacity increases logarithmically with the signal-to-noise (SNR) ratio. On the other hand, incorporating multiple numbers of fibers (either in an MCF or an MMF or bundles of SMFs layout), capacity increases linearly by the number of fibers. In long-haul submarine systems (where power is not and cannot become unlimited), SDM can lower the power demands on each FP and feed power to more FPs. In this way, we may achieve higher capacities with the same amount of power. Optical amplifiers in submarine systems use an amount of power for their own processes and then convert power to optical energy to boost fiber-optic signals. The optical power that amplifiers feed fiber-optics defines the electrical to optical power conversion efficiency (EOPCE). As EOPCE determines the number of FPs that can be powered, it also determines the total capacity of a cable system. Moreover, as pool pumps are evolving in the submarine era, amplifying several FPs get more complicated, and so the exact definition and attributes of the EOPCE must be investigated. [PERSON], [PERSON] et al. in [73] analyzed EOPCE in SDM submarine cable systems through a model. The model considered the repeaters topology, the power transfer model, several measures of pump laser efficiency, and simulation results of amplifiers conversion efficiency. Results showed that maximum pump EOPCE can be achieved from a specific range of input pump power.