# 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
\begin{table}
\begin{tabular}{c c c} \hline
**PROS** & **CONS** \\ \hline \hline \multicolumn{2}{c}{Lower capacity per FP, as FP} \\ \multicolumn{2}{c}{becomes consequently the new granularity} \\ \multicolumn{2}{c}{**C-Band**} & \multicolumn{1}{c}{} \\ \multicolumn{2}{c}{FP switching used to drop a whole} & \multicolumn{1}{c}{} \\ \multicolumn{2}{c}{FP in a branch} & \multicolumn{1}{c}{} \\ \multicolumn{2}{c}{Easier to sell FP} & \multicolumn{1}{c}{} \\ \multicolumn{2}{c}{Easier to swap FP} & \multicolumn{1}{c}{} \\ \hline \multirow{6}{*}{**(C + L) Band**} & - & Less efficient as MUX/DMUX \\  & & should be used (\(\sim\)1 dB losses) \\ \cline{3-4}  & - & Attenuation is slightly higher in L band (beyond 1600 nm) \\ \cline{3-4}  & Limits the number of FP in the cable & Interband effects between C and L bands \\ \cline{3-4}  & No need to develop big cable to & Spectrum sharing needed to \\  & contain all the FFs & sell/swap a portion of a FP \\ \cline{3-4}  & - & Higher cost for BU with WSS to \\  & & manage spectrum sharing \\  & - & L band amplifier needed \\ \hline \end{tabular}
\end{table} 
Table 2: Compare SDM with other non-SDM solutions [8]. Figure 6: Different types of submarine cables. 4.2 Multiple Spatial Channels in SDM: MCF (Multi-Core Fibers)-MMF (Multi-Mode Fibers)-Bundles of Single-Mode Fibers (Bu-SMFs)  
Regarding the number of optical spatial channels in a cable system, three options are considered for submarine SDM deployments:  
* Multiplying the number of conventional fibers (thus implementing a parallelism that consists of single-core/single-mode fibers), considering the existence of at least one element that performs spatial integration, e.g., an amplifier with sharing pumps, a switching node, or terminal equipment. * Multiplying the number of cores in MCF fibers. * Multiplying the number of modes in MMF fibers. A novel fiber option for adoption in the subsea environment are MMFs, which have more friendly production compared to MCFs but are expected to induce higher attenuation compared to their SMF counterparts. MMFs can be used in SDM terrestrial networks, supporting 3, 10, 15, or even more guided modes [9]. MMFs' major disadvantages are modal dispersion, interference, and high values of differential mode group delay (DMGD) which leads to poorer physical layer performance compared with SMFs. A major challenge for their use in transoceanic networks is to solve their mode-coupling complications. On the other hand, MCFs, which consist of multiple cores in one cladding, can be used in SDM terrestrial networks, thereby achieving transmission speeds at the order of petabits/second. MCFs suffer from extra loss from the use of fan in-out devices and possibly higher typical attenuation (compared to SMFs). MCFs and MMFs were not mature enough in 2016/2017 when the SDM submarine solution was first deployed [8]. Researchers expect that MCFs could be mature in the medium term. So, the first SDM-based system proposed by ASN uses multiple conventional fibers (bundles of parallel SMFs) which, as already mentioned, is considered to be the first generation of SDM submarine technology. Although SDM deployment in terrestrial systems is strongly favored with MMF and MCF systems, in the submarine era the requisite stronger, waterproof coatings limit the number of fibers that can be packed in a single cable. Figure 7 pictorially describes various types of fibers. Bu-SMFs incorporate a number of SMFs within a single cable. Furthermore, MMFs can support tens of guided modes whereas FMFs are manufactured to consider propagation of less guided modes. Finally, coupled core (CC) fibers consider strong mode coupling between the different cores, allowing them to attain a shorter core-to-core distance and higher spatial density compared with the uncoupled MCFs. Figure 7: Different types of fibers. Recently, thinner coatings have been tested in an effort to leave more space for fibers. Most commercial submarine cable systems up to now use bundles of SMFs which are a reliable and tested platform.