**2.2 SDM-optical fiber transmission**

Various kinds of fibers are used for SDM communication systems. As indicated above, we divide them as CDM-fibers and MDM-fibers. Considering CDM, the first technology used as SDM fibers are based on the use of Single-core Fiber bundle (fiber ribbon) where parallels single mode fibers are packed together creating a fiber bundle or a ribbon cable. The overall diameter of these bundles varies from around 10–27 mm. Delivers up to hundreds of parallel links, fiber bundles have been commercially available [21, 22], and adopted in current optical infrastructure for several years already. Fiber ribbons are also commercially used in conjunction with several SDM transceiver technologies [23]. Another scheme is based on carrying data on single cores (single mode) embedded in the same fiber known as Multicore Fibers (MCF). Hence, each core is considered as an independent single channel. The most important constraint in MCFs is the inter-core crosstalk (XT) caused by signal power leakage from core to its adjacent cores that is controlled by core pitch (distance between adjacent cores denoted usually as ʌ) [24]. There are in Principle, two main categories of MCF: weakly coupled MCFs (=uncoupled MCF) and strongly coupled MCFs (=coupled MCF) depending on the value of coupling coefficient 'K' (used to characterize the crosstalk). Using the so-called supermodes to carry data, the crosstalk in coupled MCF must be mitigated by complex digital signal processing algorithms, such as multiple-input multiple-output digital signal processing (MIMO-DSP) techniques [25]. On the contrary, due to low XT in uncoupled MCF, it is not necessary to mitigate the XT impacts via complex MIMO, (see **Table 1**). In principle, three-crosstalk suppression schemes in uncoupled MCF could be incorporated, which are trench-assisted structure, heterogeneous core arrangement, and propagation-direction interleaving (PDI) technique [26].

The first paper on communication using MCF demonstrates a transmission of 112 Tb/s over 76.8 km in a 7-cores fiber using SDM and dense WDM in the C+L ITU-T bands. The spectral efficiency was of 14 b/s/Hz [27]. The second paper [28] shows an ultra-low crosstalk level (≤−55 dB over 17.6 km), which presents the lowest crosstalk between neighboring cores value to date. Other reported works, show high capacity (1.01 Pb/s) [29] over 52 km single span of 12-core MCF. In [30], over 7326 km, a record of 140.7 Tb/s capacity are reached. Considering MDM schemes, two types of fibers are dedicated to support that strategy. One is


#### **Table 1.**

*Classification and features of multicore fibers.*

based on the use of multimode fibers (MMF) while the second exploits the wellknown few-mode fibers (FMF). The main difference between both is the number of modes (available channels). Since MMF can support large number of modes (tens), the intermodal crosstalk becomes large as well as the differential mode group delay (DMGD), where each mode has its own velocity, hence reducing the number of propagating modes along the fiber becomes viable solution. This supports FMF as a viable candidate for realizing SDM [31]. **Figure 3** recapitalizes examples of SDM optical fibers.

Due to the unavoidable attenuation over the transmission operation (i.e. degradation of the spatially multiplexed optical signals powers), SDM optical amplifiers are essential for a long-haul space division multiplexing (SDM) transmission system. Two requirements should be fulfilled by optical amplifiers, which are the large mode gain and the small difference between gains over different modes. In principle, two types of optical amplifiers, optical fiber amplifier OFA (e.g. erbiumdoped fiber amplifier (EDFA [32]), fiber Raman amplifier (FRA)) and semiconductor optical amplifier SOA. Other approach is based on electro-optical repeaters or regenerators where the amplification process is performed in electronic regime [33]. A repeater is consisting of optical receiver (i.e. optical signal to electrical signal), amplifier and Optical transmitter (i.e. electrical signal to optical signal). Three functions could be conducted over the amplifier known as 1R, 2R, and 3R.

