**2. Space division multiplexing (SDM) system**

In this section, we detail the optical fiber based SDM optical communication system. We describe essential devices/actors constructing a full SDM transmission line. We start by the emission side devices, then the SDM-fibers & amplifiers and at last the devices using for the reception of data at the receiver side. **Figure 2** illustrates a schematic representation of a generic SDM optical communication system.

#### **2.1 Emission side**

From the emission side, data (Datai) are modulated using for example a nonreturn to zero (NRZ) sequence. The electrical signal (ESi) converted into an optical signal using optical sources. These optical sources could be LED (light-emitting diode), DFB laser (distributed feedback laser), FP lasers (Fabry–Pérot laser diode), VCSEL (Vertical Cavity Surface Emitting Laser), etc. Each transmitter will couples the generated optical signal to a single mode fiber (SMFi) in order to excite the fundamental mode (i.e. namely LP01 mode) [19]. All the obtained modes are multiplexed using optical multiplexers (SDM MUX). SDM Optical multiplexers (also commonly called fan-in device) are spatial multiplexers that tend to collect modes (i.e. data carriers) from SMFs and couple them to an SDM fiber. For multiplexing various modes,

**Figure 2.** *Schematic of a generic space division multiplexing system based on optical fiber communication.*

several techniques and devices have been demonstration. Photonic lantern, Photonic integrated grating couplers waveguide optics interface, tapered multicore fibers, waveguide coupling (e.g. in case of MCFs, isolated waveguides connect each core to a particular SMF) and free space optics approaches such us phase plates, mirrors, beam splitters and special lenses [20]. In principle, the selection rule between these techniques are based on the incorporated SDM fiber (i.e. FMF, MMF MCF) and on the requirement of the lowest loss, the low susceptibility to crosstalk, the compactness, and low complexity and flexible.
