**3. OAM-SDM system over fibers: potential and challenges**

This section highlights the potential of carrying data on OAM modes and multiplexing, transmitting them over SDM fibers & de-multiplexing them. This technology is known as OAM-SDM technology. Intuitively, Incorporating OAM modes as data carriers has shown great potential in ameliorating the performances of SDM communication system. We focus on these OAM modes, what are they? How to generate and detect these kind of modes? What are the appropriate fibers that robustly support these modes? Moreover, what are the main challenges facing this technology?

#### **3.1 OAM beams**

It is well known that an electromagnetic beam (light) possess angular momentum (AM), meaning that it can rotate around the propagation direction. Light possess a total AM of *(l + s)*·*ħ* per photon, where *lħ* corresponds to the orbital angular momentum (OAM) and *sħ* is the spin angular momentum (SAM) (see **Figure 4a**). The orbital angular momentum (OAM) beam, depends on the field spatial distribution, characterized by a helical phase front of exp. (*ilϕ*), where *l* denotes the topological charge number, which is an arbitrary integer ranging from *−∞* to *+∞*. *ϕ* is the azimuthal angle, and *ħ* is the reduced Planck constant (=1.055 × 10−34 J s). The limitlessness of the topological charge number *l* indicates the unbounded states that can be modulated with OAM. In addition, two OAM lights with different *l* charge number are orthogonal. A series of wave fronts for various OAM modes are depicted in **Figure 4b**.

The sign of *l* denotes the handedness of the spiral. A clockwise rotation can be assigned to a positive *l* and an anticlockwise rotation to a negative *l*. On the other hand, the spin angular momentum (SAM) of light is related to the circular polarization state. The beam can only have bounded orthogonal states: *S = ±1*, which correspond to left or right circular polarization respectively. Intermediate values denote elliptical polarization. Benefiting by that inherent features (orthogonality & unbounded states), potential applications in diverse areas has exploited the OAM of light, including, but not limited to, optical trapping, tweezers, metrology, microscopy, imaging, optical speckle, astronomy, quantum entanglement, manipulation, and remote sensing (**Figure 5**) [13, 37–51]. As recent trend, Orbital

**Figure 4.**

*(a) The OAM and the SAM of an electromagnetic beam. (b) Helical wave fronts for a set of orbital angular momentum modes.*

angular momentum (OAM) has gained a widespread interest in the area of optical telecommunication due to its capability to elevate the transmission capacity and substantially improve the spectral efficiency (OAM could offer unlimited channels for data transmission) of optical communication in both free space and fiber optics links. Many families of light beams can carry orbital angular momentum including Laguerre-Gaussian beams (LGB) [52], Bessel beams [53], Bessel-Gaussian beams (BGB) [54], Hermite-Gaussian beams (HGB) [55], Mathieu beams [56], Ince-Gaussian beams [57], and vector vortex beams [58].
