**1. Introduction**

Over the last decade, there are unquestionably a huge demand for transmission capacity. This demand is fueling by the fast & renewed increase of the number of connected users, devices, processes, and data (e. g. According to the Annual Internet report of CISCO, there will be 5.3 billion total Internet users (66% of global population) by 2023, up from 3.9 billion (51% of global population) in 2018) [1]. This tend to create a hyper connected world. Furthermore, there are international efforts that aim to develop a concrete roadmap for "Internet Governance" targeting to both bring (i.e. to deliver) the internet to everyone (i.e. "connect the unconnected") and provide enormous boost in performance of the actual Internet network. These efforts will put much pressure on the Internet service providers/ communications actors and motivate them to reach innovative solutions and advanced technologies to deal with the growing insatiable on data capacity that will probably result in an imminent capacity crunch in the next few years [2]. On the other hand, optical fiber communication is still a milestone in the evolution of communication generally. Optical fiber is considered as the backbone of the modern

communications grid. Various research developments on optical fiber communication have been conducted showing great potential [3].

In order to cope with the huge demand of more and more data capacity, and improve the spectral efficiency, R&D optical fiber communication community has developed various technological paths based on innovative multiplexing techniques and advanced optical modulation formats. From one hand, various multiplexing techniques have been conducted based on the use of different optical signal dimensions as degrees of freedom to encode information and transmit them over optical fibers. These dimensions are the Time, as time division multiplexing (TDM: interleaving channels temporally), the polarization, Polarization division multiplexing (PDM), the wavelength, as wavelength division multiplexing (WDM: using multiple wavelength channels) and the phase (quadrature). These physical parameters help to create orthogonal signal sets even sharing the same medium (i.e. multiplexing); they do not interfere with each other (i.e. individual, separate and independent signals). **Figure 1** depicts these orthogonal dimensions. On the other hand, the improvement in modulation format is translated by the move on from the On-Off-Keying (OOK) modulation to M-ary Quadrature Amplitude Modulation (M-QAM), M-ary phase-shift keying (M-PSK) and M-ary amplitudeshift keying (M-ASK) [4–6].

Recently, researchers have oriented toward the space (Space Division Multiplexing (SDM)) as a further dimension to encode information [7]. The spatial analogue of the above cited dimensions, SDM is based on the exploitation of the spatial structure of the light (i.e. optical signal) or the spatial dimension of the physical transmission medium (e.g. optical fiber). Both strategies aim to improve the available data channels along an optical transmission link (i.e. Network). Considering these data channels, two attractive variants of SDM have shown potential interest: (1) Core Division Multiplexing (CDM) and (2) Mode Division Multiplexing (MDM). CDM is based on the increasing of the number of cores embedded in the same cladding of optical fiber [8]. These fibers are known as multicore fibers MCFs). Other classical option that has been adopted in current optical infrastructure for several years already is based on single core fibers bundles (i.e. fibers are packed together creating a fiber bundle or

#### *Multiplexing, Transmission and De-Multiplexing of OAM Modes through Specialty Fibers DOI: http://dx.doi.org/10.5772/intechopen.101340*

ribbon cable) [9]. If we assume that one core is equivalent to one data channel hence, the transmission capacity in an optical link incorporating MCFs will be multiplied by the number of embedded cores. On other side, MDM is consisting on the transmission of several spatial optical modes (various paths or trajectories) as data channels within common physical transmission medium (within the same core) targeting to boost the capacity transmission [10]. MDM could be realized by either multimode fibers (MMFs) or few mode fibers (FMFs). MMFs are dedicated to short transmission interconnect while FMFs are used for long haul transmission links. The same as CDM, if we assume that one mode is equivalent to one data channel; the transmission capacity in an optical link incorporating MMF/FMF will be multiplied by the number of supported modes. Other promising technology is based on mixing both approaches: Multicore few modes fibers (MC-FMF) where the number of channels will be proportional to the number of embedded cores and to the number of supported modes within the same core [11]. Moreover, MDM could be realized over free space link where data are carrying on multiple parallel laser beams that propagates over free space between transceivers [12].

Considering optical fiber links, numerous mode basis have been harnessed for mode division multiplexing showing its capability & effectiveness to scale up the capacity transmission and enhance the spectral efficiency. Recently, based on the feature that light can carry Angular Momentum (AM) (i.e. AM expresses the amount of dynamical rotation presents in the electromagnetic field representing the light), the capacity transmission of optical fiber has been unleashed [13]. The AM of a light beam is composed of two forms of momentums (i.e. rotation): (1) Spin Angular Momentum (SAM), which is related to the polarization of light (e.g. right or left circular polarization). SAM provide only two different states (available data channels). (2) Orbital Angular Momentum (OAM) which is linked to the spiral aspect (twisted light) of the wave front. This is related to a phase front of exp (*jlφ*) where *l* is an arbitrary unlimited integer (theoretically) that indicates the degree of twist of a beam, and *φ* is the azimuthal angle [14]. Benefiting of two inherent features of OAM modes: first, two OAM modes with different topological charge *l* do not interfere (i.e. orthogonality). Second, the topological charge *l* is theoretically unlimited (i.e. unboundedness), exploiting the OAM of light as a further degree of freedom to encode information, is arguably one of the most promising approaches that has deserved a special attention over the last decade and showing promising achievements [15, 16]. OAM modes has been harnessed in multiplexing/ de-multiplexing (OAM-SDM) or in increasing the overall optical channel capacity over optical fiber link. OAM-SDM is facing several key challenges, and lots crucial issues that it is of great importance to handle with it in order to truly realize the full potential of this promising technology and to paving the road to a robust and to a high capacity transmission operation with raised performances in next generation optical communication systems.

SDM is based on the orthogonality of spatial channels (spatial modes). Thus, mode coupling or mode mixing (e.g. channels crosstalk) is the main challenge in an SDM system and the main goals of that technology are in principle rotating around how to keep enough separation between much available modes. In order to cope with channel crosstalk, two solutions could be used. The first is the use of multiple input multiple output digital signal processing (MIMO DSP) [17] while the second is based on the optimization of fiber parameters (refractive index profile & fiber parameters) at the design stage [18]. In principle, MIMO DSP is considered as the extreme choice to decipher channels at the receiving stage since it is heavy and complex. This complexity is came from the direct proportionality between the number of required equalizer from one side and the transmission distance, the number of modes, and the difference between modes delays, from the other side. Hence, these

considering reasons allow the use of MIMO much impractical in real time and threats the scalability of optical communication SDM systems. Hence, by carefully manipulating the fiber design key parameters, it is possible to supervise/control the possible interactions between modes/channels. This better facilitates understanding each fiber parameter impact on fiber performances metrics and smooth the way of transition from the design stage to the fabrication process (e.g. MCVD as Modified Chemical Vapor Deposition) and to the deployment operation on the ground later (e.g. FTTH as Fiber To The Home and FTTX as Fiber To The x).

In this chapter, we detail the main key elements/actors (i.e. devices and parameters) that form an SDM system and allow it to become a promising approach to handle with the upcoming capacity crunch of the next generation optical communications systems. Then, we concentrate on the potential of using OAM modes over optical fibers (OAM-SDM) as a promising candidate that tend to realize the full potential for SDM technology. We provide the main generation, detection, transmission, challenges and future research directions of that technology. This aims to provide a comprehensive and deep understanding of OAM-SDM technology, which will push R&D community to derive future research directions in the field.
