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Space division multiplexing (SDM) over fibers has introduced a new paradigm in optical communication thanks to its capability to meet the ever-renewed demand of more transmission capacity and on large spectral efficiency. This ever-increasing demand is pushed by the nonstop increase of the number of connected users, devices, processes, and data (toward internet of everything IOE). One of the most promising variants of SDM, that has recently shown great potential, is based on harnessing orbital angular momentum (OAM) modes as data carriers. These OAMs are multiplexed, transmitted over special optical fibers (OAM-fibers) then de-multiplexed. In order to highlight the potential of SDM system incorporating OAM modes through fibers, in this chapter, we disassemble an SDM system and we examine its main key elements. The potential of OAM-SDM is discussed as a promising candidate for the next generation local/global communications networks. This chapter is intended to provide a comprehensive and deep understanding of SDM, which will push R&D community to derive future research directions in the field.
Faculty of Sciences of Monastir, Laboratory of Electronic and Microelectronic, Physics Department, University of Monastir, Tunisia
Habib Fathallah
Faculty of Sciences of Bizerte, Laboratory of Artificial Intelligence and Data Engineering Applications, Computer Department, University of Carthage, Tunisia
Mohsen Machhout
Faculty of Sciences of Monastir, Laboratory of Electronic and Microelectronic, Physics Department, University of Monastir, Tunisia
*Address all correspondence to: alaaeddine.rjeb@gmail.com
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 amplitude-shift keying (M-ASK) [4, 5, 6].
Figure 1.
Optical signal dimensions using as degrees of freedom to encode data.
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 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.
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.
Figure 2.
Schematic of a generic space division multiplexing system based on optical fiber communication.
2.1 Emission side
From the emission side, data (Datai) are modulated using for example a non-return 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, 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.
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].
Weakly coupled MCFs
Strongly coupled MCFs
Coupling coefficient ‘K’ [m−1]
K < 0.01
K > 0.1
Core pitch ‘ʌ’ [μm]
ʌ > 30
ʌ < 30
MIMO DSP exigence
No need
Need
Table 1.
Classification and features of multicore fibers.
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 based on the use of multimode fibers (MMF) while the second exploits the well-known 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.
Figure 3.
Several kinds of fibers used for SDM communication system.
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. erbium-doped 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.
1R: re-amplification.
2R: re-amplification + re-shaping.
3R: re-amplification + re-shaping + re-timing.
2.3 Receiver side
After propagating over the fiber, an SDM-DE-MUX which tend to disengage propagating modes (sharing the same MCF or FMF) and oriented them to particular SMFs. In principle, SDM-DE-MUX devices or techniques are the same as SDM-MUX but in the inverse sense (known also as fan-out devices).
After retrieving the optical signal (DE-MUX), optical photodectors are employed at the end of each SMFi, aiming to detect each particular mode (data carrier from each SMFi) and convert the modulated optical signal into an electrical signal. The most commonly used photodectors are semiconductor photodiodes. Semiconductor based PIN photodiode and the Avalanche photodiode (APDs) are examples of such photodectors. In principle, the selection of these devices is based on the following requirement: high responsivity, bandwidth, noise characteristics, low cost, and so on [34]. Thereafter, the obtained electrical signals are converted to digital ones using electrical-to-digital converters (ADCs). At the end, a MIMO DSP block is used to mitigate the crosstalk effects on different mode channels. The digital signal undergos a normalizing/resampling and symbol synchronizing operations. Then, the obtained signals are equalized using adaptive time-domain equalization (TDE) or frequency-domain equalization (FDE). MIMO DSP are composed of equalizers (i.e. FIR filters) of coefficients hij. The number of these equalizers is related to the number of the square of the transmitted modes (N × N), the length of the transmission link, and the difference between modes delays [35, 36].
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.
Figure 4.
(a) The OAM and the SAM of an electromagnetic beam. (b) Helical wave fronts for a set of orbital angular momentum modes.
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, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]. As recent trend, Orbital 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].
Figure 5.
Different applications of OAM.
3.2 Devices and components for OAM-SDM over fibers
In the original and the first experiment from Allen et al. in 1992 [52], helically phased LG beam was generated from Hermite-Gaussian (HG) beams. The transformation has been based on cylindrical lens (CL). The advantage of CL is its high conversion efficiency and the high purity of generated OAM. However, CL requires high construction precision. Indeed, it has poor flexibility because it requires a very precise incident field angle.
Other obvious way to implement OAM beams is to use a spiral phase plate (SPP) [59, 60, 61, 62, 63, 64]. In principle, when a Gaussian light beam passes through the phase plate, the beam experiences a different phase in the azimuth direction due to the spiral thickness of the phase plate and is converted into a helically phased beam with topological charge l. The advantage of SPP is that is very efficient, and allows the conversion of beams with relatively high power. However, since it is wavelength dependent, it needs extreme precision in manufacturing: different plate is needed for each kind of OAM mode (each l). Recent trend is the proposed adjustable spiral phase plate in [64]. Some diffractive optical devices or elements can be explored targeting to generate OAM light beams [65, 66]. Among these devices, fork grating are used for generating twisted light (holographic gratings). Fortunately, thanks to fork grating, we can generate multiple topological charges (different OAM beams) simultaneously (i.e. using vertical and horizontal superimposed fork gratings). However, this element seems to be inefficient and a variation of this technique has been proposed to improve its efficiency, using forked polarization grating [66]. Metamaterials (complex artificial materials) is another strategy that can make transformations in optical space [67, 68]. OAM modes are obtained by controlling the geometrical parameters (shape, size, direction, etc.) of the metamaterial to manipulate the phases of different azimuths and change the spatial phase of the incident light. A liquid crystal panel, q-plates is another promising and efficient way to generate twisted beams [69, 70, 71]. A light beam incident on q plate is modified to have a topological charge variation.
At last, one of the most convenient method to generate OAM beams is the use of spatial light modulator (SLM) [63, 72, 73, 74]. Made of liquid crystals, SLM is a programmable device that uses a computer [63]. It is composed of a matrix of pixels, and each pixel can be programmed to generate a given phase (there also exists SLMs that act on amplitude instead of phase). By modulating the phases of Gaussian beams, we can generate a wide range of OAM modes. SLM is a versatile component, it can be reconfigured as needed. It is even possible to send different beams on different sections of the SLM, to generate several beams simultaneously. On the other hand, due to its polarization dependent, SLM accepts only limited power. Another method to generate OAM light beams, is possible to use optical fiber. Acting as a mode selector [75] or a mode converter [76, 77], optical fiber seems to be useful in OAM mode generation. Fiber coupler [78], mechanical grating [79, 80], tilted optical grating [81], helical grating [82], multicore fibers [83, 84, 85, 86] and liquid core optical fiber [87] are example of such method. Figure 6 presents the most of examples of OAM generation devices & schemes.
Figure 6.
OAM generation devices & components & schemes.
OAM beam is doughnut shaped (never has intensity at its center). This characteristic is not sufficient to identify OAM beams and their topological charge. At the receiver of a communication system, the different OAM modes can be separated easily by exploiting the orthogonality of the helical phase fronts. A variety of methods for detecting OAM has been proposed for light. In principle, the detection operation can be performed using several techniques including those used for the generation: The operation of OAM beams detection is similar to the generation but in the inverse sens (inverse SPP [88], holographic grating [51, 89]). A common way to identify OAM is to interfere (interference method) the incident beam with a Gaussian beam, and to visualize the resulting interference pattern on a camera. Two cases are resulted: If the incident beam is Gaussian, the interference pattern will look like a series of concentric circles. If the incident beam has a helical phase front, the interference pattern will be a spiral. Then, the number of arms and the direction of the spiral indicate the topological charge and the sign of ‘l’ respectively. This technique is useful to validate the presence of OAM beam but it cannot be used for demultiplexing. Another efficient way, using a phase pattern or a fork grating on a glass plate or a SLM, to convert the incident beam back to Gaussian. A mode sorter was proposed to identify OAM modes, where the lateral position of the resulting beam tells the topological charge or the incident beam [90, 91, 92]. Recently, machine-learning-based approaches (ML) have been implemented in order to accurately identify OAM modes, after their propagation in free space [93, 94]. ML offer great potential in mode detection even after propagation in a turbulent medium. Many other OAM mode detection techniques are reviewed in [95, 96].
3.3 OAM-SDM-fibers: potentials and challenges
The utilization of OAM modes in optical fiber was a challenge to the optical communication community. This subsection focus on standard/special optical fibers designs that have been recently proposed investigated and incorporated in an OAM-SDM system. We start by the main designs and achievements and we will identify the main challenges that are facing this technology.
3.3.1 OAM-SDM-fibers: potentials
Aiming to guide robust OAM modes over an optical fiber, scientists have oriented to special fiber design (i.e. novel refractive index profiles). In principle, these OAM-fibers share common three criteria:
The refractive index profile should be ring (i.e. match the ring shape of OAM modes).
The refractive indexes between core and cladding should be high (i.e. enhance the separation between channels).
The interface between core and the cladding should be smooth (i.e. graded index profile is preferred).
Following these recommendations, various kinds of OAM-fibers have been proposed, characterized and prototyped showing potential achievements in term of capacity transmission and spectral efficiency. Moreover, the standard existing fibers have been investigating in term of their appropriateness to support OAM modes.
The investigation of already existing fibers in OAM context has been carried out by performing a comprehensive analysis of OAM modes in the standard graded index (GIF) multimode fiber (i.e. OM3) in [97]. The refractive index of GIF is shown in Figure 6a. Eventhougth, the standard step index fiber (e.g. ITU-TG.652) is usually used as a single mode fiber (SIF); it is investigated as an OAM fiber by the utilization of small wavelengths (i.e. visible bands) which tend to change the former fiber to a few mode fiber (Figure 6b) [98]. Since then, the transmission of four-OAM mode groups over OM3 MMF, the transmission of OAM modes over OM4 (8.8 km) [99], the transmission of four OAM over 5 km FMF (i.e. 4 × 20 Gbits/s QPSK data) [100], the high purity OAM modes (≥99.9%) over graded index FMF [101], and the viability of 12-OAM-GI-FMF for short/medium haul interconnect [102], have been demonstrated.
Considering the above design guidelines, specialty fibers have shown their capability to handle OAM modes. At the beginning, Ramachandran group has demonstrate the multiplexing/transmission and demultiplexing of OAM modes over a special vortex fiber [80]. The transmission of OAM modes over more than 20 m-VF [16] and 1 km-VF [103], have been demonstrated. Due to the high contrast between the air and the glass (SiO2) in term of refractive indexes, air core fibers (ACF) have been proposed, designed and prototyped (Figure 6c). An ACF supports 12 OAM modes over 2 m has been demonstrated in [104]. Two OAM modes supporting by an ACF was successfully transmitted over 1 km [105]. Another ACF fiber has been characterized in COPL at LAVAL University. This ACF supports 36 OAM states [106]. A capacity transmission of 10.56 Tbit/s has been demonstrated over an ACF using 12 OAM modes using WDM technology (OAM-SDM-WDM) [107]. Recently, over the O, E, S, C, and L bands, an ACF made by air, As2S3 and SiO2 as material for the inner core, for the outer core and for the cladding, respectively, has been designed to support more than 1000 OAM modes [55, 108].
Ring core fibers RCF (Figure 7d) are another family of OAM specialty fibers that have been extensively investigated. COPL team has manufactured a family of RCFs suitable for OAM modes [109]. The transmission of two OAM mode-group has been demonstrated over a 50 km RCF [110]. Other RCF with smoothed refractive index at the interface between the core and the cladding, known as GIRCF, have been designed (Figure 7e). A GIRCF supporting 22 OAM modes over 10 km has been demonstrated [111]. An aggregate transmission capacity of 5.12 Tbits/s and a spectral efficiency of 9 bit/s/Hz have been reported in [112]. Over 12 km GIRCF, the transmission of two OAM modes each has 12 Gbaud (8QAM) and with 112 WDM channels has been demonstrated in [113]. Hence, a transmission capacity of 8.4 Tbits/s has been reported.
Figure 7.
Various kinds of fibers that have been used in OAM-SDM systems: (a) graded index fiber, (b) step index fiber, (c) air core fiber, (d) ring core fiber, (e) graded index ring core fiber, (f) inverse parabolic graded index fiber, (g) inverse raised cosine fiber, (h) hyperbolic tangent fiber.
Other family of hybrid refractive index structure (i.e. inner core is graded while the outer core is step) have been proposed for OAM modes. Inverse parabolic graded index fiber (IPGIF) has been designed and demonstrated experimentally (Figure 7f) [114]. As a first experiment, the use of IPGIF as OAM-fiber was successfully demonstrated based on the transmission of two OAM modes over 1 km. as a second step, 3.36 Tbits/s has been achieved over a IPGIF of 10 m. In that experiment, 15 wavelengths (WDM) and 4 OAM modes have been utilized [115]. In [116], we proposed inverse raised cosine fiber IRCF (Figure 7g) for supporting moderate and robust OAM modes. The new fiber proved the support of high pure OAM modes. Recently, we demonstrated the tolerance of IRCF in bend condition. Other usual function has been incorporated as a refractive index profile, which is the hyperbolic tangent function (HTAN). The designed fiber (Figure 7h) supports high pure OAM modes, with high separation among them (low crosstalk). The fiber is resilient to bending, and characterized by low chromatic dispersion and low differential group delay [117]. Recently, we designed an inverse-HTAN-MMF supporting very large number of OAM mode group (14 MG) that outperforms those supported by OM3 [118]. We designed another OAM-FMF based inverse Gaussian (IG) function. The designed IGF is favorable to transmit OAM modes in next generation OAM-MDM multiplexing optical networks [119].
The transmission of OAM modes over MCFs has been demonstrated with the aim of further increasing the capacity of an SDM links (i.e. improve the available data channels). A 7-RCF (MOMRF) has been proposed to support 22 × 7 modes (i.e. 154 channels) [120]. Low-level crosstalk (−30 dB) has been demonstrated over 100 km long MOMRF. A trenched multi OAM ring fiber (TA-MOMRF) has been reported in [121] showing Pbit/s as transmission capacity and hundreds bit/s/Hz as spectral efficiency. Later on, a coupled multi core fiber has been proposed in [122]. The investigated supermode fiber featured low crosstalk, low nonlinearity effects and low modal loss.
3.3.2 OAM-SDM-fibers: challenges
OAM-SDM over fibers is facing several key challenges and impediments that may curbs/slow down the transition from design process to prototyping operation and then to commercialization and standardization in the market.
Mode coupling issues are the most threads that degrade the OAM-SDM system performances. Mode coupling is the physical cause of data-channels crosstalk. Keeping these modes well separated during propagation along the fiber is a challenge in order to realize a robust OAM-SDM system and avoid the employment of additional MIMO-DSP module at the receiving stage. Even by using OAM-specialty fibers that ideally tend to appropriately support the OAM channels, there are almost some perturbations and impediments along the fiber section. These perturbations include macro & micro bending, twisting, birefringence, and core ellipticity. These imperfections may cause a mode coupling. Various linear and nonlinear effects in optical fiber could be detrimental for long distance SDM systems. Concerning linear effects, material absorption cause attenuation of optical signal (i.e. power loss). Other linear effects are the effects of dispersions during propagation. Chromatic dispersion is caused by the fact that the phase velocity and the group velocity are depending on the optical frequency. Polarization mode dispersions (PMD) are occurred because of dependency between the phase velocity of propagating mode and the polarization state. Intermodal dispersion is due to the dependency between the phase velocity and the optical mode.
On the other side, due to the intensity dependence of refractive index of optical fiber, and inelastic scattering phenomenon, different kind of nonlinearity effects can occur in optical fibers. This power material-light dependency is responsible for the Kerr-effect. Several effects are manifestations of Kerr nonlinearity. Four wave mixing (parametric interaction among waves satisfying phases matching) arise when light components with different optical frequencies overlap in optical fiber. Stimulated Raman Scattering (SRS) is a nonlinear process that correspond to interaction between optical signals and molecular vibration in the glass-fiber (optical phonons). At last, stimulated Brillouin scattering (SBS) is very similar to Raman scattering that is correspond to interaction between optical signal and the acoustic vibration in the fiber (acoustic phonons).
Around a decade since the first OAM-SDM fiber, the ability of this technology has proven very fruitful in improving the optical communication networks in term of capacity, and spectral efficiency over long distances. However, it is still represent a young area of research and study that has a rich set of issues, challenges and opportunities to explore and to check it in the three regions of a communication link (emission, transmission, and reception). Starting by the emission side, important research directions are to find new materials and structures aiming to effectively generate OAM beams. These desired generation techniques or devices should feature favorable performances including low cost, high compactness, small size, high conversion efficiency, and compatibility with existing technologies. In addition, it would be important to give a significant interest in miniaturizing the devices and components at the emitter side (e.g. bringing OAM to the chip level in photonic circuits): Integrated on-chip devices on different platforms (e.g., silicon platform) could be viable candidates in next generation OAM SDM system. This helps OAM beams to be encoded & generated fast, switched freely and detected in real time. Various integrated version of devices could be widely adopted: integrated information encoders, integrated OAM modes emitters, and integrated OAM multiplexers. In spite of the price to be paid in term of cost, the development of such devices will be empowered by the rapid progress in micro and nano-fabrication technologies.
Considering optical fiber transmission phase, the perfect refractive index profile for OAM fiber is an open subject for everyone in optical communication. So far, it is unclear which kind of fiber provides the best performance in MDM, but evidently, there is no ideal OAM fiber design even if we either follow some design recommendations concluded from former proposed fibers (Section 3.3.1) or consider common electromagnetic rules. Certainly, each fiber has its pros and cons, but it is always a tradeoff between fiber key design parameters aiming to increase the number of supported modes, the separation among their refractive indexes, their purity, and their stability during transmission. Innovative designs with the former performances metrics would be an interesting direction of research. The desired designs will be motivated by the extended and the improvement of MOCVD process to support the manufacture of complex structure fibers with high refractive index contrast. Therefore, further efforts should be dedicated to develop new amplifiers. With the aim of further increasing the transmission capacity over long-haul optical fiber transmission systems, future R&D trends at the receiver side of SDM will based on the implementation of practical coherent optical communication schemes (coherent receivers) followed, if necessary, by advanced digital signal processing (DSP) techniques. It would be valuable in next generation OAM-SDM systems to explore techniques aiming to compensate both linear & nonlinear impairments (the compensation of nonlinear impairments is an interesting research area for coherent optical communications).
In addition, machine and Deep learning (ML & DL) have risen forefront in many fields. The use of ML or DL could touch various aspects from OAM-SDM systems including nonlinearity mitigation, optical performance monitoring (OPM), carrier recovery, in-band optical signal-to-noise ratio (OSNR) estimation and modulation format classification, and especially, advanced DSP. Hence, a full smart optical communication networks.
Multiplexing spatial modes (SDM) seems to be viable solution to cope with the upcoming capacity crunch. In this chapter, we attempted to focus on the different aspects from an SDM system (emission, transmission and reception) over optical fibers aiming to highlight their main key elements and components that allow this technology to be the desired one for next generation local/global optical communication systems/networks. We focused on the last trend of SDM communication research direction: OAM-SDM over optical fibers. We discussed the OAM modes and the main devices & schemes for the generation & detection and the transmission of them. OAM specialty fibers are highlighted with focus on, their key features, their main achievements (throughput & main contributions) and main challenges that face their progress. Perspectives and future research orientations that may touching SDM systems have been presented at the end of this chapter. From what we have attempted to present, SDM still unexhausted research area that optical communication R&D community have to derive/touch future research directions in the field.
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Written By
Alaaeddine Rjeb, Habib Fathallah and Mohsen Machhout
Submitted: 13 September 2021Reviewed: 21 October 2021Published: 01 July 2022
Open access peer-reviewed chapter
