**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:


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.

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

#### **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.*

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

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).

## **4. Perspectives and future research orientations**

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

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.
