**2. The roadmaps towards fiber optics**

The global proliferation of hyper photonics, intelligent photonics and frontier wireless communication need an increasing data capacity of tens of percentage each year. Services including IoT, M2M, sensor networks, and linked vehicles will need even greater bandwidth to expand capacity and find more efficient connections via high-speed optical fiber networks, as **Figure 1** shows the innovation technologies. As a result, the forthcoming growth in data transmission will exceed the SMF's maximal transmission capacity due of its low loss and optical amplification of the transmission window. Traditional SMF cannot be ignored in DWDM transmission systems with Raman amplification [23]. An increase in optical infrastructure is required to meet capacity constraints. Introducing extra optical fibers and cables is considerably simpler when contemplating an alternate technology. The future capacity constraint may be averted simply by creating more SMFs. Thus, construction/renewal of the physical infrastructure would be required, which would add to the total expenses. SDM, a fifth physical dimension, may supplement time, wavelength, frequency, and polarization multiplexing, thereby easing future capacity problems [24].

**Figure 2** shows the progress of cable density. 400-pair copper, 400-fiber ribbon, and 400 rollable fiber ribbon cables are indicated by black, blue, and red dots. A solid green line indicates the numerical limit for a hexagonally packed 250 mm fiber bundle with a 2 mm cable sheath [24].

The two spatial dimensions of mode and core are used in the design of optical fiber. MCF stands for multi-core fiber division multiplexing, while MMF stands for multi-mode fiber division multiplexing. For understanding description of a 2D representation of modes and cores inside optical fiber shown in **Figure 3**. With proper use of modes or cores, it is possible to surpass the present geometric limit of conventional optical fiber cable. Using the modes and cores in tandem will almost triple the spatial multiplicity. This recent study concludes that 6-mode and 19-core fiber can provide over 100 spatial channels [25]. It's required that a complicated transmission strategy be used because of the mode coupling and/or modedependent transmission properties in optical fiber. Also, MCF has been constantly studied and was pioneering in [24]. MCF is especially capable of using the newest

**Figure 1.** *History of optical network innovation technologies [22].*

**Figure 2.** *Evolution of communication cable density over time [7].*

**Figure 3.**

*Schematic image of the two spatial dimensions of mode and core in optical fiber [24].*

single-mode technologies. Here, we'll talk about MCF and the capacity of MCF as an SDM transmission medium.

Signal mixing and propagation time skew are two important characteristics of SDM fibers that have a direct effect on transmission performance. SDM fiber groups are provided by two mixing levels. First, there are uncoupled MCFs (UC-MCFs), few-mode fibers, or few-mode MCFs, which mix signals from several spatial channels during transmission and have minimal inter-core coupling to minimize inter-core crosstalk (XT). Both the first group randomly coupled MCF (RC-MCF) and the second group randomly coupled MCF (RC-MCF) have significant random mode mixing between modes (RC-MCF). UC-MCFs have greater spatial channel density than SMFs with traditional transceivers. To compensate for random coupling, RC-MCFs need MIMO digital signal processing (DSP). However, random coupling may decrease spatial mode dispersion and therefore the MIMO DSP's computational complexity. Other core/mode-dependent restrictions are also prevented by random coupling [26, 27].

MCF with a 125 mm cladding diameter is needed to start MCF tech. A 125 mm cladding diameter MCF design that is optically compatible with current SMF.


### *Multi-core Fiber Technology DOI: http://dx.doi.org/10.5772/intechopen.100116*

**Table 1.**

*Representative examples of reported MCFs*

 *[28].* **Table 1** depicts conventional MCFs provided by Sumitomo Electric, as well as prototype MCFs created by the company via joint research.

#### **2.1 Need for multi-core technology**

The restricted bandwidth of low-loss transmission and optical amplification, as well as transmission power limitations due to fiber non-linearity, make expanding a single optical fiber's transmission capacity challenging. A growing need for highercapacity optical fiber communications **Figure 4** shows high-capacity optical fiber transmission test results. Due to the limitations of single-mode single-core fiber, the maximum capacity is 100 Tbit/s. This high-capacity fiber capacity was achieved using MCF. SDM plus MCF or MMF may be able to outperform single-core fiber transmission systems [12, 26, 29].

## **3. MCF paradigm**

This section investigates the design and the achievements in MCF technology, which seem promising in the short term yet have certain unknown risks.

#### **3.1 Design of MCF**

Due to fiber bandwidth depletion and the development of SDMF as a possible alternative to address extra capacity, the spectrum capacity of SSMF is nearing its end. The long-term goal of SDM is to increase the number of fiber cores, guided modes, or both. MCF has many cores in a single optical cable. The core of a conventional single-core fiber is positioned in the center of a 125-m diameter cladding, limiting design freedom. The MCF's success is dependent on more than simply the number of cores. MCFs enable the designer to optimize core design, the number of cores, core arrangement, outer cladding thickness, and cladding diameter in terms of optical and mechanical properties. Fiber design is required based on the application because desirable features differ. SMFs currently have a single fiber core surrounded by 125 mm cladding and coated. Greater cores with the same cladding or larger core diameters allow for more fiber capacity [12]. Adding cores to the

**Figure 4.** *Recent reports on high capacity transmission [19].*

**Figure 5.**

*High-capacity transmission experiments using SM and FM-MCF [12].*

cladding may improve capacity, but it may need changes to the transmission system architecture.

SM–MCF transmission experiments on FMFs have shown fibers with as many as 32 and as little as 45 modes. Adding multi-core fibers that enable a few-mode core to join the fibers results in an additional 100 optical channels in each transmission and throughputs of more than 10 Pb/s [30]. The high-capacity experimental transmission utilizing SM and FM-MCF is shown in **Figure 5**.

A random coupled MCF is one that is MCF if XT is compensated by MIMO DSP. Even though the core is simple, the paired MCF is denser. Furthermore, random coupling in the connected MCF prevents the emergence of nonlinearity impairment, SMD, and mode-dependent loss/gain. In long-distance point-to-point communication, coupled MCF is utilized. Nokia Bell Labs, Sumitomo Electric, and Sumitomo Corporation collaborated to develop and launch new fibers [31].

Few-mode (FM) MCF fiber is a kind of uncoupled MCF (MCF with fiber coupled together) designed for mode-multiplexed transmission. KDDI Research, Inc. received a prototype 36-core fiber created in cooperation with NICT and Yokohama National University. The most recent accomplishments include a 19-core fiber that can be used in the whole C + L bands (1530–1625 nm) for long-distance communications [37]. This fiber achieved 10 Pbit/s per optical fiber in an experiment performed by KDDI Research [30].

The MCF has the potential to enhance data and power transmission for highpower devices. PoF, on the other hand, need MCF due to its nonlinear aberrations. Inside MCF, an eye-catching power transmission capacity was recently discovered. The placement of the cores has an impact on the MCF's performance.

Multi-core fiber architectures such as triangle, ring, square, rectangle, and hexagon were developed after analyzing the number of cores, pitch, and power spectrum. Many people are interested in the MCF fiber-optic structure and the question is how it allows the transmission of powerful signals. One-mode fiber has a lower limit imposed by the MCF and is currently limited by MCF analysis and picture processing. The placement of the cores influences the performance of the MCF.

**Figure 6.**

*Structure of multicore fiber with coupling region and with different pattern [17].*

For MCF, strong and weak coupling are illustrated in **Figure 6**. Strongly linked MCF has the smallest core-to-core distance, whereas weakly coupled MCF has the most core-to-core distance. The size of the core may vary with the pitch of the core. The effective area (Aeff) relies on the number of cores (as shown in **Figure 6**), and their configuration influence the output receiving power. MCF contains several cores with enlarged effective areas, resulting in minimal dispersion and bending losses. The suggested solution to fiber bending losses included four air core MCF. **Figure 6** shows the five distinct MCF structures.

To provide long-distance reliable signal transmission, the XT must be less than - 30 dB/100 Km [32]. To get ultra-low XT levels, make changes to MCF structures, such as trenching around the cores. Essentially, trenches are refractive index profiles with lower refractive index than the core and cladding. Trench-assisted method is one of the noteworthy techniques that lowers the coupling between the adjacent cores, therefore helping to minimize existing crosstalk.

With an MCF, if the number of cores in a restricted cladding area grows, crosstalk suppression becomes a problem. XT in MCFs is decreased by decreasing the coupling coefficient between cores. The underlying design, with strong containment of modes is critical to suppressing the mode coupling coefficient. For a higher Aeff and lower nonlinear noise, you may choose for a higher-index core with a smaller diameter.

It has three important geometrical features, as shown in **Figure 7**. The outer cladding thickness (OCT) is the distance between the outer core's center and the cladding's perimeter. Optical fiber mechanical reliability is strongly linked to cladding diameter D. A higher D value increases MCF deformations before collapse. Inter-core XT may be reduced by adjusting core and rod radius, cladding and rod relative refractive index differences, and core-to-core distance.

#### **3.2 Application**

The MCF technologies have been gradually increasing, and now we can see feasible commercial uses for the technologies. Practical use of MCFs will likely occur in near future due to continuous MCF development [28]. **Figure 8** illustrates the whole growth stages of MCF technology. Larger applications in the network such as metro and core may provide challenges to MCF implementation, because they need a complete suite of network components other than the MCF and cable

#### *Multi-core Fiber Technology DOI: http://dx.doi.org/10.5772/intechopen.100116*

**Figure 7.**

*(a) A 32 core schematic structure shows three key geometrical parameters in MCF. Schematic diagrams of (b) core index profile of heterogeneous rod rod-assisted 32-core fiber (c) trench-assisted (TA) profile, (d) holeassisted (HA) profile [33].*

#### **Figure 8.**

*Schematic image of expansion phases of MCF technology [24].*

(e.g., optical amplifier, and node management technologies). To a large extent, central offices and/or data centres are the primary target for MCF technology since they are maintained and/or optimized separately by experienced operators, making it simpler to upgrade current network components. Here, compatibility with traditional SMF is very essential, especially for connection. Then, in the second deployment phase, we use MCF P2P and/or parallel transmission technology. MCF requires a flexible connection to the optical subsystems. Submarine transmission systems may offer more promise because of their use of the newest technology, and SDM might possibly achieve a power-efficient transmission system [34]. We have finally achieved flexible and dependable SDM nodes [24].

A FORECAST predicts that data center network traffic would increase at a 25% CAGR, with most traffic (about 70%) staying within the data center [35]. Modern data centers utilize dense non-blocking topologies with point-to-point optical

transponder connections. Either on end servers or in slots of electronic switching devices, data is electrically switched. This resource-intensive paradigm may cause future data center scaling problems. Modern data center networks are increasingly relying on optical technologies like hybrid electrical-optical switching (HEOS). Recent demonstrations of a time-slotted pSSC and core-joint SDM optical switching system for edge applications [36].

SDM may improve network capacity by multiplexing SMF strands, multicore fiber cores, or even each mode of a few-mode fiber using MIMO digital signal processing [38]. Spatial Division Multiplexing-Elastic Optical Networks will then be the future of optical transport and data center networks (SDM-EON) [39]. MCF front haul multiplexes MIMO signals onto a single cable, enabling multiple optical data streams to be transmitted simultaneously. The MCF may also provide a single optical data signal to each antenna element, with varying delays and phases. Multiantenna systems need MIMO and beamforming. The MCF front haul uses MIMO signals to transmit multiple optical data streams at the same wavelength. The MCF also uses optical data transmissions with variable phase or time delays to each antenna element. 5G systems need MIMO and beamforming capabilities. **Figure 9** shows a multi-antenna MCF-based RRH-to-remote-site connection with optical beamforming and/or digital MIMO capabilities. These methods enhance system performance.

The system capacity and accessible user bitrate may be enhanced by multiplexing MIMO data streams. A 22 MIMO LTE-A transmission using MCF technology was evaluated early. This research adds 44 MIMO transmission supported by a 4-core fiber capable of feeding four AEs concurrently. The M(22) arrays allow 5G systems to control multiple groups of four AEs. MCF may be used to reduce the size and complexity of beamforming systems. A same data signal is supplied by four separate AEs with varying delays, as shown in **Figure 9**. MCF aligns all optical lines in the beamforming system, simplifying the network [40].

In [36], Making fiber bundles revealed a 19-core MCF. A 7-core MCF Micro-lens array (MLA) claims 47.8 dB return loss and 0.87 dB insertion loss. Tapers were made, then cut apart to make fused fiber. Non-mode-selective, in which the modes spin on the device itself, and mode-selective, with minimal unitary rotation between modes. The most common components were lamps, phase plates, PLCs, and then mode selective PLCs. Ultrafast laser inscription can produce low loss 3D waveguides in conventional optical glass for MCFs. 3 mode FM-MCF fiber with average IL 0.92 and homogeneity 0.1.

SDM enthusiast offered considerable flexibility in fiber light mixing, integrated sensors and controllers. MCF technology has also been used to construct optical fiber sensors, which make them excellent for industrial applications. High

#### **Figure 9.**

*(a) A multi-antenna site application scenario in which the RRH is linked to the antenna components through MCF. Examples of four-antenna systems with (b) spatial multiplexing using 44 MIMO (4 distinct data streams coded in 4 layers) and (c) 41 beam forming (4 antennas transmitting the same data with different delays) [40].*

#### *Multi-core Fiber Technology DOI: http://dx.doi.org/10.5772/intechopen.100116*

temperature sensing to 1000°C using MCF optical sensors with a typical temperature sensitivity of 170 pm/°C Mach–Zehnder (MZ) interferometers may be fabricated by employing MCFs since the slopes of the resultant interference peaks are steeper. Until quite recently, many MCF optical sensors used inefficient methods to throw light into the multi-arm MZ, causing substantial losses for QI processing [41] use novel tapering methods to construct the multi-arm MZ directly into a specially built MCF.
