**5. Limitation**

Researchers all around the globe are working to reduce problems in order to attain ultra-low signal distortions in fiber optic technology [45]. However, high-capacity transmission systems place extra importance on network reliability [46]. It has gotten a lot of attention as a promising technique for dealing with the capacity limitations that are associated with single-core SMFs and cable size limitations like in datacenter networks and Passive Optical Networks (PONs) (which require high fiber count and high-density). Also, MCFs provide redundant signal lines and primary signal lines, which enable them to construct extremely dependable networks [47].

#### **5.1 Cross talk**

MCF is currently actively researched for SDM. SDM-based long-haul transmission requires low-crosstalk (XT) architecture. MCF transmission presents an

**Figure 14.** *(a) Mechanism of inter-core crosstalk, (b) time evolution of XT, (c) histogram of XT [12].*

immense difficulty because to crosstalk, which may decrease the quality of optical data, caused by unintentional coupling between cores running in the same direction and wavelength, as shown in **Figure 14**.

Transmission is hampered by crosstalk in an MCF-based optical network, which may be reduced via power and mode coupling. So cladding widths vary. The cladding's strength decreases with increasing diameter. Each of these parameters must be changed. The MCF inter-core crosstalk is now calculated using coupled-mode and coupled power theories. First, MCF systems must agree on allowable crosstalk per length. Modern coherent optical communication systems have MCF crosstalk requirements regardless of transmission distance. Tolerable crosstalk has penalties for capacity, reach, universality transponder installation, and system link implementation [48, 49].

Ratio-reach trade-off of optical transponders capable of fine-tuning their modulation format to the channel circumstances through methods like probabilistic constellation shaping (PCS) for Nyquist pulses,

$$\text{SE} = 2.\log\_2\left[\mathbf{1} + \frac{\mathbf{1}}{\eta\_{TRX}} \frac{P\_S}{\left(\eta\_L P\_{ASE} + \chi P\_S^3 + kP\_s\right)}\right] \tag{1}$$

where SNR <sup>¼</sup> *PS <sup>η</sup>LPASE*þ*χP*<sup>3</sup> ð Þ *<sup>S</sup>*þ*kPs* **.**

Ps is the per-channel (dual-polarization) signal launch power, and PASE is the per-channel (dual-polarization) amplified spontaneous emission power. For example, non-perfect amplification causes noise enhancement, beginning with PASE as the ASE from ideal distributed amplification. The parameter represents nonlinear interference noise (NLIN) and is calculated utilizing [50] formalisms. It indicates the average XT power due to other signals co-propagating at the same wavelength in different MCF cores. In the low coupling regime studied here, XT may be represented as AWGN, k increases linearly with distance, and interactions between XT and fiber nonlinearities can be disregarded.

MCF optical network crosstalk research for spectrum and fiber core allocations are many. As a consequence, to reduce crosstalk, all of the methods suggested reduced network capacity. A nearby core is already transmitting data on the same wavelength, therefore they do not send data on it. Recent work in [51] shows that optical signal counter-propagation across MCF cores may decrease crosstalk.

**Figure 15.** *Schematic diagram explaining stochastic behaviors and statistical parameters of XT in MCF [28].*

**Figure 15** shows XT's stochastic nature. The ensemble average X is characterized by the unpredictability of its behavior. The values of X are present in the MCF literature, but not explicitly stated. The instantaneous frequency of light is constant while the spectrum of the signal light is flat. Components of XT components that are sufficiently broad act as Gaussian noise, also called ASE noise, or nonlinear interference noise.

#### **5.2 Non linearity**

As a result of the numerous nonlinear distortions, the traditional single mode fiber properties are harmed. Many kinds of SDM fibers have been proposed. In all optical transmission and network scenarios, no single SDM fiber seems to be better than parallel SMF or ribbon fiber. To mention a few, connected core MCFs (CC-MCFs) have shown to be more resistant to non-linearity, resulting in a longer transmission distance as well as processing and amplification benefits. However, FMFs or FM-MCFs have more spatial channels per cladding diameter, making them more suited for short-distance, high-capacity connections [12, 17].

#### *5.2.1 Self phase modulation (SPM)*

The intensity dependence of the refractive index is the main source of SPM (i.e. optical Kerr effect). The change in refractive index as the signal passes through the fiber. Positive and negative refractive index gradients have leading and trailing edges. The total load at the user end of a PoF connection is dictated by the PV cell's conversion efficiency (detector). Eq. (2) states the mathematical connection, Eq. (3).

$$P\_{load} = P\_{in} \* \eta\_{pv} \tag{2}$$

$$P\_{in} = \frac{P\_{load}}{\eta\_{pv}}\tag{3}$$

The nonlinear phase change due the SPM is given by the Eq. (**4**).

$$
\rho\_{nl} = k\_{nl} \* P\_{in} \* L\_{\rm eff} \tag{4}
$$

Where knl denotes the nonlinear component of the propagation constant and Leff denotes the effective length. The following phase equation may be recast in terms of Pload, demonstrating that the nonlinear phase distortion in Eq is caused by load power (5).

$$
\rho\_{nl} = k\_{nl} \* \frac{P\_{load}}{\eta\_{pv}} \* L\_{\epsilon f} \tag{5}
$$

#### *5.2.2 Cross phase modulation (XPM)*

It is a nonlinear optical phenomenon produced by intensity changes in refractive index. XPM is guided by SPM since both rely on the refractive index and intensity of separate transmission pulses. Asymmetric spectrum broadening and signal distortion are caused by power and refractive index changes. The effective refractive index is given by Eq. (6).

$$
\eta \epsilon \sharp f' = \eta \text{ l} + \eta \text{nl} \left(\frac{P\_{in}}{A\_{\epsilon f}}\right) \tag{6}
$$

This is the linear component of the refractive index profile. Similarly, as seen in Eq., the propagating constant is defined as linear and nonlinear (7)

$$k\_{\sharp \overline{f}} = k\_l + k\_{nl} \* A\_{\sharp \overline{f}} \tag{7}$$

The effective refractive index and propagation constant are proportional to the effective area. It is possible that the number of cores required for high power applications has a substantial effect on the link's nonlinear distortion. Also in Eq., the core multiplicity factor [21] and N-number of core and D-cladding diameters define the Aeff (8)

$$A\_{\rm eff} = \left[ \text{CMF} \ast \left( \frac{D}{2} \right)^2 \ast \pi \right] / N \tag{8}$$

Thus the Eq. (7) can be re written

$$
\eta\_{\rm eff} = \eta\_l + \eta\_{nl} \frac{N}{\text{CMF} \ast \left(D/2\right)^2 \ast \pi} \tag{9}
$$

According to Eq. (9), the effective refractive index (cause of XPM) is likewise affected by the number of cores in MCF.

#### *5.2.3 Stimulated Raman scattering (SRS)*

MCF has a large doped area where several optical beams may propagate. SRS is created when nonlinear acoustic vibrations interact with optical photons. The overlapping of the signal and pump electric fields at different excitation settings determines SRS efficacy. The effect of SRS for MCF for PoF connection has not been investigated. Because Aeff and Leff influence threshold power, the number of cores, cladding diameter, and input pump power impact output power. Longdistance transmission weakens power signals, and optical beams' frequency changes downstream, producing signal loss [52].

#### *5.2.4 Stimulated Brillouin scattering (SBS)*

The performance of every optical link is affected by scattering. The number of cores improves the fiber's high power transmission capacity while decreasing the back scattered photon power, which influences the medium's nonlinearity and the acoustic photon. Within the core region, both weakly and strongly connected cores may be linear, triangular, rectangular matrix, tightly spaced hexagonal, or any other symmetric or asymmetric structure. The small core pitch type fiber has the greatest crosstalk and possible photo interaction. A PoF connection's maximal optical power transmission is limited by this interaction. Since the large numerical aperture (NA) is responsible for beam diffraction, the number of cores determines the SBS threshold. SBS changes depending on the medium's characteristics (homogeneous or birefringent) and the optical source. The thermally generated photon field affects the spectral breadth. The temperature of the fiber and its surroundings induce heat dispersion. The strain produced by internal heat may damage the fiber, reducing the output optical power. SBS has a lower effect on MCF than single mode fiber [53].

#### *5.2.5 Optical pulse compression*

MCF uses pulse compression and combination extensively. All MCF cores combine the injected optical signal. Structure and density determine signal compression. The MCF's nonlinearity produces self-focusing, anomalous dispersion, and wave collapse at high power levels. Due to the constant distance between MCF cores, the spatial non-uniformity of coupling is very important. The coupling coefficient, which determines Gaussian statistics, fluctuates with distance. Inhomogeneity in coupling causes phase mismatches and pulse delays that require special care [54].

#### *5.2.6 Capacity wastage*

If network capacity is bidirectional, overusing data centers wastes considerable capacity. To minimize MCF network effects, we asymmetrically distribute the fiber cores. It minimizes inter-core interference and allows for varying the amount of fiber cores on each side of a fiber connection. This minimizes network capacity wastage owing to mismatched bidirectional traffic demand. The suggested approach is tested on the MCF optical network's routing, spectrum, and core assignment (RSCA) problems. Two ILP models and a graph-based heuristic method are suggested to improve network spectrum usage [51].

#### **6. Conclusion and future direction**

Over the last decade, it has become apparent that MCF technologies are the only viable solution to the optical network's "capacity crunch" and other issues. Due to fiber nonlinearity, which limits growing transmission power and amplifier bandwidth. MCF should be operating by 2025. SDM's endurance and demand for telecommunications services must be shown. MCF just exceeded SSMF's maximum capacity. Increased capacity, dependability, and cost-effectiveness are required to allow broad use. New possibilities in multi-mode, spatial coding, and efficient DSP are anticipated to improve the performance of next-generation optical communication systems. This chapter examines the realities of multi-core fiber-based SDM optical wiring. The most common SDM fiber is the UC-MCF. SDM fibers use MIMO DSP to cope with modal XT.

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

Bidirectional traffic demand asymmetry is growing, leading to substantial capacity waste while building and running an optical transport network. Asymmetric and counter-propagating MCF fiber core allocation is advised for MCF optical networks. Assigning a flexible number of fiber cores in opposing directions to reduce network capacity waste owing to asymmetric traffic demand.

Inter-core crosstalk and traffic demand imbalance are significant factors in MCF optical network design. This network's design reduces inter-core crosstalk and capacity waste owing to bidirectional traffic demand imbalance.

Assemblies and PIC fabrication procedures are all part of SDM. When light couples the fibers, the cores converge and link the PICs. It includes extending the cores and attaching them to the PIC entrances through photonic wire bonding. Complicated handling and fusing are needed, but time-control introduces propagation delays.

Power over fiber technique uses multicore fiber structures. This chapter examined various MCF variations. The hexagonal MCF form is recommended for high power applications. Our MCF losses were also addressed. Nonlinear distortions in MCF act differently than in SMF. Some nonlinearity compensating methods, such pre-distortion, may also help reduce the impact of such distortions. The fiber cores, modes, or a mix of both provide new difficulties and possibilities for future research.

## **Acknowledgements**

We thank Dr. Muneer Azam for assistance with particular for comments that greatly improved the manuscript.

### **Author details**

Muhammad Irfan Anis\* and Hamdan Ali High Performance Research Group, FEST, Iqra University, Karachi, Pakistan

\*Address all correspondence to: mirfananis@iqra.edu.pk

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
