**1. Introduction**

Internet traffic infrastructure is underpinned by optical transmission systems and networks [1]. However, continuous supply of new services like video on demand, Virtual Reality and Augmented Reality have rising data volumes needed to satisfy the requirements of industry, academics, governments, and people presents new challenges to optical communication infrastructure [2]. Fiber-optic communication systems based on conventional single mode single core fibers (SMF) are almost saturated due to amplifier bandwidth, nonlinear noise, and fiber fuse phenomena due to this it's capacity consumption will be beyond capacity limitations by the year 2022 [3]. As a result, researcher has led to a steady push for new, higher bandwidth optical fibers that can replace the SMF.

Space division multiplexing (SDM) methods is one of the potential capacity expansion strategies for an optical transport network. It is transmission fibers that enable concurrent parallel data transmissions on multiple cores in a single cladding or several cores inside a single core to improve speed and data rate [4]. The first demonstration of an SDM link consisting of standard cladding diameter surpassing the typical size of 125 μm 7-core MCFs, highly efficient MC-EDFAs, and MCF connectors transmission above 100 Tbit/s across a 316 km has occurred and considerably greater capacity tests such as over 1 Pbit/s and 1 Ebit/skm were performed using single mode multi-core fiber [5]. SDM fibers are defined as multicore fibers (MCFs) and few-mode fibers (FMFs) or multi-mode fiber (MMF) [6]. MCF and multi-mode fiber technologies provide for additional fiber capacity proportional to the number of cores and modes per fiber [7]. Single-mode cores, contained in a shared cladding, are employed independently in the former. An FMF has one core, which enables several optical modes, each of which may transmit data independently. MCF is a promising technology for providing enormous bandwidth and capacity with regard to information [8]. The MCF help facilitate the data transmission and the transmission of power in high power devices. Multi-core fibers have many positive attributes over conventional fibers: they have significantly decreased core separation and are very regular when compared to free-standing fibers, and they also provide a monolithic package with several fiber features [9]. Moreover, multi-core fibers that allow a few-mode core to combine the fibers results in an extra 100 optical channels in each transmission, as well as throughputs of over 1 Pb/s [10]. Recently MCF-based fiber-optic transmission, a capacity of 1 Pb/s per 32-core fiber has been achieved [11].

Multi-Mode-Multi-Core Fiber (MM-MCF) significantly increases the number of spatial channels to 114 or more, and transmission of 10 Pbit/s was achieved utilizing this multi-mode MCF. Despite these benefits, the MCF may have limitations such as crosstalk (XT), non-linearities, dispersion, and so forth. Over long distances, the accumulation of MCF crosstalk may be the most limiting issue influencing the performance of an optical communications system. As a consequence, in recent years, research in this field has been driven by the development of ultralow crosstalk MCFs [12]. The impact of XT on MCF system capacity and range has recently been studied [13]. However, the results vary with modulation format and transmission reach, leading to the general notion that different network applications, from short-range to ultra-long haul, need different MCF designs. Standardization and mass production are essential for widespread commercial usage of emerging technologies like MCF. An XT standard per unit length of 55 dB/km has been proposed [14]. In an MCF system, the performance penalty must be evaluated against a non-XT system, regardless of unit size (i.e., a fiber bundle instead of an MCF). Capacity and reach penalties are required. Calculation on an optimal MCF core density for long-distances-independent crosstalk specifications have been done. The crosstalk process was originally described in [15], although the majority of crosstalk on a fiber is continuous, it is at discrete places where crosstalk amplifies the most, when core matching circumstances occur. Since the locations and phases of these sites may change randomly, crosstalk in MCFs follows a random chi-square distribution with 4 degrees in time and wavelength [16].

Using MCF's nonlinear distortions for power-over-fiber operations poses a number of challenges. The structure of MCF, which enables for high-power signal transmission via the fibers, has lately received attention. It is recommended that image processing be used; therefore, the present limitation of single mode fiber must be overcome [17].

A 7-core MCF with reduced inter-core crosstalk was used for trans-oceanic transmission. Using MCF and a spectrum efficient modulation scheme, 201 x 100 Gbit/s transmission across 7326 km produced a capacity-distance product surpassing 1 Exabit/skm [18]. These systems propose the MCF as one of many optical transmission techniques.

To transmit 52.2 Tbit/s across 10230 km, the CDP for SM-SCF transmission is 534 Pbit/s/km. The transmission rate was 1.03 Ebit/s km/h [19]. A preliminary test using seven spatial channels and PDM-QPSK yielded 53.3 Tb/s [20]. Using 8 spatial channels with PDM-8PSK, the capacity was 83.33 Tb/s. MMF allows transmission distances up to 1200 km (3 spatial modes x 40 Gbit/s DP-QPSK) and 13.9 Pbit/skm (CDP with MMF)[21]. Uncoupled MCF allows for considerably longer transmission. The 1500 km transmission used a propagation-direction interleaved design to minimize interference between neighboring cores [19].

This chapter investigates MCF-based novel technologies for creating nextgeneration optical networks. We first examine the roadmaps towards optical fiber and examine why there is a need for MCF. We also highlight the newest reports' in MCF paradigm covering design and application. Looking into the main technology as a key functional building blocks for next generation optical communication. Next, we demonstrate the experimental setup of MCF. In last section describe the MCF limitation before conclusion.
