**4.1 Space division multiplexer/demultiplexer**

MCF technology uses SDM MUX/DEMUX. There are now numerous options, each with its own footprint, cost, capabilities, multi-mode affinity, etc. An SDM multiplexer or demultiplexer effectively links light between SMF fibers and SDM fiber modes or cores. Spatial MUXs are needed for SDM studies and may be used to link SMF and SDM networks in the future. This connection has been suggested in many creative ways. Direct and indirect coupling methods are widely classified. The optical signal is fully confined inside a waveguide during connection. **Figure 10** shows two typical layouts. As illustrated in **Figure 10**, the SMF cladding diameter is tapered to splice a bundle of SMFs to the SDM fiber. A photonic lantern is made by compressing MCF or SMF cores into an FMF. Alternatively, an unneeded

waveguide may be used. Inscribed light-guiding cores on a tiny glass block form the waveguide [12].

**Figure 10** shows the inscribed cores in the waveguide output plane separated by the MCF core separation. Input and output SMF arrays are connected using UVcured glue. In addition to fiber producers, Chiral Photonics uses the fiber bundle technique often. Optoscribe's commercialized 3D waveguide technology is easy to incorporate with photonic integrated circuits (PICs). Indirect coupling uses bulk optics like lenses and prisms [12].

#### **4.2 Transceiver**

Transceivers are devices that combine the operations of a transmitter and a receiver into a single unit. They connect the network to a computer module in both directions. Their duty is to generate optical signals and then convert them to electronic data. Aside from the form factor and connectors, the optical and electrical properties are significant factors to consider throughout the selection process. The transmitter determines the wave properties of the transmission. The wavelength, spectral breadth, and transmission power are all critical parameters. Other transmitter characteristics include wave modes, reflections, and so on. To receive an optical signal, the receiver must be set to the proper wavelength. Furthermore, the signal's polarization and power must be compatible. Photo-diodes and other lightsensitive semiconductors transform optical impulses into electrical signals that may be monitored for data extraction. The received power must be within the detector's permitted range. If the power is too low, it is impossible to differentiate between signal and thermal noise, resulting in a poor signal-to-noise ratio. The detector gets overwhelmed if the received power is too high. Modulations of the luminous flux are not detectable. Overloading may permanently harm the detector and should therefore be avoided [42].

#### **4.3 Connectors**

Connectors are devices that connect optical wires. Connectors are required for SDM systems such as fusion splicing in terrestrial and submarine trunk networks. For a variety of cable types and transmission methods, many connection types have been created. Due to the fiber break, the link transmission is lossy. Lenses, end polishing, and forms are utilized to decrease attenuation. M-type connections were used for 7-core MCFs with an IL of 0.13 dB and a 500-fold improvement in MTBF. A multi-fiber MPO connection with over 40 dB return loss and 0.85 dB IL. A 7-core MCF connection with a return loss of 45 dB and an MPO connector for four 7-core fibers with a return loss of 0.3 dB are also shown in the study.

#### **4.4 Amplifiers**

Erbium-doped MCF amplifiers may be constructed utilizing separate pump lasers. Sharing pumps across multiple cores enhances power efficiency. Another approach is to pump the MCF's cladding, which is outfitted with multi-mode lasers. To achieve greater efficiency than an array of SSMF EDFAs, more power must be injected [43].

#### **4.5 Switches**

A network's heart is comprised of switches. Switches manage signal paths between nodes. In traditional copper networks, this routing is based on data packet IDs. Routing in optical networks, on the other hand, may be based on physical

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

**Figure 11.** *Core switching with spectrum contiguity within an optical network that uses multi-core fiber transmission.*

signal properties. This may be the wavelength in WDM or the core in multi-core fiber transmission, for example.

**Figure 11** is an example of a signal route in a switch. All the linked wires has seven cores. Every signal has its own core. When routing a signal, it may be done dynamically or statically. The physical routing technique is switch-dependent, which means that different switch types will have different methodologies. Due to physical coupling effects between the various cores, the range of multi-core fiber technology is limited to km. The limited flexibility of single-core fibers is distinct from that of multi-core fibers [44].

Low-speed SDM optical switches are already promising technologies, and preliminary work on SDM optical switches based on MEMS or LCoS technologies has been done. This will benefit WAN networks that need high-layer packets to be routed directly into the optical domain. Optical fast-switching networks have never achieved broad adoption owing to building difficulties, quicker signal degradation, and lower-cost electronics.

These switching granularities arise from the spatial component of SDM networks: Space granularity (joint switching) is needed when all modes intermix. Fibers like FMFs have full wavelength granularity in fractional space. Recently, several papers on SDM optical switches have appeared. In [36], a heterogeneous WSS switches spatial channels in an FMF, SSMF array, and SC-MCF. [36] claims a three-port four-core MCF WSS for SDM with 34 dB crosstalk and IL under 2.2 dB. Reference depicts a silicon PIC with a 7 7 switching matrix (MZIs) with an insertion loss of [4.5, 7.0] dB. Acoustic-optical crystals may also be used to create SDM optical switches. **Figure 12** depicts a CJ-AOM switch for 7 spatial channels. A 10 second switching time with an insertion loss of 10 dB.

There are spectrum resources in each core in SDM-EONs. All Spectrum slots are created equal. Following the spectrum contiguity requirement implies the whole service must utilize the same spectrum slots along the lightpath. To keep spectrum continuity constraint in a fiber, service spectrum slots must be continuous in the spectral dimension. The OFDM method should be used for each core to enhance spectrum efficiency. Spatially and spectrally resolved optical switching fibers are made as shown in **Figure 13**. In the optical switching fabric, core, fiber, and spectrum switching may be accomplished, which enables flexible channel addition, removal, and wavelength-level granularity channel switching. A transceiver pool supplies the necessary sub-transceivers for the different communication

#### **Figure 12.**

*(a) Core-joint electro-absorption switch diagram, (b) core-joint acousto-optical modulator switch diagram, and (c) core-joint mirror switch diagram [36].*

**Figure 13.** *Spatially and spectrally resolved optical switching fabric.*

requirements. To overcome the spectrum contiguity restriction, spectrum slots in the switch fabric may be swapped between various cores. To summarize, it is possible to flexibly move signal cores without losing spectrum.
