Figure 1. Multi-Core Fiber Geometry
This paper aims to make understand the fundamentals and recent advancement in Multi-core Fiber Technology using Space Division Multiplexing. Few Mode Multi-core Fiber (FM-MCF) that enable Space Division Multiplexing (SDM) have greater potential to improve the transmission capacity compared to Single Spatial Mode Fiber (SSMF). The concept of Heterogeneous Few Mode Multi-core Fibers has paved its way in optical communication system by replacing Homogeneous Few Mode Multi-core Fibers which were previously opted. The uncoupled Multi-core Fibers (MCFs) which can utilize multiple cores are arranged in a fiber as spatial transmission channels and then is used for the SDM transmission. Design of 36 core and 3 mode is also demonstrated. Measurement of Inter-core XT for different bending radius is studied. And to give the readers a glimpse of recent development in Multi-core Fiber (MCF) technology, some noticeable research papers have also been discussed. System implementations based on MCF are mentioned along with future research directions.
Optical fibers have low loss and are used for large capacity transmission, therefore they are spread remarkably. The architecture of optical fiber communication consists of a transparent core which is surrounded by a transparent cladding material with a lower refractive index. Then, the light is made to propagate in the core by the phenomena of total internal reflection which makes the fiber to act as the waveguide. Also, optical fibers are playing a vital role as a transmission line of communication system which enable us to exchange the information between the overseas countries.
Advantages of optical fiber are as follows:-
In optical fiber communication, we have many issues related to capacity limits and to overcome this problem we need to increase the spatial efficiency within the available fiber cross section. And to achieve this, an effective solution of MCF is used in conjunction with SDM (Space Division Multiplexing). So, it is one of the most promising and an attractive technology for further enlargement of fiber capacity. Figure 1 illustrates the Geometry of Multi-Core Fiber.
Figure 1. Multi-Core Fiber Geometry
In both the technologies SDM/MDM transmission, and the channel counts are increased and enhanced by multiplication of spatial and modal channel counts, i.e. N x M.
Optical communication technology is growing rapidly for several decades and supports our Increasing information driven society and economy. Commercial systems now utilize multiplexing in time, wavelength, polarization, and phase to send more information. Also, it is possible to manufacture fibres supporting hundreds of spatial modes or containing multiple cores, which can be exploited as parallel channels for independent signals. Therefore, Space Division Multiplexing is regarded as the next frontier in the optical communication system and its architecture is shown in Figure 2. The relation between the transition of introduced multiplexing techniques and transmission capacity of a fiber is shown in Figure 3.
Figure 2. Architecture of an N*N SDM Transmission System utilizing cohorent MIMO digital signal processing. MUX/DEMUX: Multiplexer/Demultiplexer, CoRx: Cohorent Receiver
Figure 3. Increase of Capacity of an Optical Fiber
A comparison of different types of SDM techniques is summarized in Table 1 based on the devices and techniques currently available. It can be seen that, despite of its disadvantages like high complexity due to the MIMO algorithm, SDM technique based on FMF has several advantages such as reduced number of devices like amplifier, ROADM and most importantly, the N times channel capacity that can enable large future capacity of Terabit and beyond optical networks.
Table 1. Comparison of Different SDM Technologies
In order to realize high capacity transmission using Heterogeneous MCFs, it is required to increase the count of cores in a fiber. To compare the core density of MCFs, Spatial Efficiency (SE) or Core Multiplicity Factor (CMF) can be used, which is defined as:
where n is a number of cores with effective area (Aeff ) in a cladding and CD is a cladding diameter. The CMF is used to indicate the core area ratio in a cladding. The expression for calculation of cladding diameter is
The novel input and output devices, called Fan-in / Fanout (FI/FO) devices, for MCFs are indispensable to construct the SDM transmission system using MCFs. Since the MCFs have multiple cores which are separately arranged in two dimensions to avoid the coupling between cores in a fiber, and these fibers cannot be directly connected to conventional optical devices[3, 8, 9]. Several kinds of FI/FO devices, so far, have been developed such as fiber bundle type, femtosecond laser inscribed three dimensional waveguide, mosquito polymer waveguide, and so on.
The amplifier of the MCFs is inevitable to realize the long haul SDM transmission. In the same way as the single core transmission, the Erbium-doped Fiber Amplifier (EDFA) also can be used in MCF transmission. The multicore EDFA have been already demonstrated in 2011 [10] 28]. However, MC-EDFA is not suitable for the large count spatial channels transmission from the viewpoint of cost and power consumption, because the number of the pump source of amplifier increased as the number of cores. To overcome this matter, the cladding-pumped MC-EDFA, in which the light signals in the separated cores are amplified simultaneously using only one laser diode was proposed [29, 30]. The number of cores to be multiplexed in a fiber is determined by the core-to-core distance and outer cladding diameter is fixed. The core-to-core distance is small in uncoupled MCFs and this results in large crosstalk between neghbouring cores. The suppression of crosstalk between cores is small and is one of the critical issues for practical use of uncoupled MCFs.
A brief description of different MCF types according to their number of cores and structures are enlisted in Table 2.
Table 2. Parameters, Number of Cores and Core Geometries of Different MCF Types
In Figure 4, it can be seen that in 7-core structure, the large core-to-core distance is acceptable, but the CMF becomes low. Also, on the other hand, higher core density is achieved by using 13-core or 19-core structure. However, the maximum allowable core-to-core distance is approximately 43µm and 37µm, respectively, in 13-core fiber and 19-core fiber structures respectively. DMD can be decreased by the use of supermodes.
Figure 4. Relation between Number of Cores and Condition of Core-to-core Distance in MCFs with Hexagonally Close Pack Structure, where the Cladding Diameter (CD) and Cladding Thickness (CT) are assumed to be 230 µm and 40 µm, respectively
The applications of MCF Include:
A number of core arrangements are there for uncoupled MCFs, including homogeneous MCFs with multiple identical cores, quasi-homogeneous MCFs with numerous types of slightly different cores, and heterogeneous MCF’s with several kinds of different cores.
In addition, each core is designed to support not only a single mode, but also a few modes or even multiple modes. These are termed as Multi-Core Single Mode Fiber (MC-SMF), Multicore Few Mode Fiber (MC-FMF), and Multi-Core Multi-mode Fiber (MC-MMF) respectively. The classification of these fibers is summarized in Figure 5.
Figure 5. Types of MCFs
MCFs are categorized into two types, i.e. homogeneous and heterogeneous MCFs. In homogeneous MCF, all cores are similar to each other and the core to core distance dominates the core density of Multi-Core Fiber (MCF), which in turn reduces crosstalk level along a propagation length to an acceptable level [10]. As it is a well-known fact that maximum power transferred between the cores goes down prominently if cores have the difference in their core radius. Therefore, in case of heterogeneous MCF, nonidentical cores are arranged which in turn ensures small crosstalk between any pair of cores and cores are more efficiently packed as compared to homogeneous MCFs.
Figure 6 demonstrates the progress of high capacity transmission experiments using MCFs, FMFs, MMFs, and FMMCFs in recent years. The configurations and the colors of the dots are used to represent the types of the fiber and classification of the transmission scheme used. The different shapes, i.e. circular, triangular, rectangular, and pentagram dots indicate the MCF, FMF, MMF, and FM-MCF, respectively.
Figure 6. Increase of Transmission Capacity of MCFs
The blue, red, and yellow dots corresponds to the SDM, MDM, and SDM/MDM, respectively [1], [2], [4-7], [21], [31-42].
In Figure 7, different types of heterogeneous fibers are demonstrated. In a 7-core heterogeneous MCF, three different types of single mode cores are arranged in the form of triangular lattice arrangement as shown in Figure 7 (a). The core pitch (Λ) is 40 mm and core to core distance between similar types of cores is 69.3 mm. Figure 7 (b) shows another arrangement of heterogeneous MCF, called as rectangular lattice arrangement. The core pitch (Λ) is 35 mm and core to core distance is 70 mm . Figure 7 (c) and Figure 7 (d) shows the 19-core triangular arrangement and 12-core rectangular arrangement respectively.
Figure 7. Different Types of Heterogeneous Fibers
36 cores consists of 3 different core types which are arranged in a 4-layer hexagonal lattice, are shown in Figure 8 (b) and Figure 8 (c). Also, index profile of the three core types displayed in Figure 8(a). The relative refractive index difference of each core (Δ+ ) from the outer cladding is different for all three core types.
Figure 8. FM-MCF Structure (a) Refractive Index Profiles of Cores (b) Cross-sections of the Fabricated MCF (c) Schematic Design Illustrating Core Arrangement, Core Types, Marker Position, and Identification Number
For type A it is 0.74%, for type B it is 0.64% and for type C it is 0.54%. All three core types support LP01 mode and LP11 mode. Type A cores are arranged on the outermost vertices as it has strongest mode confinement which allows it to use a thinner outer cladding design to keep the attenuation below 0.001 dB/km due to coupling to the coating. The core pitch is 34 µm, and the cladding diameter is 306 µm. In Table 3 the optical properties of fabricated 5.5 km FM-MCF is listed for three core types. Measurement of effective areas for all core types and cutoff wavelengths show similar results. This is suitable for C-band transmission.
Table 3. Optical Properties of the Fabricated MCF At 1550 Nm (Parentheses Indicate Core ID Numbers)
The inter-core XT is suppressed when the fiber bending radius is larger than the critical bending radius which is of the order of a few cm. It has been evaluated with coupled power theory based on the measured index profile of the cores which is shown in Figure 9. Above 100 mm, XT becomes practically independent of the bending radius and below 30 mm, bending radius becomes critical. Radius of the fiber spool is calculated as 140 mm. The highest inter-core crosstalk of about -31 dB occurs between the LP11 modes for transmission distance of 5.5 km. While inter-core XT of the FM-MCF is tolerable from the viewpoint of transmission, both intercore and mode XT are caused by the MUX/DEMUX and fiber splices. Due to such mode XT, MIMO processing is necessary to be performed among all the three modes. Lower Differential Mode Delay (DMD) is preferable to minimize the MIMO complexity.
Figure 9. Inter-Core Crosstalk Versus Bending Radius for Different LP Mode Combinations of Core Types B and C. (The Dashed Vertical Line Indicates the Fiber Spool Radius)
There are some relevant papers which further demonstrates an emerging technology, i.e. "heterogeneous Few Mode MCF". There descriptions are mentioned below
This is an invited paper in which weakly coupled MCF technology for the application of high capacity SDM transmission has been described. Multi-Core Fibers (MCFs) are expected to be as a good candidate for overcoming the capacity limit of a current optical fiber communication system. It also demonstrates that the recent progress on, the MCFs for space division multiplexing to be utilized in for future large capacity long distance transmission systems. Trade off issues between low crosstalk and high core density in MCFs is described and prospect of large space multiplicity of MCFs is mentioned.
The design and features of a novel three mode 12-core fiber is demonstrated in this paper. The fiber has low intercore crosstalk (IC-XT) with low Differential Mode Group Delay (DMD). In order to produce such a fiber, three new techniques are introduced to few mode multi-core fiber. To minimize IC-XT, a heterogeneous core arrangement with two types of cores, is used.
A square lattice structure allows the inclusion of 12 cores within a cladding diameter of 230 µm. Optimum design of the fiber using these techniques is determined from analysis and calculations. Finally, detailed characteristics of a three-mode 12-core fiber based on the design are reported. A fabricated 40-km fiber is also confirmed to have a DMD (Differntial Mode Delay) of less than 530 ps/km over the C + L band and an estimated worst case Intercore Crosstalk (IC-XT) of less than 55 dB/100 km at 1550 nm. Therefore, this low value of inter-core crosstalk allows for the transmission of 32 QAM signals.
Design and characterization of a heterogeneous three mode, 36-core fiber with three core types is described in this paper. Inter-core crosstalk for LP11 modes is estimated to be below -31 dB for 5.5 km propagation at a core pitch of 34 mm. Feasibility of 108 space/mode division multiplexed transmission is investigated using free-space multiplexing/ demultiplexing technologies, 40 Wavelength Division Multiplexed (WDM), 25 GBd, 93.4- Gb/s dual-polarization QPSK signals, and coherent detection with a sparse 66 MIMO equalizer is also determined. The total transmission capacity results to 403.7 Tb/s.
This paper describes the design and characteristics of a 3- type heterogeneous 36-core, 3-mode fiber with record spatial channel count and density and performs transmission measurements in all 108 spatial channels using 40-100 GHz spaced 25 GBaud DP-QPSK signals. Also it consists of fabrication and characterization of a 36-core 3- mode fiber based on a three type heterogeneous core structure. Spatial channel count of 108 and density of 18.0 compared to SSMF is believed to be the highest for any optical transmission fiber reported to date.
This paper reviews recent progress of space division multiplexing technologies using few mode and multicore fibers. Heterogeneous design has been shown as a key technology for dense core arrangement. Highest spatial channel density and over 100 spatial channel counts are realized with a heterogeneous 36-core 3-mode fiber. Total 36 cores can be hexagonally packed with a 34µm core pitch, while suppressing LP11 mode inter-core crosstalk down to around -31 dB, expectedly. It is also expected that such FM MCFs will eventually afford future optical networks a great increase and enhancement in the throughput.
This paper reviews the recent progress in Space Division Multiplexed (SDM) transmission, the first demonstration of dense Space Division Multiplexing towards ultra-high capacity optical transport systems is mentioned. Reducing inter-core crosstalk and increasing Aeff simultaneously is necessary to realize ultra high capacity transmission systems with MCFs and MC-FMFs. Hence, it can be concluded that few mode fiber is much more efficient for information transmission compared to multimode fiber.
This paper reports the first 1024 QAM polarization multiplexed transmission at 3 Gsymbol/s over a 55 km 7- core fiber, with a total bit rate of 420 Gbit/s (60 Gbit/sx7 cores). The potential spectral efficiency per core reached 15.6 bit/s/Hz, which corresponds to a total spectral efficiency as high as 109 bit/s/Hz in a multicore single mode fiber. The obtained results indicate that the influence of MCF crosstalk is still negligible over this distance, and thus a longer transmission reach can be expected by employing a multi-core EDFA.
In this paper, the concept of supermode is described for long distance optical transmission systems. The super modes uses coupling between the cores of a multi-core fiber, in which the core-to-core distance is much shorter than the conventional multi-core fiber. Using supermodes can lead to a larger mode effective area and higher mode density than the conventional multi-core type of fiber. Through simulations, it is proved that the coupled multicore fiber allows mode coupling, lower modal dependent loss and differential modal group delay than few mode fibers. These properties prove that the coupled multi-core fiber can be a good candidate for both spatial division multiplexing and single mode operation.
This paper demonstrates a method to estimate the capacity limit of fiber optic communication systems based on information theory. Also, this paper is divided into two parts. Part (1) reviews fundamental concepts of digital communications and information theory. It treats digitization and modulation followed by information theory for channels both without and with memory. It also provides explicit relationships between the commonly used signal to noise ratio and the optical signal to noise ratio. It further evaluates the performance of modulation constellations such as combinations of amplitude shift keying and phase shift keying, quadrature carrier multiplexing also called (QAM), exotic constellations and several concentric rings for the additive white Gaussian noise channel which uses coherent detection. Part (2) is devoted specifically to the channel of fiber. It provides the review of the physical phenomena present in transmission over optical fiber networks, including sources of noise, the necessity for optical filtering in optically routed networks, and, most critically, the presence of fiber Kerr non-linearity. It also describes various transmission scenarios and impairment mitigation techniques, and define a fiber channel considered to be the most relevant for communication over optical networks. Then it proceeds to determine the capacity limit estimate for the fiber channel using ring constellations. Several scenarios are examined, including uniform and optimized ring constellations, several different fiber dispersion maps, and varying transmission distances. It further presents evidences that point to the physical origin of the fiber capacity constraints and provide a comparison of recent record experiments with the capacity limit estimation.
This paper describes a new type of optical fiber called heterogeneous Multi-Core Fiber (heterogeneous MCF) towards future large capacity optical transport networks and its design principle. In the heterogeneous MCF, identical as well as non-identical cores, which are single mode and isolated from each other, are arranged so that the crosstalk between any pair of cores is sufficiently small. As the maximum power transferred between non-identical cores goes down drastically, cores are more closely packed in definite space compared to a conventional, homogeneous Multi-Core Fiber (homogeneous MCF) composed of only identical cores.
In this section, future research directions for heterogeneous few mode MCF based systems have been presented. They are as follows:-
Minimization of inter-core crosstalk is a major research issue in present day MCF systems. Crosstalk is the phenomena in the MCF systems in which signal propagating in one core affects the propagation of signals travelling in other cores. It has been also observed that most of the previous works are concerned with the reduction of crosstalk of 7-core fibers. This can be applicable to the reduction of the crosstalk in high density core MCFs such as 12-core and 19-core homogeneous as well as heterogeneous MCFs.
Coupling loss is still a critical issue in designing of MCF based systems. It is observed from the previous work that most of the coupling devices are bulky that cannot fit easily into compact MCF systems. Further research works can be implemented in the development of miniature sized coupling devices that can be integrated with other waveguide type devices such as Wavelength Division Multiplexing filters.
Regeneration of optical signals across a long-haul link and placement of optical amplifiers is a major research interest in today’s SSMF systems. This concept can be further extended in MCF based systems. It is also observed that further research can be performed in the development of hybrid amplifier for MCF based system that can operate over entire C and L band.
It can also be expected that the transmission capacity of the FM-MCF can be further enlarged and maximized by the development of the technologies related to the mode multi/demultiplexing and processing which can support large number of modes.
In this paper, the authors have studied recent progress on MCF researches to date for high capacity SDM transmission focusing on the uncoupled MCFs. Because of the limitations of the outer cladding size of MCFs related to their mechanical reliability, the number of cores as well as the core arrangement have to be carefully determined based on the required modulation format and transmission distance. In addition, the combination of MCF and FMF, which is FM-MCF, is a very promising and advanced approach to realize capacity-distance product over Exa b/s/fiber km. In order to realize relative spatial efficiency of larger than 20, FM-MCF is required and further development on related devices such as Fan-in/Fan-out and amplifier for FM-MCF transmission is highly expected. Multi-Core Fiber (MCF) based Space Division Multiplexing system can be an answer to the increasing demand of bandwidth. Also, the design of 36-core, 3-mode fiber is analyzed which gives transmission capacity of 403.7 Tb/s. So, it can be further expected that we can enhance the following parameters by varying the number of modes, cores and other MCF geometries using a known optical system simulator named Optiwaves OptiSystem
The authors would like to thank the supervisors Mr. Sharad Mohan Shrivastava and Mr. Vikas Sahu for their cooperation, encouragement and advice during entire paper work, and also to anonymous reviewers for their helpful comments and suggestions.