Figure 1. Transmission Capacity on Optical Fiber [3]
This paper aims to make understand the fundamentals and recent advancement in Dense Space Division Multiplexing (DSDM). Similar to Dense WDM (DWDM) with dense wavelength spacing and high count of over several tens of wavelength channels, it is shown that DSDM with high spatial density and large spatial multiplicity is effective for greatly expanding transmission capacity. Multicore fibers and few-mode fibers have potential application in realizing Dense Space Division Multiplexing system. As the main objective was to study the most important parameter that establishes the performance of optical fiber in distance and capacity. As in MCF, many cores tightly packaged together is inevitable that signals cause undesired effects from one core to another. Beside the physical space available for core, the maximum tolerated crosstalk dictates the maximum quantity of cores we can use. Design of single-mode 31 core fiber with Quasi Single Mode (QSM) core is demonstrated. Measurement of Spatial Channel Count (SCC) for different number of cores is studied. And to give the readers a glimpse of recent development in DSDM and MCF technology, some noticeable research papers have also been discussed.
The rapid increase in internet traffic, demands for much higher capacity will increase in optical communication networks for high videos and new data communication services. Figure 1 shows the growth in transmission capacity per optical fiber as well as commercial optical communication systems [15]. Advance in WDM (Wavelength Division Multiplexing) technology allowed the multiplexing of a large number of wavelengths of over 30. With 30 wavelength channels and 100 GHz spacing, the bandwidth is 3 THz, which is in agreement with the amplification range of an Erbium-Doped Fiber Amplifier (EDFA) in the C-band [6,16,17]. Such system was denoted as Dense Wavelength Division Multiplexing (DWDM). Recently digital coherent technology is used to improve the spectral efficiency by using multi level modulation formats and high performance compensation in optical fiber transmission lines [3]. As a result, the transmission capacity per fiber has reached 100 Tb/s in research, and 10 Tb/s in commercial systems. These three major technologies have led to increase in optical fiber transmission capacity by a factor of more than 100,000 times over the past three decades. As the capacity is growing at an annual rate of 1.4 times, and is anticipated to grow at an even faster rate, research and development continues to target larger capacity.
Figure 1. Transmission Capacity on Optical Fiber [3]
For higher capacity, new multiplexing technologies are needed that can offer an addition multiplicity of around ten to hundred times and full compatibility with current Time Division Multiplexing (TDM), Wavelength Division Multiplexing (WDM), and Digital Coherent Transmission technologies [7,10,13]. The use of Space Division Multiplexing (SDM) with Multi Core Fiber (MCF) or Multi Mode Fiber (MMF) has been proposed as the potential next generation mutiplexing technology for optical fiber communication. SDM have high capacity transmission exceeding petabits/s. Further there is an advancement in spatial multiplexing technology, a Dense Space Division Multiplexing (DSDM) with more than 30 spatial channels. Further advances in DSDM studies are expected to realize ultra high capacity and long haul transmission systems. The study toward dense SDM is shown in Figure 2.
Figure 2. Dense SDM Optical Communication [5]
The issues for all few mode multi core transmission includes reducing DMD and inter core crosstalk in a few mode core MC-FMF. The commonly used method to reduce DMD in FMF transmission using proper refractive index profile design along with splicing several FMFs with positive and negative DMDs [4, 21]. However, adjusting DMD by managing different types of fiber makes the transmission line complicated. The authors employed novel parallel MIMO signal processing for handling DMD, which allowed us to transmit DSDM signals without the need for fabricating several MC-FMFs with opposite DMD characteristics and optimizing fiber combinations. Figure 3 shows an example setup of multi core and multi mode based DSDM transmission. Fiber have low loss and low crosstalk multi core few mode fiber and high precision low loss FI/FO devices have been developed. As regards device technology, various types of mode (MUX/DEMUXs) multi/demultiplexers have been used [18, 20]. The future ultra high capacity and long distance DSDM transmission should find further advances in signal processing, transmission scaling, device integration, and power efficient multi core and multi mode amplification technologies.
Figure 3. DSDM Transmission System using Multi-core and Multi-mode [21]
Spatial efficiency, is defined as the spatial multiplicity divided by the fiber cross sectional area, was previously employed to examine efficiency in terms of the use of space per fiber [2,19]. In this work, the expressions to be used are the SCC, SE, and Relative SE (RSE) as a measure of the spatial multiplicity of MCFs and FMFs [11,12].
where CC is the Core Count in a fiber, MC is the Mode Count in a core, and DC is the cladding diameter of a fiber.
DMD is a major issue for long distance few mode transmission, since signal processing complexity increases with DMD. Various fiber designs have been used for FMF transmission, such as Step Index (SI), multi-SI refractive index profile, and Graded Index (GI) refractive index profiles. SI FMF typically has a DMD of 3 ns/km, but it can be less than 25 ps/km for three-mode GI-FMF. A low DMD of <63 ps/km has been used for GI type MC-FMF in the C-band. Signal processing complexity is to reduce the DMD in an FMF. Similar to the case of chromatic dispersion in an SMF, fiber management is reducing the maximum DMD in an FMF. DMD compensation that combines FMF spools with DMDs with opposite signs has been reported to suppress the overall maximum DMD. For the optical fiber, a 40.4 km low loss, low crosstalk, and low DMD 12 core 3 mode MC-FMF composed of two types of heterogeneous cores is employed. The two types of cores were designed to have a multi step index profile and a trench structure.
A heterogeneous core is quit effective for reducing XT. However, precise process control of the core structure is required for preparing heterogeneous cores. The authors demonstrated a high core count MCF with homogeneous structure [8, 9].
QSM (Quasi Single Mode) transmission is a technique that uses only the fundamental mode of an FMF. QSM transmission experiments over the FMFs have been successfully demonstrated. The advantage of QSM transmission is the reduction of the non-linearity of transmission lines. An advantage of applying QSM transmission to MCF designs is the XT reduction owing to the high confinment of the LP01 mode in a few mode core. The suppression of XT enables a dense core arrangement within a limited range of cladding diameters. The undesired remaining LP11 modes have been reported. The direct coating absorption of the inner core is far smaller than that of the outer most cores. However, the power of an LP11 mode is expected to leak to a coating through the XT from the inner cores to the outer cores. The authors designed a QSM MCF using the above concepts. The fiber contains 31 trench assisted homogeneous cores in a hexagonal close packed structure with a cladding diameter of 230 m. Figure 4 shows a cross section and the layer/core assignments of the fiber.
Figure 4. A Fabricated QSM 31-Core fiber, (a) A Cross-sectional View, (b) The Denition of the Layer and Core Numbers [2]
MCFs consist of coupled MCFs and uncoupled MCFs. In the case of coupled MCFs, the transmission LP-modes in each core are strongly coupled among the cores, and supermodes are generated. DMD can be decreased by the use of supermodes. In case of uncoupled MCFs, the cores are used as independent waveguides to reduce the signal processing load. FM-MCFs have been mainly based on uncoupled MCF technology [1]. In designing these uncoupled FM-MCFs, the following issues relating to the MCFs and FMFs designed: (a) IC-XT, and (b) DMD. Different types of fibers are shown in Figure 5.
Figure 5. Types of MCFs [21]
There are some relevant papers which further demonstrates an emerging technology, 'Spatial Multiplicity Enhancement for Dense Space Division Multiplexing'. Their descriptions are mentioned below.
This is an invited paper, where multi core fibers and few mode fibers have potential application in realizing Dense Space Division Multiplexing systems. However, there are some trade off requirements for designing the fiber. In this paper, the trade off requirements such as spatial channel count, crosstalk, differential mode delay, and cladding diameter are discussed. Further, the design concept and transmission characteristics of high core count single mode multi core fiber are discussed. A heterogeneous multi core fiber with 30 cores and quasi single mode multi core fibers with 31 cores are developed.
In this paper, recent progress in Space Division Multiplexed (SDM) transmission, and the authors demonstration of Dense Space Division Multiplexing (DSDM), the ultra high capacity SDM transmission system with high spatial density and spatial channel count of over 30 per fiber have been discussed. The authors introduced the SDM transmission matrix, various types of multi-core multi-mode transmissions according to the type of light propagation in optical fibers and how the spatial channel are handled in the network. For each category in the matrix, latest advances in transmission, studies and evaluates the transmission performance by spectral and spatial efficiencies. The authors expound on technologies for multi-core or multi-mode transmission, optical fiber, signal processing, spatial multi/ demultiplexer, and amplifier, in configuring DSDM transmission system and review the first DSDM transmission experiment over a 12 core 3 mode fiber.
Few- Mode Multi Core Fibers (FM-MCFs) that enables Dense Space Division Multiplexing (DSDM) have the potential to drastically improve the fiber capacity. In designing the FM-MCFs, several issues that originate from the few mode fibers and multi mode fibers must be considered. In this paper, these design issues such as Inter-core Crosstalk (IC-XT) and Differential Mode Delay (DMD) are discussed. Three-mode 12-core fiber with low DMD and low IC-XT achieves long haul DSDM transmission over 500 km. The design concept, fiber design, and characteristics of the labricated three mode 12-core fiber are also described.
In this paper, super mode is introduced for long distance optical transmission systems. The supermodes exploit coupling between the cores of a multi core fiber, in which the core-to-core distance is much shorter than conventional multi core fiber. The use of supermodes lead to a larger mode effective area and higher mode density than the conventional multi core fiber. Through simulations, it is shown that the proposed coupled multi core fiber allows lower modal dependent loss, mode coupling, and differential modal group delay than few mode fibers. These coupled multi core fiber is a good candidate for both spatial division multiplexing and single mode operation.
This paper reviews the 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 count are realized with a heterogeneous 36-core 3-mode fiber. Total 36 cores can be hexagonally packed with a 34 m 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 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 defines 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 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 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 are the prospect of large space multiplicity of MCFs is mentioned.
In this paper, the authors have studied the recent progress on MCF researches to date for high capacity DSDM transmission focusing on the number of cores and counts to increases the spatial channels. FMF transmission can outperform in terms of the fiber cost, average of photonic integration device, easy fusion splicing and low non-linear limit. Lower Differential Mode Delay (DMD) is preferrable to reduce the MIMO complexity. Supermode [14] is introduced to reduce the DMD. FM-MCF is required and further development on related devices such as Fanin/ Fan-out and amplifier for FM-MCF transmission is highly expected. The maximum transmisable capacity through a Single Mode Fiber (SMF) has increased approximately 1000, 100, and 10 times through the use of various multiplexing technologies, namely Time Division Multiplexing (TDM), Wavelength Division Multiplexing (WDM), and digital coherent technologies. Multi Core Fiber (MCF) based space division multiplexing system can be an answer to the increasing demand of bandwidth. So, it can be further expected that the authors can enhance the following parameters by varying the number of modes, cores and other MCF geometries using a known optical system simulator named Opti System by Optiwave:
The authors would like to thank the supervisors Mr. Vikas Sahu and Mr. Sharad Mohan Shrivastava for their co-operation, encouragement and advice during their entire paper work, and also to anonymous reviewers for their helpful comments and suggestions.