Space Division Multiplexing Research Articles

Multi-Associated Parameters Aggregation-Based Routing and Resources Allocation in Multi-Core Elastic Optical Networks

Space division multiplexing (SDM), as a potential means of enhancing the capacity of optical transmission systems, has attracted widespread attention. However, the adoption of SDM technology has also additionally increased resource dimensions, introduced complex crosstalk, and made it difficult to integrate multi-dimensional fragments. These factors force the transmission constraints to be more complicated. Especially, some factors have a mutual restraint relationship, and excessive consideration of certain factors will cause the deterioration of other ones. Therefore, how to comprehensively consider the associated factors to achieve trade-offs and improve network performance is a problem worthy of study. This paper exploits the advantages of self-organizing feature mapping (SOFM) model to process multi-dimensional data with relevant features. Firstly, multiple constraints will be input into SOFM as mode vectors from the core level. Then, by judging the similarity between the competition layer neuron and the pattern vector, the position of the winning neuron is located, which determines the transmission level of each core. Finally, a routing, core, and spectrum allocation scheme is proposed by preferentially locating the core with higher transmission quality. Along the selected core, the available slots will be classified twice respectively by the number of adjacent cores and crosstalk direction to quickly find the spectrum blocks with relatively small crosstalk. Results indicate the scheme can reduce blocking probability and the resource fragmentation. Further, it can increase the resource utilization within tested network load.

Priority-aware traffic routing and resource allocation mechanism for space-division multiplexing elastic optical networks

In recent years, the introduction of new technologies and applications connected to the Internet has demonstrated the physical incapacity of current optical backbone networks in providing resources soon. One of the main proposals to deal with this problem today is space-division multiplexing elastic optical networks. However, to transport the massive amount of data that is being generated today, these backbone networks should provide efficient resilience mechanisms. We propose a routing algorithm with a protection mechanism that considers traffic priority for elastic optical networks space-division multiplexing. The results obtained demonstrate the proposed algorithm’s high efficiency in establishing resilient high priority connections compared with recently proposed algorithms in the literature.

Optical Time Domain Reflectometry for Simultaneously Characterizing Forward and Backward Crosstalk Along Multi-Core Fibers

Uncoupled multi-core fibers (MCFs) are promising transmission media for realizing long-haul and large-capacity transmission using space division multiplexing. When the assignment of the signal transmission directions in adjacent cores is unidirectional or bidirectional, the forward and backward crosstalk respectively limits the capacity and distance of the transmission system using uncoupled MCFs. Understanding both the forward and backward crosstalk is crucial for determining an optimal transmission scheme according to traffic demands. In this paper, we propose a novel method based on optical time domain reflectometry for simultaneously characterizing the forward and backward crosstalk along uncoupled MCFs. We measured the forward and backward crosstalk along a four-core fiber by the proposed method. The forward and backward crosstalk obtained with the proposed method agreed well with those measured with unidirectional and bidirectional transmission methods. Since the proposed method enables us to simultaneously evaluate the forward and backward crosstalk along uncoupled MCFs from one end, it would be useful for testing those characteristics during the construction of MCF transmission lines.

Broadband dispersion compensating ring-core fiber for orbital angular momentum modes.

A well designed ring-core fiber can theoretically support numerous orbital angular momentum (OAM) modes with low crosstalk for space-division-multiplexing (SDM) data transmission, which is considered as a promising solution for overcoming the capacity crunch in optical communication network. However, the accumulated chromatic dispersion in OAM-fiber could limit the data speed and transmission distance of communication systems. A potential solution is to insert a dispersion compensation ring-core fiber with opposite-sign of dispersion in the transmission fiber along the fiber link. In this work, we propose a triple ring-core fiber with broadband negative dispersion. A highest negative dispersion of -24.47 ps/(nm·km) at 1550 nm and an average dispersion slope in the C band from -0.182 ps/(nm2·km) to 0.065 ps/(nm2·km) can be achieved to compensate multi-order dispersion. The effects of Ge-doping concentration fluctuation in the high-index ring core and fabrication errors on fiber geometric structures are also investigated. Furthermore, the effective mode area decreases as the widths of high-index rings increase due to the enhanced confinement ability. The designed triple ring-core fiber could offer potential for compensating OAM fiber links with positive dispersions.

High-efficiency cladding-pumped 4-core erbium-doped fiber with a pedestal for space division multiplexing amplification.

A cladding-pumped 4-core erbium-doped fiber (4C-EDF) with a pedestal structure has been firstly, to the best of our knowledge, proposed and fabricated for space division multiplexing (SDM) amplification. The numerical simulation shows that the index-raised pedestal around the fiber core can improve power conversion efficiency (PCE) by enhancing pump power usage. Compared with conventional 4C-EDF, the 4C-EDF with a pedestal has a gain improvement of 4.5 dB and a PCE enhancement of 91.8%, according to the experimental results (pedestal fiber: 9.55%, conventional fiber: 4.98%). For a 6 dBm total input signal power at L-band and a 7.8 W pump power at 976 nm, the pedestal 4C-EDF shows an average gain of 25 dB and an average noise figure (NF) of 6.5 dB over all cores in the wavelength range of 1570.41 nm to 1610.87 nm. The core-to-core gain variation is less than 2 dB.

Influence of the Width of Launch Beam Distribution on the Transmission Performance of Seven-Core Polymer-Clad Silica Fibers

We propose a space division multiplexing (SDM) in a newly constructed multicore polymer-clad silica fiber (PCSF) with seven cores arrayed in a hexagonal array, each carrying a centrally launched beam. This enables a higher SDM capacity at longer fiber lengths in the proposed seven-core PCSF if compared with previously proposed angular division multiplexing (ADM) in single-core (SC) PCSF. As a result, the SDM is not limited to short fiber lengths in the proposed seven-core PCSF, as it is in the case of the ADM channels due to mode coupling in the SC PCSF. In addition, the time-independent power flow equation (TI PFE) is used to analyze the effect of the width of the launch beam distribution on the equilibrium mode distribution (EMD) and steady state distribution (SSD) in each of the seven cores of the investigated PCSF. The width of the launch beam distribution has a considerable impact on the fiber length at which the EMD and SSD are attained, according to our numerical results. Thus, by decreasing the full width at half maximum (FWHM) of the launch beam distribution from 20 to 2°, the length at which EMD is established increases from Lc = 1020 to 1250 m, and the length at which SSD is attained increases from zs = 2650 to 3250 m. A narrow launch beam distribution leads to higher bandwidth at small and intermediate fiber lengths. On the other hand, at shorter fiber lengths, a wider launch beam distribution induces a bandwidth change from 1/z proportional to 1/z1/2 proportional curve, e.g., a slower bandwidth reduction. When building a multicore optical fiber transmission system for SDM, such characterization of multicore PCSFs under various launch conditions should be taken into account.

Influence of launch beam distribution on power flow and angular division multiplexing in seven-core silica optical fibers

We analyze the effect of launch beam distribution on space-division multiplexing (SDM) performance in multimode multicore silica optical fibers (MM MC SOF) with seven cores. The time-independent power flow equation (TI PFE) is used to explore the effect of the width of the distribution of the Gaussian launch beam on power flow in each of the seven cores. We show that the optical fiber length at which the equilibrium mode distribution (EMD) and steady-state distribution (SSD) are obtained is greatly influenced by the width of the Gaussian launch beam distribution. We further show that when the width of the Gaussian launch beam distribution widens, the optical fiber length at which angular division multiplexing (ADM) in each of the seven cores can be realized with minimal crosstalk between neighboring angular optical channels decreases. We demonstrate that, for increasing the capacity of an optical fiber transmission system, an SDM system with two- and three-channel ADM and multicore optical fiber multiplexing can be implemented with the proposed seven-core MM MC SOF at optical fiber lengths up to ≈1 km (2 ADM channels × 7 cores) and ≈200 m (3 ADM channels × 7 cores), respectively. Such characterization of MM MC SOFs under various launch conditions is important for building a multicore optical fiber SDM transmission system.

Design and characteristics of ellipse-assisted ring-core fiber for space division multiplexing

• Elliptical-assisted ring-core fiber with central low-refractive-index elliptical region. • 9 effectively separated spatial modes have uniform Δn eff with values greater than 3.14 × 10 −4 at 1550 nm. • The level of polarization separation is lower than 3.33 × 10 −6 . • Good scalability and fabrication error tolerance. • This fiber design could be promising candidates toward high-capacity SDM systems. We propose a novel ellipse-assisted ring-core fiber characterized by an elliptical region of low refractive index in the core. The proposed fiber proves effective in eliminating the spatial degeneracy of the LP mn mode groups and keeping the birefringence properties at a low level. This fiber, with optimized structural parameters, can guide 9 modes over the entire C-band. The simulation results show that the effective refractive index difference (Δn eff ) between adjacent modes at 1530–1565 nm falls within the scope of (3.1 ∼ 6.64) × 10 −4 . At the same time, the polarization separation level of each mode is lower than 3.23 × 10 −6 , which is 2 orders of magnitude lower than the degree of spatial mode degeneracy separation. This fiber design has good application prospects in space division multiplexing systems.

Learning to transmit images through optical speckle of a multimode fiber with high fidelity

Multimode fibers provide a unique opportunity for exploring the spatial degrees of freedom for high throughput light transmission. However, the modal dispersion prevents from the straightforward application of multimode fibers for space division multiplexing, such as image transmission. Herein, we propose and experimentally demonstrate a deep neural network termed multimode fiber inverse-scattering net for overcoming the modal dispersion induced scrambling in multimode fibers. Such a network is capable of transmitting grayscale image through the multimode fiber with high fidelity. 256-level grayscale images with 128 × 128 spatial channels encoded in the input wavefront can be retrieved from the output optical speckle patterns, where the average Pearson correlation coefficient and structural similarity index are as large as 0.97 and 0.95, respectively. Our results demonstrate that the proposed deep neural network has an excellent ability for learning the relationship between the input and output optical fields of a multimode fiber, which might facilitate the realization of high throughput space division multiplexing through multimode fibers.

Rate and Power Scaling of Space-Division Multiplexing Via Nonlinear Perturbation

A nonlinear perturbation model for fiber-optic channels is used to analyze achievable rates of combined space-division multiplexing (SDM) and wavelength-division multiplexing (WDM). The analysis shows that, for a large number S of SDM channels, the power of the nonlinear interference is at most quadratic in S. The achievable rates scale at least as the cubic root of S and linearly with the optimized launch power for large S. The expressions are verified numerically for systems with up to 16 SDM channels and 5 WDM channels.

Relaying System Based on Few-Mode EDFA for Space Division Multiplexing Wireless Optical Communication

Relay assisted transmission can compensate atmospheric turbulence, improve transmission distance, and realize communication between non-linear targets in free space optical communication (FSO) system. Space division multiplexing (SDM) greatly improves the channel capacity of FSO system. This paper focuses on the relay assisted transmission oriented to SDM wireless optical communication system. We develop a dual-hop SDM FSO communication model with two spatial mode channels. At the relay, the received optical signals are amplified by the few mode EDFA and forwarded to the destination. Simulation results show that the bit error rate (BER) of the dual-hop SDM system with a few mode EDFA based relay outperforms the single-hop SDM system. Under BER = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \times {10}^{ - 5}$</tex-math></inline-formula> , the power budget is increased by 0.53 dB, 1.2 dB, and 4.88 dB, when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\boldsymbol{C}}_{\boldsymbol{n}}^2 = \ 4 \times {10}^{ - 15}$</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$7 \times {10}^{ - 15}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \times {10}^{ - 14}$</tex-math></inline-formula> , respectively. Moreover, the BER performance is the best when the relay is (originally in the middle of the link) slightly closer to the source.

1-Pbps orbital angular momentum fibre-optic transmission

Space-division multiplexing (SDM), as a main candidate for future ultra-high capacity fibre-optic communications, needs to address limitations to its scalability imposed by computation-intensive multi-input multi-output (MIMO) digital signal processing (DSP) required to eliminate the crosstalk caused by optical coupling between multiplexed spatial channels. By exploiting the unique propagation characteristics of orbital angular momentum (OAM) modes in ring core fibres (RCFs), a system that combines SDM and C + L band dense wavelength-division multiplexing (DWDM) in a 34 km 7-core RCF is demonstrated to transport a total of 24960 channels with a raw (net) capacity of 1.223 (1.02) Peta-bit s−1 (Pbps) and a spectral efficiency of 156.8 (130.7) bit s−1 Hz−1. Remarkably for such a high channel count, the system only uses fixed-size 4 × 4 MIMO DSP modules with no more than 25 time-domain taps. Such ultra-low MIMO complexity is enabled by the simultaneous weak coupling among fibre cores and amongst non-degenerate OAM mode groups within each core that have a fixed number of 4 modes. These results take the capacity of OAM-based fibre-optic communications links over the 1 Pbps milestone for the first time. They also simultaneously represent the lowest MIMO complexity and the 2nd smallest fibre cladding diameter amongst reported few-mode multicore-fibre (FM-MCF) SDM systems of >1 Pbps capacity. We believe these results represent a major step forward in SDM transmission, as they manifest the significant potentials for further up-scaling the capacity per optical fibre whilst keeping MIMO processing to an ultra-low complexity level and in a modularly expandable fashion.

Reconfigurable Microwave Photonic Filter Based on Space-Division Multiplexing Powered by Artificial Neural Networks

Space-division multiplexing (SDM) techniques bring new approaches for various applications in microwave photonic signal processing. In this work, an all-fiber reconfigurable microwave photonic filter (MPF) is realized based on SDM techniques using multicore fiber (MCF) and multimode fiber (MMF). The multimode interference (MMI) generated from the proposed MCF-MMF-MCF structure can be manipulated in dimensions of space and wavelength simultaneously, thus configuring the power distribution in all the taps of the MPF. The trained artificial neural network can accurately predict the operating wavelength and spatial input core for a target power distribution. In a low-cost, light-weight, and compact way, SDM fibers combined with MMI allow for greater flexibility and diversity in the response of the reconfigurable MPF.

Inside Cover

By adopting the polarization phase shifting and space division multiplexing in digital holographic microscopy, a reflectance quantitative phase microscopy system capable of dynamic quantitative phase imaging and depth‐resolved imaging is developed. The compact design of on‐axis near common‐path Fizeau polarization interferometry offers a single‐shot microscopy system with robust phase stability in comparison to other techniques. The proposed system has the advantages of label‐free, singleshot, and depth‐selective imaging features, which will be a prospective tool in the quantified investigations in medical diagnosis and in vivo imaging in optical phase microscopy.Further details can be found in the article by Hanzi Liu, Xiaoyan Wu, Guodong Liu, Hongliang Ren, Vinu R. V., Ziyang Chen, and Jixiong Pu (e202100400)image

Power-Over-Fiber in Support of 5G NR Fronthaul: Space Division Multiplexing Versus Wavelength Division Multiplexing

We investigatethe potential of space division multiplexing (SDM) for the power-over-fiber (PWoF) technique, in terms of weakly-coupled multicore fiber (WC-MCF) design and experimental verification. Our simulation results indicate that, the SDM-enabled PWoF scheme is superior to the wavelength division multiplexing (WDM) enabled counterpart, due to the scalability of the energy light delivery and the save of wavelength division multiplexers. Then, we experimentally demonstrate the co-transmission of optically carried fifth generation new radio (5G NR) signals at 1550-nm and the 60-W energy light at 1064-nm over the 1-km weakly-coupled seven-core fiber (WC-7CF) fronthaul link. Since the optical power delivery characteristic per the WC-MCF core is similar to that of the standard single-mode fiber (SSMF), the SDM-enabled PWoF scheme can evenly distribute the high-power energy light into multiple spatial channels, to maximize the efficiency of the optical power transition efficiency (OPTE). Consequently, the total collected optical power of the energy light over the 1-km WC-7CF is 4.6 times higher than that of the 1-km SSMF, mainly due to the insertion loss of the fan-in and fan-out (FIFO) devices. Meanwhile, the error-vector magnitude (EVM) of 1.5-Gbit/s 5G NR signal is 0.28%, when the received electrical signal power is −25 dBm. The EVM value of the co-transmitted 5G NR signal with the 60-W energy light only has a fluctuation of 0.01% over six hours. The SDM-enabled PWoF is useful for the optically powered 5G pico-cells, with the capability of high-speed access and centralized management.

MIMO Processing With Linear Beat Interference Cancellation for Space Division Multiplexing Self-Homodyne Coherent Transmission

We investigate self-homodyne coherent transmission employing space division multiplexing (SDM), where a continuous wave laser source is divided to produce spatially multiplexed signals and a co-propagating pilot-tone (PT), which serves as a local oscillator for self-homodyne detection at the receiver side. The beat interference caused by the spatial coupling between PT and the signals during propagation is investigated through derivations and numerical modelling. Both conventional multiple-input–multiple-output (MIMO) and unreplicated parallel interference canceler (UPIC)-assisted MIMO suffer from the effects of beat interference and additional mode dependent loss introduced by PT-signal crosstalk, which limits the performance of SDM-based self-homodyne coherent systems. We propose two novel MIMO processing schemes employing digital conjugate input branches to handle the beat interference and demonstrate system performance improvement in both simulations and experiments. For a 240-Gbps self-homodyne coherent system transmitting mode-multiplexed QPSK signals over a 3-mode multimode fiber, after adapting the two enhanced MIMO schemes, simulations show a up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 4 dB improvement in PT-signal crosstalk level tolerance, and experiments show undistorted constellations as well as BER improvements from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim 10^{-2}$</tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim 10^{-5}$</tex-math></inline-formula> with PT-signal delay alignment after the mode demultiplexer.

Integration in the C-band between quantum key distribution and the classical channel of 25 dBm launch power over multicore fiber media

The quantum-classical coexistence can be implemented based on wavelength division multiplexing (WDM), but due to Raman noise, the wavelength spacing between quantum and classical signals and launch power from classical channels are restricted. Space division multiplexing (SDM) can now be availably achieved by multicore fiber (MCF) to reduce Raman noise, thereby loosening the restriction for coexistence in the same band and obtaining a high communication capacity. In this paper, we realize the quantum-classical coexistence over a 7-core MCF. Based on the SDM, the highest launch power of 25 dBm is achieved which has been extended nearly 19 times in previous work. Moreover, both the quantum and classical channels are allocated in the C-band and the minimum wavelength spacing between them is only 1.6 nm. The coexistence system eliminates the need for adding a narrowband filter.

Investigation of crosstalk and BER in multicore fiber optic transmission link for Space Division Multiplexing

The explosive growth of optical communication (i.e., 6G or beyond 5G) will transform the way of communication. Advanced modulation schemes, guided media, high data rate, minimum dispersion, low transmission noise, and signal processing techniques are available to enrich optical networks. The multicore fiber (MCF) will play a significant role in spatial division multiplexing. The major bottleneck of the MCF is the crosstalk in different cores. This article analyzed the crosstalk in multicore fiber (i.e., 2, 3, 4, and five core) as a transmission length function and compared the Q factor, bit error rate, and output power by considering all nonlinearities of the fiber. Maximum BER obtained for 5, 4, 3, and 2 core MCF is 10-14, whereas minimum BER for 5, 4,3, and 2 core MCF are 10–248, 10–248, 10–281, and 10–242, respectively. Maximum crosstalk occurs for 5 core MCF i.e.3166.32 dBm and minimum for 2 core MCF i.e. −44.083dBm. The highest Q factor is obtained at 35.8 for 3 core 10 km, and the minimum Q factor is ∼ 7.351 for 5 core MCF.

Modulation format recognition with transfer learning assisted convolutional neural network using multiple Stokes sectional plane image in multi-core fibers.

A modulation format recognition (MFR) scheme based on multi-core fiber (MCF) is proposed for the next generation of elastic optical networks (EONs). In this scheme, multiple Stokes sectional planes images are used as signal features which are typed into a transfer learning (TL) assisted convolutional neural network (CNN) to realize MFR. Compared with the traditional Jones matrix, the Stokes space mapping method is insensitive to polarization mixing, carrier frequency skew and phase offset, therefore, it has better feature representation ability. TL is introduced to transfer the model used in standard single-mode fiber (SSMF) to MCF transmission, reducing the required training data and complexity. In addition, multiple Stokes sectional planes images are input simultaneously, which improves the accuracy of the neural network. Experimental verifications were performed for a polarization division multiplexing (PDM)-EONs system at a symbol rate of 12.5GBaud by 5 km MCF. Nine modulation formats, including three standard modulation formats (BPSK, QPSK, 8PSK), three uniformly shaped (US) modulation formats (US-8QAM, US-16QAM, US-32QAM) and three probabilistically shaped (PS) modulation formats (PS-8QAM, PS-16QAM, PS-32QAM), were recognized by our scheme. The experimental results show that the scheme achieves high recognition accuracy even at low optical signal-to-noise ratio (OSNR). Moreover, the required number of training samples is less 40% compared to the traditional CNN. The proposed scheme has a high tolerance to the crosstalk damage of MCF itself and can realize the short training time of large-capacity space division multiplexing (SDM)-EONs. Our findings have the potential to be used in the next generation of a SDM fiber transmission system.

SDM-WDM ROADM Based on Direct-Laser-Writing SDM Core Separator

Weakly-coupled space division multiplexing (SDM) techniques based on multi-core fibers (MCF) have attracted great attention for the capacity enhancement of optical interconnect systems and networks. In this paper, we propose a novel direct-laser-writing (DLW) core separator device and apply it in a reconfigurable optical add/drop multiplexer (ROADM) structure for two-dimensional SDM and wavelength division multiplexing (WDM) switching. The proposed core separator matching an 8-core MCF is fabricated. Then assisted with MCF rotators, independent adding and dropping functionalities for arbitrary core are experimentally demonstrated with slight power penalty for both core and wavelength dimensions. The influence of crosstalk for cascading core separators is also investigated and below 0.35-dB receiving sensitivity penalty is obtained when three cores are dropped one by one. The compact DLW core separators can reduce the complexity of SDM nodes and will be beneficial for high-speed optical interconnects.