Space Division Multiplexing Research Articles

Scheduled lightpath demand provisioning in fiber space division multiplexing optical networks

Space division multiplexing (SDM) and spatial super-channel (Spa SCh) technology offer scalable solutions to meet the increasing capacity demands in optical networks. This study focuses on multi-fiber optical networks integrated with the innovative Spa SCh technology, which creates different granularities of Spa SChs by spatially combining the same spectrum across multiple fibers, known as fiber-SDM (FSDM). Most existing studies have concentrated on provisioning dynamic or static lightpath demands in FSDM networks. However, for several applications in real-world networks, scheduled lightpath demands—where setup and teardown times are pre-determined—are more common. To address this, we consider the provisioning of scheduled lightpath demands in FSDM networks and aim to solve the routing, fiber, spectrum, and time-slot assignment (RFSTA) problem for these demands. We propose two integer linear programming (ILP) models: one optimizing only the spectrum resource utilization (SRU) and the other jointly optimizing both SRU and capital expenditure (CAPEX). In addition, we develop several efficient heuristic algorithms for large-scale optical networks, based on the concepts of the spatial spectrum window (SSW), SSW plane, and time-slot window (TW). Furthermore, an evolutionary joint optimization strategy is introduced to explore optimal Spa SCh configurations, aiming to jointly optimize network SRU and CAPEX. Simulation results demonstrate that the proposed algorithms closely match the performance of ILP models in small networks. Among the heuristic approaches, the adaptive least-load algorithm performs best under an optimal Spa SCh configuration, which bundles half of the fibers to form link fiber bundles.

Analysis of crosstalk drift and optimization of spatial light modulator based mode-selective multiplexers for multimode fibers.

Adaptive mode-selective multiplexers offer the potential to control the modal content within multimode fibers for space division multiplexing (SDM). To such an end, spatial light modulators allow programmable control over the phase, amplitude, and polarization of optical wavefronts. One of the major challenges is to precisely match the manipulated beam to the waveguide modes in the multimode fiber. Achieving precise alignment of optical components within the free space system is crucial for accurate mode multiplexing while active alignment may be necessary to overcome environment induced drift. In this paper, we investigate, through theory, simulations and experiments, the impact of misalignment errors in a free space telescopic Fourier system, including phase mask and lenses misposition and angular misalignment in fiber collimation. Mode multiplexing is achieved using phase holograms in the Fourier domain while mode demultiplexing is achieved through off-axis holography and Fourier domain processing. Furthermore, we analyze the crosstalk drift with time in SDM transmission over a 45 mode OM3 fiber. System stability is experimentally evaluated over a 9-hour transmission period.

High spectral-efficiency, ultra-low MIMO SDM transmission over a field-deployed multi-core OAM fiber

Space-division multiplexing (SDM) systems based on few-mode multi-core fibers (FM-MCFs) utilize both spatial channels (fiber cores) and modes (optical modes per core) to maximize transmission capacity. Unlike laboratory FM-MCFs or field-deployed single-mode multi-core fibers (SM-MCFs), SDM transmissions over field-deployed FM-MCFs in outdoor settings have not been reported. Therefore, concerns remain that environmental interference and cabling stress could worsen inter-core and intra-core modal crosstalk and impact the performance of SDM systems over FM-MCFs. In this paper, we demonstrate successful bidirectional SDM transmission over a 5-km, field-deployed seven ring-core fiber (7-RCF) with a cladding diameter of 178 μm. Our measurements show no significant differences in attenuation and mode coupling compared to pre-cabling conditions, confirming the fiber’s resilience to environmental disturbances and adaptability to cable deployment. Using the field-deployed 7-RCF, bi-directional SDM transmission is implemented, achieving spectral efficiency (SE) of 2×201.6 bit/(s Hz) which sets a new record in field-deployed fiber cables that is a tenfold increase over previous systems. Furthermore, these results were achieved using a small-scale 4×4 multiple-input multiple-output (MIMO) scheme with a time-domain equalization (TDE) tap number not exceeding 15. These results demonstrate the substantial potential of using SDM techniques to significantly enhance SE and expand capacity in practical fiber-optic transmission applications.

Research on center-assisted ring-core few-mode fiber with an eccentric circle for mode degeneracy separation in space division multiplexing

An eccentric-circle-assisted ring-core fiber (ECRF) structure is proposed to effectively separate spatial-degenerated linear polarization (LP) modes while maintaining birefringence at the 10−6 level, which provides more degrees of freedom in the design of few-mode fiber employment in space division multiplexing (SDM) systems. Our simulation analysis demonstrates that the effective refractive index difference (Δneff) between the spatially degenerate modes in this fiber falls within the range of (2.44−6.42)×10−4 at 1530–1565 nm, given the refractive index difference level typical of conventional fiber core and cladding. In addition, the polarization separation level for each mode is on the order of 10−6 and below. The design requirements of significant separation of spatial degeneracy and basically no separation of polarization degeneracy are achieved. Based on the characteristics of spatially degenerate mode in this fiber, we extend the regulation law for spatially degenerate mode separation of the LP m n mode groups in center-assisted ring-core fibers. In comparison to the existing center-assisted ring-core fiber structure, the ECRF can further reduce the fabrication difficulty and increase the feasibility of preparation.

Polarization-resolved transmission matrices of specialty optical fibers.

Transmission matrix measurements of multimode fibers are now routinely performed in numerous laboratories, enabling control of the electric field at the distal end of the fiber and paving the way for the potential application to ultrathin medical endoscopes with high resolution. The same concepts are applicable to other areas, such as space division multiplexing, targeted power delivery, fiber laser performance, and the general study of the mode coupling properties of the fiber. However, the process of building an experimental setup and developing the supporting code to measure the fiber's transmission matrix remains challenging and time consuming, with full details on experimental design, data collection, and supporting algorithms spread over multiple papers or lacking in detail. Here, we outline a complete and self-contained description of the specific experiment we use to measure fully polarization-resolved transmission matrices, which enable full control of the electric field, in contrast to the more common scalar setups. Our exact implementation of the full polarization experiment is new and is easy to align while providing flexibility to switch between full-polarization and scalar measurements if desired. We utilize a spatial light modulator to measure the transmission matrix using linear phase gratings to generate the basis functions and measure the distal electric field using phase-shifting interferometry with an independent reference beam derived from the same laser. We introduce a new method to measure and account for the phase and amplitude drift during the measurement using a Levenberg-Marquardt nonlinear fitting algorithm. Finally, we describe creating distal images through the multimode fiber using phase-to-amplitude shaping techniques to construct the correct input electric field through a superposition of the basis functions with the phase-only spatial light modulator. We show that results are insensitive to the choice of phase-to-amplitude shaping technique as quantified by measuring the contrast of a razor blade at the distal end of the fiber, indicating that the simplest but most power efficient method may be the best choice for many applications. We also discuss some of the possible variations on the setup and techniques presented here and highlight the details that we have found key in achieving high fidelity distal control. Throughout the paper, we discuss applications of our setup and measurement process to a variety of specialty fibers, including fibers with harsh environment coatings, coreless fibers, rectangular core fibers, pedestal fibers, and a pump-signal combiner based on a tapered fiber bundle. This demonstrates the usefulness of these techniques across a variety of application areas and shows the flexibility of our setup in studying various fiber types.

Few-Mode Fiber with Low Spontaneous Raman Scattering for Quantum Key Distribution and Classical Optical Communication Coexistence Systems.

In this paper, the theoretical model of spontaneous Raman scattering (SpRS) in few-mode fiber (FMF) is discussed. The influence of SpRS on quantum key distribution (QKD) in FMF is evaluated by combining wavelength division multiplexing (WDM) and space division multiplexing (SDM) techniques. On this basis, an improved ring-assisted FMF is designed and characterized; the transmission distance can be increased by up to 54.5% when choosing different multi-channels. The effects of forward and backward SpRS on QKD are also discussed.

Unlocking mode programming with multi-plane light conversion using computer-generated hologram optimisation

Abstract Programmable optical devices provide performance enhancement and flexibility to spatial multiplexing systems enabling transmission of tributaries in high-order eigenmodes of spatially-diverse transmission media, like multimode fibre (MMF). Wavefront shaping with spatial light modulators (SLMs) facilitates scalability of the transmission media by allowing for channel diagonalization and quasi-single-input single-output operation. Programmable mode multiplexing configurations like multi-plane light conversion (MPLC) utilise the SLM and offer the potential to simultaneously launch an arbitrary subset of spatial tributaries in any N -mode MMF. Such programmable optical processor would enable the throughput of space-division multiplexing (SDM) systems to be progressively increased by addressing a growing number of tributaries over one MMF and in this way meet a growing traffic demand – similarly to the wavelength-division multiplexing evolution path. Conventionally, MPLC phasemasks are calculated using the wavefront matching algorithm (WMA). However, this method does not exploit the full potential of programmable mode multiplexers. We show, that computer-generated hologram algorithms like direct search enable significant improvement compared to the traditional WMA-approach. Such gains are enabled by tailored cost functions with dynamic constraints concerning insertion loss as wellas mode extinction ratio. We show that average mode extinction ratio can be greatly improved by as much as ≈ 15 dB at the expense of a small insertion loss35
deterioration of < 3 dB. One particular feature of programmable mode multiplexers is the adaptability to optimised transmission functions. Besides conventional LP modes transmission, we employ our approach on Schmidt modes, which are spatial eigenchannels with minimum crosstalk derived from a measured transmission matrix

First real-time single-span 106-km field trial using commercial 130-Gbaud DP-QPSK 400 Gb/s backbone OTN transceivers over deployed multi-core fiber cable

Weak-coupled space-division multiplexing (SDM) technique using multi-core fibers (MCF) has attracted great research interests due to its huge capacity potential and compatibility with high-speed transceivers. In this paper, we demonstrate the first real-time 128 Tb/s and 224 Tb/s single-span 106-km field trial over deployed 4-core and 7-core MCF cable with 65 multi-core fusion splicing using commercial DP-QPSK 400 Gb/s backbone optical transport network (OTN) transceivers. The 4-core and 7-core transmission systems still reserve with more than 5.5-dB and 3.5-dB OSNR margins respectively thanks to the 130-Gbaud DP-QPSK modulation format enabled by optoelectronic multiple-chip module (OE-MCM) packaging technique. The MCF cable has a length of 17.69 km and the contained MCFs are with standard 245-μm coating, which enables the compatibility with standard cabling processing. This field trial marks the maturity of MCF-based weakly-coupled SDM transmission systems and is an important milestone towards the implementation of MCF in high-speed terrestrial cable systems.

Ultra-high-capacity band and space division multiplexing backbone EONs: multi-core versus multi-fiber

Both multi-band and space division multiplexing (SDM) independently represent cost-effective approaches for next-generation optical backbone networks, particularly as data exchange between core data centers reaches the petabit-per-second scale. This paper focuses on different strategies for implementing band and SDM elastic optical network (BSDM EON) technology and analyzes the total network capacity of three sizes of backbone metro-core networks: ultra-long-, long-, and medium-distance networks related to the United States, Japan, and Spain, respectively. Two BSDM strategies are considered, namely, multi-core fibers (MCFs) and BSDM based on standard single-mode fiber (SSMF) bundles of multi-fiber pairs (BuMFPs). For MCF-based BSDM, we evaluated the performance of four manufactured trench-assisted weakly coupled (TAWC) MCFs with 4, 7, 13, and 19 cores. Simulation results reveal that, in the regime of ultra-low (UL) loss and inter-core crosstalk (ICXT), MCF-based throughput can be up to 14% higher than SSMF BuMFP-based BSDM when the core pitch exceeds 43 µm and the loss coefficient is lower than that of standard single-mode fibers. However, increasing the number of cores with (non-)standard cladding diameters, UL loss, and ICXT coefficient is not beneficial. As core counts increase up to 13 for non-standard cladding diameters (<230µm), the core pitch and loss coefficient also increase, leading to degraded performance of MCF-based BSDM compared to SSMF BuMFP-based BSDM. The results indicate that, in scenarios with 19 MFPs, SSFM BuMFP-based BSDM outperforms 19-core MCF-based scenarios, increasing the throughput by 55% to 73%, from medium-backbone networks to ultra-long ones.

Performance evaluation of a two-stage few-mode EDFA for high-capacity SDM systems.

Space division multiplexing (SDM) systems using few-mode fibers (FMF) are essential for next-generation fiber optic communications. Optical amplifiers with low noise, minimal differential modal gain (DMG), and minimal differential spectral gain (DSG) are essential for these systems. In this work, we present a method to design and optimize a two-stage few-mode erbium-doped fiber amplifier (FM-EDFA) using a joint DMG-DSG minimization approach. This methodology involves the pumping profile design and the gain flattening filter design. Simulation results show the two-stage FM-EDFA achieves DMG and DSG below 2.8 dB and 0.42 dB, respectively, with an optical signal-to-noise ratio above 19 dB across the C-band, enabling a system capacity of 48.6 Tbps. This work reveals the effectiveness of this two-stage FM-EDFA for optical amplification in the context of SDM systems.

Scaling wavelength selective switches for multi-band and space-division-multiplexed networks

This paper investigates the practical scalability of wavelength selective switching technology for the emerging multi-band and space-division-multiplexed (SDM) networks. Wavelength selective switching architectures are introduced for multi-band SDM networks. The switching capacity is analyzed for both weakly coupled and strongly coupled SDM networks. Key bottlenecks for scaling up toward multi-band and more spatial modes are identified. Contrary to the conventional view that liquid crystal on silicon (LCOS) was the only technological obstacle, the manufacturability of free-space optics with high numerical apertures and constraints on the optical dimensions also brought significant challenges for the development of highly integrated wavelength selective switches for multi-band SDM networks.

Neural network-assisted meta-router for fiber mode and polarization demultiplexing.

Advancements in computer science have propelled society into an era of data explosion, marked by a critical need for enhanced data transmission capacity, particularly in the realm of space-division multiplexing and demultiplexing devices for fiber communications. However, recently developed mode demultiplexers primarily focus on mode divisions within one dimension rather than multiple dimensions (i.e., intensity distributions and polarization states), which significantly limits their applicability in space-division multiplexing communications. In this context, we introduce a neural network-assisted meta-router to recognize intensity distributions and polarization states of optical fiber modes, achieved through a single layer of metasurface optimized via neural network techniques. Specifically, a four-mode meta-router is theoretically designed and experimentally characterized, which enables four modes, comprising two spatial modes with two polarization states, independently divided into distinct spatial regions, and successfully recognized by positions of corresponding spatial regions. Our framework provides a paradigm for fiber mode demultiplexing apparatus characterized by application compatibility, transmission capacity, and function scalability with ultra-simple design and ultra-compact device. Merging metasurfaces, neural network and mode routing, this proposed framework paves a practical pathway towards intelligent metasurface-aided optical interconnection, including applications such as fiber communication, object recognition and classification, as well as information display, processing, and encryption.

Omnidirectional optic fiber shape sensor for submarine landslide monitoring

In-situ submarine landslide monitoring is imperative for ensuring the safety of ocean infrastructures, although it poses greater challenges compared to conventional landslide monitoring methods. In response to this need, we developed an omnidirectional optic fiber shape sensor (OFSS) and a self-contained submarine landslide monitoring system. Wavelength division multiplexing (WDM) and space division multiplexing (SDM) are used to build 10 omnidirectional curvature sensors with 3 fiber Bragg gating (FBG) arrays. Difference curvature demodulation algorithm is introduced to eliminate bending-irrelative effects. The omnidirectional curvature transmission matrix (OCTM) is obtained through calibration process with standard curvature molds. Frenet algorithm is used to recovery the shape. Laboratory experiments demonstrate OFSS’s ability to monitor 3D shape in real time, achieving the deflection error of 0.23 % and the bending direction error of 0.06 rad in the simple bending shape recovery experiment. In the S-shape recovery experiment, the OFSS’s deflection error is 0.77 % and the bending direction error is 0.04 rad. Subsequently, the system underwent testing in shallow water within a ship dock. In this experiment, the OFSS is successfully penetrated into the seabed and remained embedded in the soil for 90 min. The maximal shift observed at the end of the OFSS is 4.61×10-3m with the standard deviation of 9.95×10-4m, while the bending direction exhibited a maximal shift of 3×10-3 rad with the standard deviation of 7.51×10-4rad over the course of the 90-minute duration. These results validate the system’s effectiveness in providing accurate and reliable in-situ monitoring of submarine landslide.

Fading suppression and noise reduction of a DAS system integrated multi-core fiber.

Multi-core fiber (MCF) has attracted increasing attention for application in distributed fiber sensing owing to its unique properties of independent light transmission in multiple spatial channels. Here, we report a distributed acoustic sensing (DAS) system integrated MCF to suppress coherent fading, which overcomes an inevitable challenge in DAS systems. Because the parallel spatial cores in MCF allow the use of space-division multiplexed (SDM) technology, we propose that fading can be effectively suppressed by merging different signals with the spatial rotated-vector-average (SRVA) method. We theoretically analyze the principle of SRVA in fading suppression, and identify that it can effectively reduce phase noise with preventing phase unwrapping failures. In our experiment, a DAS system with 2.58-km length MCF have been investigated, the fading rate of Rayleigh backscattered signals is effectively reduced by three orders of magnitude and the amplitude fluctuation range is decreased by 21.9 dB. Compared with the conventional spectrum extraction and remix method (SERM), SRVA reduces the noise level by 9.5 dB, which also shows excellent low-frequency signal recovery ability. Benefiting from its fading suppression, the false alarm of localization is mitigated and the phase recovery can be distortionless. The proposed and verified method is helpful for the application of SDM-based MCF in long-distance distributed fiber sensors and accelerates the progress of integrated sensing and communication.

Uniform intensity chiral optical field by multifocal synthesis.

Chiral optical beams that carry orbital angular momentum (OAM) have a broad range of applications such as optical tweezers, chiral microstructure fabrication, and optical communications. However, some chiral optical beams have inhomogeneous intensity distribution that limits the application in these fields. In this Letter, two different types of chiral optical fields with uniform intensity and arbitrary length were proposed based on the amplitude encoding method and multifocal synthesis. The intensity distribution of the chiral optical fields is determined by the distance between the focal points that can greatly extend the modulation length of the chiral optical field. Moreover, since each focal point contains modulable amplitude and phase, an arbitrary interception of the optical field can be realized by selectively retaining a part of the focal points. By partitioning the chiral optical field and assigning different topological charges, the OAM space-division multiplexing and independent tunability of the topological charges can be realized. In addition, the composite multi-petal vortex array formed by combining two different chiral optical fields can greatly enhance the information capacity of the optical communications and may have potential applications in fields such as particle manipulation.

How can we consider multi‐core fibre standard?

AbstractThe authors provide an example roadmap for realising the space division multiplexing (SDM)‐based ecosystem. The authors investigate whether the new standard for multi‐core fibre can be specified by considering a 125 μm standard cladding diameter and backward compatibility with the existing single‐core single‐mode fibre. The authors propose that well‐planned standardisation activities need to be initiated considering mandatory technologies for realising the terrestrial SDM ecosystem in the near future.

Single-exposure multi-wavelength optical diffraction tomography based on space-angle dual multiplexing holography.

We present a space-angle dual multiplexing holographic recording system for realizing single-exposure multi-wavelength optical diffraction tomographic (ODT) imaging. This system is achieved by combining the principle of single-exposure multi-wavelength holographic imaging technique based on angle-division multiplexing with the principle of single-exposure ODT imaging technique based on microlens array multi-angle illuminations and space-division multiplexing. Compared with the existing multi-wavelength ODT imaging methods, it enables the holographic recording of all the diffraction tomography information of a measured specimen at multiple illumination wavelengths in a single camera exposure without any scan mechanism. Using our proposed data processing method, the multi-wavelength three-dimensional (3D) refractive index tomograms of a specimen can be eventually reconstructed from single recorded multiplexing hologram. Experimental results of a static polystyrene bead and a living C. elegans worm demonstrate the feasibility of this system.

Spatial and mode selective switch for orbital angular momentum mode division multiplexing.

In analogy to a wavelength selective switch in wavelength-division multiplexing (WDM) optical fiber communication systems, a spatial and optical mode selective switch (SMSS) would be an important component in future ultrahigh capacity optical fiber communication systems based on space and mode division multiplexing. In this work, a free-space SMSS for orbital angular momentum (OAM) mode-division multiplexing (MDM) is proposed and experimentally demonstrated. The SMSS consists of a separating part for transforming OAM modes to spatial modes and a recombining part for selecting and recombining the modes to any spatial channel. The SMSS is able to implement strictly non-blocking switching between a total of 36 SDM/MDM channels configured as four spatial channels each supporting nine OAM mode channels.

Deep-reinforcement-learning-based RMSCA for space division multiplexing networks with multi-core fibers [Invited Tutorial

The escalating demands for network capacities catalyze the adoption of space division multiplexing (SDM) technologies. With continuous advances in multi-core fiber (MCF) fabrication, MCF-based SDM networks are positioned as a viable and promising solution to achieve higher transmission capacities in multi-dimensional optical networks. However, with the extensive network resources offered by MCF-based SDM networks comes the challenge of traditional routing, modulation, spectrum, and core allocation (RMSCA) methods to achieve appropriate performance. This paper proposes an RMSCA approach based on deep reinforcement learning (DRL) for MCF-based elastic optical networks (MCF-EONs). Within the solution, a novel state representation with essential network information and a fragmentation-aware reward function were designed to direct the agent in learning effective RMSCA policies. Additionally, we adopted a proximal policy optimization algorithm featuring an action mask to enhance the sampling efficiency of the DRL agent and speed up the training process. The performance of the proposed algorithm was evaluated with two different network topologies with varying traffic loads and fibers with different numbers of cores. The results confirmed that the proposed algorithm outperforms the heuristics and the state-of-the-art DRL-based RMSCA algorithm in reducing the service blocking probability by around 83% and 51%, respectively. Moreover, the proposed algorithm can be applied to networks with and without core switching capability and has an inference complexity compatible with real-world deployment requirements.

Auxiliary management and control channel aided crosstalk online monitoring in multi-core fibers

As a stochastic perturbation, the inter-core crosstalk (IC-XT) severely distorts the signal in multi-core fibers (MCF), especially for long-haul transmission. How to quickly measure and monitor the IC-XT online for an MCF-based space division multiplexing (SDM) system is of special importance. In this paper, we introduce the technology of auxiliary management and control channel (AMCC) to online monitor the IC-XT of MCF, in which the unique advantage of low-frequency auxiliary management and control signal is fully utilized with the limited influence on high-speed data transmission. Specifically, two orthogonal sequences are chosen as monitoring signals for the signal-channel core and the crosstalk-channel core, respectively, followed by digital signal processing (DSP) for the received signal to evaluate the real-time crosstalk accurately. The experimental verifications of XT online monitoring confirm the effectiveness of our proposed method with very small monitoring error (mostly < 0.5 dB) for both heterodyne XT and homodyne XT in the C and L bands, showing its great potential for future SDM systems.