Space Division Multiplexing Transmission Research Articles

Long-Haul Mode Multiplexing Transmission Enhanced by Interference Cancellation Techniques Based on Fast MIMO Affine Projection

In space division multiplexing (SDM) transmission utilizing mode division multiplexing (MDM) technologies, transmission performance of each spatial channel is highly dependent on a spatial mode that delivers information symbols over few-mode fibers (FMFs) or multi-mode fibers (MMFs). This still remains as a dominant barrier to realize a future deployable long-haul MDM transport systems. In this article, we review the transmission issues we face when transmitting MDM signals over long distances. We also present the review of key techniques that we have recently developed to overcome mode-relevant linear phenomena. By developing the projection technique for a signal detection application in multiple-input multiple-output (MIMO) systems, a novel linear filtering adaptation technique is proposed to enhance the performance of our interference canceling scheme with decreased complexity. Experimental evaluation using dense SDM transmission results over multicore FMFs verifies a fast convergence property brought by the proposed projection technique.

Spatial Density and Splicing Characteristic Optimized Few-Mode Multi-Core Fiber

Few-mode fiber design suitable for long-haul dense space division multiplexing (SDM) transmission is proposed, where the spatial density and the splicing characteristics of the fiber is optimized. We numerically investigate the impact of the number of modes per core in a multi-core structure on the total number of spatial channels possible and the splice loss property. It is shown that increased number of mode per core enables us to improve the spatial density while maintaining feasible splice-induced differential modal attenuation. We then successfully fabricate a 10-mode 12-core fiber with a cladding diameter of 217 μm, that achieves a relative core multiplicity factor (RCMF) of 101.7 and maximum number of spatial channels among the possible few-mode multi-core fiber designs with a cladding diameter of less than 250 μm. Experiments confirm that its splice-induced differential modal attenuation characteristics were better than those of previously reported 114-spatial-channel fiber.

Design of High Speed Optical Fiber Cables and Transmission Techniques in Advanced Optical Networks

In the present paper, design of high velocity fiber-optic communication networks with ultra wide-dense wavelength division multiplexing (UW-DWDM), space division multiplexing (SDM) and optical transmission techniques is presented. All connects in the (SDM) are split into subgroups of various chemical and geometrical characteristics .The parameters have been designed at: i) Δn, the relative refractive index difference for both the core and also the clad, ii) x%, the percentage of doping in silicon material by Germanium material. The Δn and x% parameters are also preserved during their technological boundaries of attention. UW-DWDM has 1200 channels that transferred at the fiber-optic wavelengths from 1450 nm up to 1650 nm within 140 connects from UW-SDM wherever every connect is prepared to own the same compressed chromatic dispersion. From this technique, it is noticed that the connect design parameters suffer more nonlinearity with the number of connects. So, two different propagation techniques have been used to investigate the transmitted bit-rate as a criterion to enhance the system performance. The first technique is Soliton propagation, where the control parameters lead to equilibrium between the pulse spreading due to dispersion and the pulse shrinking because of nonlinearity. The second technique is the maximum time division multiplexing (MTDM) technique where the parameters are adjusted to lead to minimum dispersion. Two cases are investigated; no dispersion cancellation and dispersion cancellation. The investigations are conducted over an enormous range of the set of controlling parameters. Thermal effects are considered through three basic quantities, namely: the transmission bit rate, the dispersion characteristics and the spectral losses.

Advanced DSP for Coherent Optical Fiber Communication

In this paper, we provide an overview of recent progress on advanced digital signal processing (DSP) techniques for high-capacity long-haul coherent optical fiber transmission systems. Not only the linear impairments existing in optical transmission links need to be compensated, but also, the nonlinear impairments require proper algorithms for mitigation because they become major limiting factors for long-haul large-capacity optical transmission systems. Besides the time domain equalization (TDE), the frequency domain equalization (FDE) DSP also provides a similar performance, with a much-reduced computational complexity. Advanced DSP also plays an important role for the realization of space division multiplexing (SDM). SDM techniques have been developed recently to enhance the system capacity by at least one order of magnitude. Some impressive results have been reported and have outperformed the nonlinear Shannon limit of the single-mode fiber (SMF). SDM introduces the space dimension to the optical fiber communication. The few-mode fiber (FMF) and multi-core fiber (MCF) have been manufactured for novel multiplexing techniques such as mode-division multiplexing (MDM) and multi-core multiplexing (MCM). Each mode or core can be considered as an independent degree of freedom, but unfortunately, signals will suffer serious coupling during the propagation. Multi-input–multi-output (MIMO) DSP can equalize the signal coupling and makes SDM transmission feasible. The machine learning (ML) technique has attracted worldwide attention and has been explored for advanced DSP. In this paper, we firstly introduce the principle and scheme of coherent detection to explain why the DSP techniques can compensate for transmission impairments. Then corresponding technologies related to the DSP, such as nonlinearity compensation, FDE, SDM and ML will be discussed. Relevant techniques will be analyzed, and representational results and experimental verifications will be demonstrated. In the end, a brief conclusion and perspective will be provided.

Optimal design of a bend-insensitive heterogeneous MCF with differential inner-cladding structure and identical cores

Abstract We propose a scheme of differential inner-cladding structure and identical cores to design a kind of bend-insensitive heterogeneous multi-core fiber (MCF) with high density of cores and ultra-low crosstalk (XT), which have not been previously reported. Eight cores are arranged in a ring layout in a fiber diameter of 150 μm. The impact of differential inner-cladding structure on fiber performances has been simulated and evaluated in detail. The numerical analyses show that the proposed hetero-MCF achieves an ultra-low XT of less than −50 dB/100 km, mode field area ( A eff ) of larger than 90 μm2, cut-off wavelength ( λ cc ) of less than 1350 nm, bending loss of less than 0.5 dB/100 turns at 1625 nm when bending radius ( R b ) is set to be 30 mm, and relative core multiplicity factor (RCMF) of 6.77. Our proposed hetero-MCF is capable for long distance and high capacity space division multiplexing (SDM) optical transmission systems.

Evaluation of Device Cost, Power Consumption, and Network Performance in Spatially and Spectrally Flexible SDM Optical Networks

To keep pace with the rapidly increasing network traffic demand anticipated in the coming years, space-division multiplexing (SDM) has become a promising technology for overcoming the limitations of the current wavelength-division multiplexing based optical networks. The key advantage of SDM-based optical transmission is that it increases the network capacity by extending the available spatial dimensions. In this context, several innovative technologies have been proposed to support spatially and spectrally flexible all-optical transmission. One typical technology of this kind is called spatial and spectral super-channel (Spa & Spe SpCh) transmission. Newly designed Spa & Spe SpCh transceivers can support high-speed transmission with excellent spectral efficiency by allowing the optical carriers to be flexibly distributed in both the spatial and spectral domains without guardbands. In addition, SDM-based reconfigurable optical add/drop multiplexers (ROADMs) with varying spatial switching granularities and add/drop ports have been developed to support super-channel transmission (e.g., routing and add/drop). Moreover, ROADMs with spatial lane change support, which can further improve the routing flexibility in the network layer, have also been proposed in recent years. Different modes of implementation of the aforementioned technologies have distinct advantages and disadvantages. These technologies can be combined in various ways to support different types of SDM transmission, leading to variable network performance. In this paper, we evaluate the device cost, power consumption, and network performance of SDM transmission systems based on different implementations of these technologies to serve as a reference for future research.

Digital monitoring and compensation of MDL based on higher-order Poincaré spheres.

We present a digital technique able to monitor and compensate for the mode-dependent losses (MDL) in space-division multiplexing (SDM) transmission systems. The working principle of the technique is based on the analysis of the received signal samples in the higher-order Poincaré spheres (HoPs). When an arbitrary pair of tributaries is represented in the respective HoPs, the effect of the MDL can be modeled as a simple shift of the constellation points in a such sphere. Therefore, the MDL can be estimated by computing those shifts over all the HoPs and the induced signal distortions can be compensated by re-centering all the constellations in the respective HoPs. It should be highlighted that the proposed technique is scalable with an arbitrary number of spatial channels, modulation format agonistic and free of training sequences. The HoPs-based MDL monitoring (compensation) technique allows the MDL estimation (compensation) up to values of ≈ 6 dB. The proposed technique can partially compensate the MDL distortion, making a MDL sensitive algorithm in an insensitive one. When the proposed technique assists a HoPs-based space-demultiplexing algorithm, it provides signal-to-noise ratio (SNR) enhancements of 2, 4 and 8 dB for PM-QPSK, PM-16QAM and PM-64QAM signals, respectively, for the particular case of a SDM-based transmission system with a spatial diversity of 2 and 2 dB of MDL.

Design of 125-μm cladding diameter multicore fibers with high core multiplexing factor for wideband optical transmission

Abstract We propose improved design of heterogeneous trench-assisted multicore fibers (MCFs) for wideband space division multiplexing (SDM) optical transmission. By adopting different parameters for heterogeneous cores, the trade-off among the effective area (Aeff), cut-off wavelength (λcc), and crosstalk (XT) was balanced. Different optimization approaches for long-haul and short-reach transmission applications have been adopted. A 125-μm cladding diameter 6-core fiber with Aeff = 90 μm2, λcc = 1300 nm and XT

228-Spatial-Channel Bi-Directional Data Communication System Enabled by 39-Core 3-Mode Fiber

We demonstrated simultaneous usage of 228 spatial channels (effectively) within a strand of fiber. We fabricated a 13-km 39-core 3-mode fiber and constructed a bi-directional space division multiplexing (SDM) transmission setup by using cascaded spatial multiplexers on both fiber ends. Aggregated intercore crosstalk and return loss of the system were below -35 and above 31 dB, respectively, over the C and L bands. The feasibility of using bi-directional SDM transmission was investigated using 10 GBaud, 37.38-Gb/s dual-polarization QPSK signals at 1550 nm and coherent detection with a 6 × 6 multi-input-multi-output equalizer. Excellent transmission properties were obtained for most spatial channels.

Iterative Unreplicated Parallel Interference Canceler for MDL-Tolerant Dense SDM (12-Core × 3-Mode) Transmission Over 3000 km

We propose a novel multiple-input multiple-output (MIMO) detection scheme based on iterative unreplicated parallel interference cancelling (UPIC) technique for mode dependent loss (MDL)-impaired few-mode fibre (FMF) transmission. An optical MIMO channel over an FMF exhibits a mode-dependent property where each mode division multiplexed signal is passed through a different lossy channel, known as an MDL phenomena. One challenging issue for realising future deployable space division multiplexing (SDM) systems is establishing reliable and robust signal transmission techniques in the presence of MDL. The main contributions of this paper include first, a proposal of newly developed iterative interference cancellation technique with parallel processing for MDL-impact mitigation, second, a comparative study on performance of MIMO detection schemes, and third, a demonstration of the longest dense SDM transmission over multicore (MC)-FMF. The proposed UPIC with iterative processing is experimentally shown to improve Q-factors by more than 3 dB, and record-long dense SDM transmission reach exceeding 3000 km over 12-core × 3-mode MC-FMF was achieved.

Characteristics of homogeneous multi-core fibers for SDM transmission

We describe optical data transmission systems using homogeneous, single-mode, multi-core fibers (MCFs). We first briefly discuss space-division multiplexing (SDM) fibers, observing that no individual SDM fiber offers overwhelming advantages over bundles of single-mode fiber (SMF) across all transmission regimes. We note that for early adoption of SDM fibers, uncoupled or weakly coupled fibers which are compatible with existing SDM infrastructure have a practical advantage. Yet, to be more attractive than parallel SMF, it is also necessary to demonstrate benefits beyond improved spatial spectral efficiency. It is hoped that the lower spread of propagation delays (skew) between spatial channels in some fibers can be exploited for improved performance and greater efficiency from hardware sharing and joint processing. However, whether these benefits can be practically harnessed and outweigh impairments or effort to mitigate cross talk between spatial channels is not yet clear. Hence, focusing on homogeneous MCFs, we first describe measurements and simulations on the impact of inter-core cross talk in such fibers before reporting experimental investigation into the spatial channel skew variation with a series of the experimental results including a comparison with SMF in varying environmental conditions. Finally, we present some system and transmission experiments using parallel recirculating loops that enable demonstration of both multi-dimensional modulation and joint digital processing techniques across three MCF cores. Both techniques lead to increased transmission reach but highlight the need for further experimental analysis to properly characterize the potential benefits of correlated propagation delays in such fibers.

Vector mode based optical direct detection orthogonal frequency division multiplexing transmission in short-reach optical link

As one solution to implement the large-capacity space division multiplexing (SDM) transmission systems, the mode division multiplexing (MDM) has gained much attention recently. The vector mode (VM), which is the eigenmode of the optical fiber, has also been adopted to realize the optical communications including the transmission over free-space optical (FSO) and optical fiber links. Considering the concerns on the short-reach optical interconnects, the low cost and high integration technologies should be developed. Direct detection (DD) with higher-order modulation formats in combination of MDM technologies could offer an available trade-off in system performance and complexity. We review demonstrations of FSO and fiber high-speed data transmission based on the VM MDM (VMDM) technologies. The special VMs, cylindrical vector beams (CVB), have been generated by the q-plate (QP) and characterized accordingly. And then they were used to implement the VMDM transmission with direct-detection orthogonal frequency division multiplexing (DD-OFDM). These demonstrations show the potential of VMDM-DD-OFDM technology in the large-capacity short-reach transmission links.

Hybrid cladding-pumped multicore EDFA/Raman amplification for space division multiplexing transmission systems.

We propose and demonstrate a hybrid cladding-pumped multicore erbium-doped fiber amplifier (EDFA) and distributed Raman amplification for space division multiplexing transmission systems. The cladding-pumped multicore EDFA is used to efficiently amplify signals in multiple cores simultaneously, while Raman pumping is used to control loss in each core individually. We construct an in-line amplified 7-core transmission line, and show that distributed Raman amplification can compensate loss variation between cores. Furthermore, we transmit 46 WDM PDM-16QAM signals over a long distance of greater than 1000 km and demonstrate good transmission performance.

Six-Mode Seven-Core Fiber for Repeated Dense Space-Division Multiplexing Transmission

We propose the optimum design of high spatial density few-mode multicore fiber (MCF) for repeated space-division multiplexed (SDM) transmission. Numerical analyses reveal that a hexagonally arranged six-mode seven-core structure is potentially better than the other possibility, the six-mode multicore structure, in terms of realizing high spatial density and low mode-dependent loss (MDL) characteristics at the splice point; its benefits will enable us to construct long-haul dense SDM transmission links. We fabricate a 171 μ m cladding six-mode seven-core fiber and successfully demonstrate low loss, differential mode delay (DMD), crosstalk, and splice-induced MDL characteristics while maintaining the relative core multiplicity factor of more than 50. Finally, we demonstrate repeated optical 12 × 12 MIMO transmission of more than 100 km using our six-mode MCF and erbium doped fiber amplifier (EDFA).

Advanced MIMO Signal Processing Techniques Enabling Long-Haul Dense SDM Transmissions

Space division multiplexing (SDM) transmission technology is the prime candidate for next technological breakthrough in fiber-optic communication systems, as spatially structured multicore fibers and multimode fibers allow exploitation of the spatial degrees of freedom. Although the use of multi-input multi-output (MIMO) signal processing has significantly contributed to the development of MIMO-SDM transmission, differential mode delay (DMD) and mode dependent loss (MDL) remain as restricting factors of long-haul MIMO-SDM transmission. This paper starts by reviewing the recent progress demonstrated by SDM transmission experiments. Issues with MIMO-SDM transmission including DMD and MDL are then discussed with their methodologies based on optical and digital approaches. Next, brief descriptions of linear and nonlinear MIMO signal processing techniques that enable robust and reliable MIMO-SDM transmission are provided with complexity and performance comparisons, followed by a discussion of their adaptation in coherent MIMO-SDM transmission. We also elucidate the experimental evaluation of frequency selective channels in a multicore few-mode fiber with experimental evaluations, and discuss their impact on MIMO-SDM signal transmission from a signal processing perspective.

10.16-Peta-bit/s Dense SDM/WDM Transmission over 6-Mode 19-Core Fiber across the C+L Band

Space-division multiplexing (SDM) is an attractive technique for dramatically enhancing the transmission capacity in a single optical fiber. Recently, ultradense SDM transmission experiments with a spatial multiplicity of over 100 have been reported by using few mode multicore fibers (FM-MCFs). Considering the maximum capacity of around 100 Tb/s reported in single-mode single-core fiber transmission experiments, the capacity in FM-MCF transmission with more than 100 spatial channels is expected to reach 10 peta-b/s; however, the maximum capacity has been limited to 2 peta-b/s. In this paper, we demonstrate ultradense SDM transmission of 739 WDM 12-Gbd dual polarization—64-quadrature amplitude modulation (QAM)/16-QAM signals over 11.3-km 6-mode 19-core fiber using the C+L band, achieving a record fiber capacity of 10.16 peta-b/s with an aggregate spectral efficiency of 1099.9 b/s/Hz.

SDM for Power-Efficient Undersea Transmission

We discuss space division multiplexing as a means of increasing link capacity for power limited transmission in undersea communication. Theoretical study of erbium-doped fiber amplifiers-based space division multiplexing (SDM) transmission shows the existence of optimal values of spectral efficiency, signal-to-noise ratio, and span length that maximize power efficiency. We experimentally confirm the existence of an optimal spectral efficiency and demonstrate power efficient SDM transmission using 12-core multicore fiber. Modulation format, amplifier, and link design are discussed in connection to SDM power efficient transmission.

Maximum Optical Power Efficiency in SDM-Based Optical Communication Systems

We demonstrate the existence of an optimal spectral efficiency that maximizes the power efficiency and/or capacity of a power limited erbium-doped fiber amplifier (EDFA)-based space division multiplexing transmission system. Optimal spectral efficiency observed experimentally is in good agreement with a theoretical estimate.

Crosstalk-aware routing, spectrum, and core assignment in space-division multiplexing optical networks with multicore fibers

The transmission capacity of existing optical fiber will reach its physical limitation due to increasing network bandwidth. Recently, space-division multiplexing (SDM) technology has been proposed to increase the fiber transmission capacity. Multicore fiber (MCF) is a strong candidate for SDM transmission, but the intercore crosstalk will be a critical issue with MCF transmission. We make a specific analysis of transmission crosstalk in elastic optical networks. Then, the concept of spare spectrum availability is introduced to accommodate connection requests. Based on it, two different crosstalk-aware routing, spectrum, and core assignment algorithms are presented at different network states. Simulation results show that our proposed scheme improves the spectrum resource utilization and reduces blocking, with a cost of additional algorithm complexity and set-up delay.

Coupled Few-Mode Multicore Fiber With Low Differential Mode Delay Characteristics

We investigate a coupled few-mode multicore fiber (FM-MCF) design to realize an ultrahigh spatial density space division multiplexing (SDM) transmission line with low differential mode delay (DMD) characteristics. We describe a super-mode-based FM-MCF design that takes account of the DMD between modes in the inter-linearly polarized (LP) mode group (e.g., between LP01- and LP11-like super modes). We also show a randomly coupled FM-MCF design based on fiber bending and twisting to reduce the DMD between modes in the intra-LP mode group. We fabricated a coupled three-mode seven-core fiber and realized a low DMD value of less than 0.2 ns/km for all modes where the intercore nonuniformity induced DMD was successfully suppressed owing to distributed random mode coupling. The fabricated fiber has 21 spatial modes within a 125 μm cladding, and a relative core density of 21, which is the highest of any reported 125 μm cladding SDM fiber.