Space Division Multiplexing Research Articles

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.

A transmission length limit for space division multiplexing in step-index silica optical fibres

ABSTRACTBy solving the power flow equation, we investigate the influence of mode coupling on space division multiplexing capability of three multimode step-index silica optical fibres with a different strengths of mode coupling. Results show that mode coupling significantly limits the length of these fibres at which the space division multiplexing can be realized with a minimal crosstalk between the neighbour optical channels. This is most pronounced in silica optical fibres with the strongest mode coupling. The two and three spatially multiplexed channels in the investigated step-index silica optical fibres can be employed with a minimal crosstalk up to the fibre lengths of few hundred of meters and few tens of meters, respectively. These lengths are much shorter than kilometer lengths at which these fibres are usually employed without space division multiplexing. Such characterization of optical fibres should be considered in designing an optical fibre transmission system for space division multiplexing.

Channel model and optimal core scrambling for multi-core fiber transmission system

Space division multiplexing (SDM) is a potential candidate to increase the capacity of the conventional single mode fiber based transmission systems. Several multi-core fiber (MCF) structures have been proposed, each one is impaired by different core dependent loss (CDL) resulting from the fiber structure, crosstalk, splicing in the optical fiber link and inline components. One of the solutions to mitigate the CDL is the core scrambling. In this paper, we introduce three deterministic core scrambling strategies for different MCFs. The strategies show their efficiency to reduce the CDL compared to the random scrambling. Moreover, in order to estimate the CDL level and predict the system performance for any MCF structure, we propose a theoretical channel model depends on the system configuration and the number of core scrambling installed in the transmission link. Lastly, the optimal deterministic core scrambler is obtained for further reduction of the scramblers number.

Enhanced Modal Dispersion Estimation Enabled by Chromatic Dispersion Compensation in Optical Vector Network Analysis

Component characterization is fundamental for understanding the limits of optical devices, sub-systems, and transmission systems. With the introduction of space division multiplexing in optical fiber transmission systems, new impairments, such as mode dependent loss and differential mode dispersion arise. Spatially-diverse optical vector network analyzers are capable of measuring these characteristics in a fast single sweep over a very large bandwidth. As a result of this large bandwidth, these analyzers are sensitive to differential chromatic dispersion within the interferometric measurement setup. This study discusses the influence and compensation of differential chromatic dispersion in such systems. Partial chromatic dispersion compensation is demonstrated to improve the representation and accuracy of impulse response measurements obtained from optical vector network analyzers for fibers and components with large differential chromatic dispersion. Analysis of a 39-core few-mode multi-core fiber is discussed, reporting variances of -2.9-0.1 ps/nm, and 0.6-6.9 ps/nm for the two mode groups, respectively, between the few-mode cores. A correlation with the total impulse response is observed. Furthermore, a maximum propagation skew of 20 ns between cores is observed after 13.6 km.

Multiple Core Fibers versus Multiple Fibers Enabled Space Division Multiplexing Based Elastic Optical Networks: A Performance Comparison

Multiple Core Fibers versus Multiple Fibers Enabled Space Division Multiplexing Based Elastic Optical Networks: A Performance Comparison

Recent progress in optical devices for mode division multiplex transmission system

• Review in mode conversion for mode multiplexing. • Recent progress in mode multiplexing for mode division multiplexing. • Mode amplification in mode division multiplexing. • Few-mode fibers and their challenges in mode division multiplexing. • Space division multiplexing. The idea of adopting the space domain as the next frontier for optical communication has received increasing attention in the last few years. Optical devices are the integral parts of a mode division multiplexing (MDM) transmission. Therefore, conducting an intensive study on the devices is paramount to the successful realization of the overall system. This paper presents a review of the recent advances in the inline components of an MDM system, consisting of mode converters, spatial (de) multiplexers, optical amplifiers, and few-mode fibers (FMFs). Also presented are different mode conversion and multiplexing schemes. Recent techniques of minimizing differential mode gain (DMG) in the optical amplifiers are also reviewed. The review covers other types of amplification schemes and their current standing in the MDM system. These include optical semiconductor amplifiers (OSAs), and the Raman amplifiers (RAs). Finally, the review also highlights the role of FMF, multicore fiber and their relationship with fan-in/fan-out devices.

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.

All-angle polarization-insensitive negative refraction in high-dielectric photonic crystal

To obtain all-angle polarization-insensitive negative refraction, a 2D square array photonic crystal has been systematically investigated here. The presented structure is composed of air holes in a high-dielectric (ε=36) background and provides all-angle polarization-insensitive negative refraction in the first band along the ΓM direction, in a bandwidth of Δω/ωc=2.4%. The structure is applicable in polarization beam splitters used in integrated space-division multiplexing and has potential applications in highly efficient negative refraction-based phenomena, such as in angle and polarization-independent cloaking and invisibility. The results achieved by the finite difference time domain and the plane wave expansion methods here are in good agreement.

Calibrated phase-shifting digital holography based on space-division multiplexing

This paper proposes a calibrated phase-shifting digital holography by using a single camera. In the proposed system, two types of information are recorded simultaneously in a single image based on the space-division-multiplexing technique. One type of information is used to reconstruct the object image, while the other is used to calculate the phase-shifting amount. As the proposed method can detect the phase-shifting amount accurately without using any additional electronic devices, a simple low-cost high-quality image reconstruction phase-shifting digital holography system can be developed. The effectiveness of the proposed system is demonstrated through experiments.

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.

Soliton-comb structures in ring-shaped optical microresonators: generation, reconstruction and stability

Characteristic features of soliton-comb structures in optical microresonators are investigated in normal and anomalous dispersion regimes, when the detuning parameter is varied over a broad range of values. The study rests on the assumption that soliton combs are self-organized ensemble of co-propagating coherently entangled states of light, and depending on the group-velocity dispersion they can result from space-division multiplexing of single-bright and single-dark solitons. Their analytical and numerical reconstruction schemes are discussed, while a linear-stability analysis leads to a $2\times 2$ Lam\'e eigenvalue problem whose boundstate spectrum is composed of a Goldstone-type translation mode and stable internal modes, as well as unstable decaying modes and growing modes. A power-spectral analysis of the three distinct possible soliton crystals enables us probe their inner structures in the frequency domain, and unveil the existence of structural defects in their power spectra.

Highly efficient subwavelength imaging in mid-IR range using high dielectric hexagonal lattice photonic crystals

Two-dimensional hole-type hexagonal lattice photonic crystals with low and high dielectric materials have been investigated for the aim of achieving all-angle negative refraction for subwavelength imaging. Structures composed of Si (eSi = 12 at IR) and PbTe (ePbTe = 36 at mid-IR) have been regarded, as sample materials. All-angle negative refraction has been achieved for the TM polarization in the second band under the light line in a broad bandwidth of 26% and 31% for low and high dielectric materials, respectively. The optimized radii for low and high dielectric PCs equal to 0.31a and 0.35a and provide the spatial full width at half maximum of 0.37λ and 0.38λ, respectively. The structures present TE gap in the operating wavelength range, and this feature of the structures can be applied to design polarization beam splitters in integrated space division multiplexing.

Statistical Assessment of Open Optical Networks

In order to cope with the increase of the final user traffic, operators and vendors are pushing towards physical layer aware networking as a way to maximize the network capacity. To this aim, optical networks are becoming more and more open by exposing physical parameters enabling fast and reliable estimation of the lightpath quality of transmission. This comes in handy not only from the point of view of the planning and managing of the optical paths but also on a more general picture of the whole optical network performance. In this work, the Statistical Network Assessment Process (SNAP) is presented. SNAP is an algorithm allowing for estimating different network metrics such as blocking probability or link saturation, by generating traffic requests on a graph abstraction of the physical layer. Being aware of the physical layer parameters and transceiver technologies enables assessing their impact on high level network figures of merit. Together with a detailed description of the algorithm, we present a comprehensive review of several results on the networking impact of multirate transceivers, flex-grid spectral allocation as a means to finely exploit lightpath capacity and of different Space Division Multiplexing (SDM) solutions.

High-Capacity 5G Fronthaul Networks Based on Optical Space Division Multiplexing

The introduction of 5G mobile networks, bringing multi-Gbit/s user data rates and reduced latency, opens new opportunities for media generation, transport and distribution, as well as for new immersive media applications. The expected use of millimeter-wave carriers and the strong network densification resulting from a much reduced cell size—which enable the expected performance of 5G—pose major challenges to the fronthaul network. Space division multiplexing (SDM) in the optical domain has been suggested for ultra-high capacity fronthaul networks that naturally support different classes of fronthaul traffic and further enable the use of analog radio-over-fiber and advanced technologies, such as optical beamforming. This paper discusses the introduction of SDM with multi-core fibers in the fronthaul network as suggested by the blueSPACE project, regarding both digitized and analog radio-over-fiber fronthaul transport as well as the introduction of optical beamforming for high-capacity millimeter-wave radio access. Analog and digitized radio-over-fiber are discussed in a scenario featuring parallel fronthaul for different radio access technologies, showcasing their differences and potential when combined with SDM.

OAM Modes Exchanging by Controlling Polarization of Vortex Beams Propagating Through an FMF-Based Polarization Controller

In order to exchange OAM modes in a space-division multiplexing (SDM) system, an OAM-mode-exchange concept between the first-order vortex beams (L = and −1), which is achieved by controlling the polarization of vortex beams propagating over a few-mode-fiber-based polarization controller (FMFPC), is proposed. A step-index fiber is also designed and manufactured for switching the propagated OAM beams. The feasibility of the proposed concept is not only exploited and simulated in principle, but is also verified through experiments. The simulated results show that SDM-based vortex beams (L = and −1) can be mutually exchanged by appropriately controlling the phase difference (with respect to the polarization of propagated vortex-beams) between two first-order-linear-polarized-mode beams ( $\text {LPM}_{\text {11ax/ay}}$ and $\text {LPM}_{\text {11bx/by}}$ ). An experimental setup is also proposed and established for verifying the feasibility of the proposed concept in practice. The measured experimental results are consistent with the simulation, and demonstrate that the proposed scheme is correct and reliable. To evaluate the performance of the proposed scheme, bit error rates (BERs) and constellation graphs (CGs) with respect to the exchanging ways, loop numbers, and fiber lengths, are employed and investigated, respectively. The measured BERs and CGs demonstrate that the performance can be further improved with the decrease of fiber length and crosstalk, and also can be enhanced when the coiled loops is set as an appropriate value.

Wavelength Demultiplexer Designs Operating Over Multiple Spatial Modes of a Rectangular Waveguide

Wavelength demultiplexers are instrumental in wavelength division multiplexing (WDM) communications. In advance of space-division multiplexing (SDM) WDM communications, we investigate wavelength multiplexer/demultiplexer designs operating over multiplexed spatial modes simultaneously, when the modes are arranged along one transverse direction of a rectangular waveguide. The demultiplexer waveguides’ modes match those of rectangular core fibers, enabling simple and efficient edge-coupling. We describe the challenge of demultiplexing mode division multiplexed (MDM) channels, and suggest guided optics solutions for its implementation. We analyze the spatial–spectral interplay at output waveguide coupling due to mode dependent phase delays in arrayed waveguide grating router demultiplexer design, and offer low mode dependent loss solutions. The proposed schemes allow for efficient and compact MDM-enabled WDM multiplexing, paving the way to SDM-WDM communication networks.

400-Gb/s DMT-SDM Transmission Based on Membrane DML-Array-on-Silicon

We report on space-division multiplexing (SDM) transmissions of up to 400 Gb/s over a homogeneous four-core fiber using discrete multitone (DMT) modulation for intra-datacenter applications and 200/400 GbE links. The transmission system is enabled by a compact, SDM-channel scalable, multi-core fiber (MCF) pluggable, and energy-efficient SDM transmitter composed of a 4-channel 1.3-μm membrane directly modulated laser (DML) array-on-silicon integrated with a fiber-bundle type fan-in with loss of less than 1 dB and negligible optical and electrical crosstalk (XT). By simultaneously modulating all four lasers, 200-Gb/s (50-Gb/s/channel) and 400-Gb/s (100-Gb/s/channel) are achieved over a 425-m MCF link for KP4 and soft-decision forward error correction, respectively, with a power reduction of ~6× compared with conventional DML transmitters. Additionally, numerical simulations based on a detailed rate-equations model predict the feasibility of the system for MCF transmissions up to 10 km even with moderate levels of XT.

Octave-wide supercontinuum generation of light-carrying orbital angular momentum.

Nonlinear frequency generation of light-carrying orbital angular momentum (OAM), which facilitates realization of on-demand, frequency-diverse optical vortices, would have utility in fields such as super-resolution microscopy, space-division multiplexing and quantum hyper-entanglement. In bulk media, OAM beams primarily differ in spatial phase, so the nonlinear overlap integral for self-phase matched χ(3) processes remains the same across the 4-fold degenerate subspace of beams (formed by different combinations of spin and orbital angular momentum) carrying the same OAM magnitude. This indistinguishable nature of nonlinear coupling implies that supercontinuum generation, which substantially relies on self/cross-phase modulation, and Raman soliton shifting of ultrashort pulses typically results in multimode outputs that do not conserve OAM. Here, using specially designed optical fibers that support OAM modes whose group velocity can be tailored, we demonstrate Raman solitons in OAM modes as well as the first supercontinuum spanning more than an octave (630 nm to 1430 nm), with the entire spectrum in the same polarization as well as OAM state. This is fundamentally possible because spin-orbit interactions in suitably designed fibers lead to large effective index and group velocity splitting of modes, and this helps tailoring nonlinear mode selectivity such that all nonlinearly generated frequencies reside in modes with high spatial mode purity.

Performance of Spectrum Allocation Strategies under the Worst Case Crosstalk Estimation method in SDM-EONs

Present work considers Space Division Multiplexing (SDM) Elastic Optical Networks (EONs) with Multi core fiber (MCF), in which light paths are established on multiple cores. The problem of inter core crosstalk occurs when the adjacent cores use the same spectral portion. Due to the crosstalk in the adjacent cores, the transmission reach which a modulation format can support is reduced, thus affecting the spectral efficiency of the networks. A commonly used method for the crosstalk calculation is the worst case crosstalk estimation method. This method is widely used due to its simplicity. However, the performance of applied spectrum allocation strategies varies under the consideration of this method. Thus, the aim of this paper is to evaluate the performance of the spectrum allocation strategies under the worst case crosstalk estimation method in SDM-EONs with MCF. We apply first fit and first last fit spectrum allocation strategies in this work.

256 Gb/s Four-Channel SDM-Based PAM4 FSO-UWOC Convergent System

Given the rising demand for free-space with underwater environmental monitoring, calamity precaution, and developed industry applications, a free-space optical (FSO)-underwater wireless optical communication (UWOC) convergence is constructed to be an attractive architecture for affording a high transmission capacity. A 256-Gb/s four-channel space-division-multiplexing (SDM)-based four-level pulse-amplitude modulation (PAM4) FSO-UWOC convergent system that exploits two-stage injection-locked red-light vertical-cavity surface-emitting laser transmitters is, thus, offered and practically demonstrated. With a four-channel SDM scheme and PAM4 modulation, the total transmission capacity is drastically enhanced to 256 Gb/s. To the best of our understanding, this architecture is the leading one to build a high-speed 256 Gb/s FSO-UWOC convergent system with competent performances. It directs an approach to expedite extensive applications in the integration of high-speed free-space with underwater links.