Inter-core Crosstalk Research Articles

Crosstalk-Aware Shared Backup Path Protection in Multi-Core Fiber Elastic Optical Networks

Elastic optical networks employing multi-core fibers (MCF-EON) have the potential to expand significantly the transmission capacity of optical transport. However, wide deployment of such networks depends on addressing effectively two critical challenges: inter-core crosstalk, which may cause serious signal performance degradation in an MCF link, and survivability against network failures that may cause enormous data loss. In this article, we consider the design of MCF-EONs with shared-backup path protection (SBPP), one of the most efficient techniques for protecting network traffic. Specifically, we tackle the crosstalk-aware routing, core, and spectrum assignment (CA-RCSA) problem with the objective of jointly minimizing the network spectrum resources used and the total inter-core crosstalk. We formulate the problem as an integer linear programming (ILP) model subject to strict inter-core crosstalk limits for each provisioned lightpath, and we also propose an auxiliary graph (AG) based heuristic algorithm for lightpath provisioning. Simulation studies show that our algorithm is effective in terms of the objectives, and it is efficient to perform close to the ILP model in small networks, for which solving the ILP is feasible.

BER performance limitations due to inter-core crosstalk in a multi-core fiber-optic transmission system

An analytical approach is developed to evaluate the amount of crosstalk due to inter-core coupling and its limitations on the Bit Error Rate (BER) performance of a fiber link length of 1000 m with a homogeneous multi-core optical fiber (MCF) with a direct detection receiver. Coupling coefficients due to inter-core coupling in 9, 7 and 3 core MCF are evaluated numerically for different core pitch. Crosstalk power, Signal to Crosstalk plus Noise Ratio (SCNR) and BER at any output core with light launched into other core of the MCF are determined numerically. Crosstalk is found to be more pronounced in 9 core and 7 core MCF compared to 3 core. It is found that there is significant deterioration in BER performance due to inter-core coupling induced crosstalk. Power penalty due to inter-core crosstalk at a BER of 10−9for a link length of 1000 m are found to be 2.5 dB, 1.50 dB and 0.5 dB for 9, 7 and 3 core MCF respectively. There is a minimum attainable BER corresponding to a given pitch and number of cores.

Outage Probability Due to Crosstalk from Multiple Interfering Cores in PAM4 Inter-Datacenter Connections

In this work, we propose to use four-level pulse amplitude modulation (PAM4) and multi-core fibers (MCFs) to support very high capacity datacenter interconnect (DCI) links. The limitations imposed by inter-core crosstalk (ICXT) on the performance of 112 Gb/s up to 80 km-long optically amplified PAM4 inter-DCI links with intensity-modulation and direct-detection and full chromatic dispersion compensation in the optical domain are analyzed through numerical simulation for high and low skew-symbol rate product (SSRP). With only one interfering core, we show that those PAM4 inter-DCI links achieve an outage probability (OP) of 10−4 with a maximum ICXT level of −13.9 dB for high SSRP and require an ICXT level reduction of about 8.1 dB to achieve the same OP for low SSRP. Due to using full dispersion compensation, for an OP of 10−4, the maximum acceptable ICXT level shows only a 1.4 dB variation with the MCF length increase from 10 km to 80 km. When considering the ICXT induced by several interfering cores, the maximum ICXT level per interfering core for an OP of 10−4 decreases around 3 dB when doubling the number of interfering cores. This conclusion holds for high and low SSRP regimes. For two interfering cores, we show that a single interfering core with low SSRP is enough to induce a severe reduction of the maximum acceptable ICXT level.

High-Power Jamming Attack Mitigation Techniques in Spectrally-Spatially Flexible Optical Networks

This work presents efficient connection provisioning techniques mitigating high-power jamming attacks in spectrally-spatially flexible optical networks (SS-FONs) utilizing multicore fibers. High-power jamming attacks are modeled based on their impact on the lightpaths’ quality of transmission (QoT) through inter-core crosstalk. Based on a desired threshold on a lightpath’s QoT, the modulation format used, the length of the path, as well as a set of physical layer characteristics, each lightpath can potentially tolerate a high-power jamming attack. In this paper, an integer linear program is thus formulated, as well as heuristic algorithms to solve the problem of attack-aware routing, spectrum, modulation format, and core allocation in SS-FONs, aiming to both efficiently provision the network in terms of network resources, as well as minimize the impact of high-power jamming attacks on the established lightpaths. Extensive simulation results are obtained for several algorithm variants with different objectives, demonstrating the validity and efficiency of the proposed techniques that can effectively mitigate high-power jamming attacks, by minimizing the number of inter-core interactions, while at the same time establishing connections with high spectral efficiency.

Routing, Core and Wavelength Allocation in Multi-Core-Fiber-Based Quantum-Key-Distribution-Enabled Optical Networks

Quantum-key-distribution (QKD) enabled optical networks can provide secure keys for different kinds of applications to ensure the high security of communication processes. As the number of wavelength channels in a single-core fiber is limited, space division multiplexing (SDM) has been considered to be a valuable technique to provide more wavelength resources, which solves the problem of insufficient channel capacity. Due to the rapid expansion of data volume, the SDM-based QKD network has been regarded as an important paradigm. At the same time, resource allocation in multi-core optical networks is a problem worthy of studying. In this work, four routing, core, and wavelength allocation (RCWA) algorithms are proposed in the multi-core optical network for quantum service requests and classical service requests, which are referred to as RCWA without perception (RCWA-WP), RCWA with crosstalk perception (RCWA-XTP), RCWA with spectrum perception (RCWA-SP), and RCWA with core perception (RCWA-CP). Simulations are carried out to evaluate the network performance in terms of blocking probability, key utilization, and average crosstalk intensity. The comparative results indicate that the RCWA-SP algorithm can improve network performance and reduce the blocking probability, while the RCWA-XTP algorithm can reduce inter-core crosstalk during transmission.

On Throughput Optimization in Software-Defined Multi-Dimensional Space Division Multiplexing Optical Networks

The capacity expansion of a single time division multiplexing (TDM) or wavelength division multiplexing (WDM) domain is difficult to cope with long-term capacity pressure. The emergence of space division multiplexing (SDM) technology provides a new dimension for the expansion of optical network capacity, namely the spatial dimension. However, this also poses a challenge to both network control and resource optimization. Therefore, to improve the flexibility of network control, we propose an architecture of software-defined multi-dimensional SDM optical networks based on architecture on demand (AoD) nodes and multi-core fiber (MCF) links in this paper. Next, we mathematically describe the system model including a model of the SDM network, a model of multi-type service requests, and a MCF model. Then, an integer linear programming (ILP) is developed to minimize the index of multi-dimensional resources occupied by multiple types of services in the time, frequency, and space domains. To reduce inter-core crosstalk between homogeneous cores, the core classification algorithm is designed to build multi-granular network layers. Also, for adjacent heterogeneous cores, we utilize the AoD to customize different functions and use different spectrums to transmit different types of services. Finally, we propose routing and multi-dimensional resource allocation algorithms to optimize network throughput. The simulation results show that the proposed algorithms can effectively improve the throughput of the software-defined multi-dimensional SDM optical networks compared with the benchmark algorithms.

New Expression for Evaluating the Mean Crosstalk Power in Weakly-Coupled Multi-Core Fibers

A closed-form expression is proposed for evaluating the mean inter-core crosstalk (ICXT) power in weakly-coupled multi-core fibers (WC-MCFs), with cores’ propagation constants perturbed by bending, twist, and random structure fluctuations. This expression generalizes the expression proposed by Koshiba et al. by taking the dependence of the propagation constants on the longitudinal coordinate of the fiber (induced by constant bending and twist along the MCF) into account in a more rigorous way. We provide a physical interpretation of the proposed expression as the convolution of the spectrum of the $z$ -dependent component of the propagation constant induced by the bending and the spectrum of the perturbation induced by the structure fluctuation. The accuracy of the proposed closed-form expression is assessed by comparing simulation results of the coupled-mode equations with the theoretical predictions from the proposed expression. Very good agreement of the mean ICXT power estimates is shown in the whole range of tested bending radius, difference of intrinsic effective refractive indexes of cores, twisting period and correlation length, for fiber lengths longer than one half of the twisting period and five times the correlation length. We show that the proposed expression enables evaluating the mean ICXT power in WC-MCFs with improved accuracy relative to Koshiba's expression: remarkable differences of mean ICXT power, that may exceed $\text{6}\;\text{d}\text{B}$ , can occur between the estimates provided by the Koshiba's expression and the closed-form expression derived in this article, particularly for high correlation length and twisting rate. Differently from the expression proposed by Koshiba et al. , the proposed closed-form expression predicts that the twisting period may have a significant influence on the mean ICXT power, in qualitative agreement with experimental results reported by other authors.

Protected Resource Allocation in Space Division Multiplexing-Elastic Optical Networks with Fluctuating Traffic

In this paper, we introduce a Protected Routing, Modulation, Spectrum, and Core Allocation (RMSCA) algorithms with fluctuating traffic in Space Division Multiplexing-Elastic Optical Networks (SDM-EONs). This RMSCA algorithm is called Traffic Awareness Cross-talk Interference Avoidance (TACIA-RMSCA). We investigate the efficiency of efficient allocation of spectrum resources to minimize the impact of crosstalk (XT), and blocking probability. We first assess the impact inter-core crosstalk in Multi-Core Fiber (MCF) with the peak load of the fluctuating traffic. This is usually caused by signal leaks generated by the adjacent cores. It can occur whenever the transmitted optical signals overlap their spectrum segments. We design the Triangular Iterative Core Assignment (TICA) strategy to overcome XT, and to minimize the blocking probability. We evaluate the performance of TACIA-RMSCA with the benchmark algorithm (GAK-RMSCA). Experimental results demonstrate that the proposed algorithm can achieve promising performance with the peak load of the fluctuating traffic.

Semi-analytical approach for calculation of intercore crosstalk in multicore fibers

In this paper, a semi-analytical method to calculate intercore crosstalk (ICXT) in bent and twisted multicore fibers (MCFs) is proposed. The modified refractive index of bent MCF is first computed using geometrically exact beam theory (GEBT) and conformal mapping technique. The computed refractive index profile is used to obtain mode field profiles of bent MCF by solving the wave equation using vectorial finite element method in COMSOL. To take twisting of MCF into account, outer cores of the MCF are approximated to be helically bent around the central core. Using the mode field profiles, mode coupling coefficient between the MCF cores is calculated for different bend radii which is then used to calculate ICXT by using closed form analytical expressions reported in the literature. The ICXT values obtained using our proposed approach are in very good agreement with the experimental values reported in the literature. The effect of change in core pitch and change in relative refractive index contrast of MCF on the mode coupling coefficient between the MCF cores is analysed. The influence of twisting rate of MCF on the ICXT values for different bend radii is studied and it is shown that, ICXT is constant with respect to twisting rate of MCF for bending radii greater than the critical bending radius of the MCF. The proposed approach is also extended to calculate ICXT between different modes of trench assisted few-mode MCF (FM-MCF). The ICXT values obtained for the trench assisted FM-MCF follow the expected trend.

Analysis of Inter-Core Crosstalk in Weakly-Coupled Multi-Core Fiber Coherent Systems

The influence of the inter-core crosstalk (ICXT) on the performance of weakly-coupled multi-core fiber (WC-MCF) systems with coherent detection and arbitrary inter-core skew is studied numerically and analytically. We analyze the evolution of the instantaneous ICXT power, induced by polarization division multiplexed quadrature amplitude modulation signals, short term average crosstalk (STAXT), and detected ICXT components along the time to get insight on how the ICXT affects the coherent system performance. Numerical results show that, with low skew-symbol rate product (4:1), the STAXT can have large fluctuations along the time and the variance of the detected ICXT can also have large fluctuations along the time, causing large variations of the bit error rate (BER) along time. With large skew-symbol rate product (≫1), the STAXT is almost constant along the time and, although the detected ICXT varies along time, the detected ICXT variance is almost constant along time leading to very small fluctuations of BER along time. Analytical expressions for the variance of the coherently detected ICXT, average BER and optical signal-to-noise ratio (OSNR) penalty are proposed and shown to agree with Monte-Carlo simulation results for arbitrary skew, modulation format order, and roll-off factor of the transmitted signals. Numerical results show that the maximum allowable ICXT level for 1 dB OSNR penalty increases when the skew augments and can reach 1.3 dB for a roll-off factor of 1. For most cases of interest of low roll-off factor, the increase of the maximum allowable ICXT level is very small. It is shown that the OSNR penalty estimates agree quite well with other authors' experimental results (with nearly zero roll-off factor).

On-demand fragmentation-aware spectrum allocation in space division multiplexed elastic optical networks with minimized crosstalk and multipath routing

Elastic optical networks are extremely effective networks for high speed optical communication owing to excessive data rate requirements in future years. Applying the space division multiplexing (SDM) technology with multi-core fibers (MCFs) provides possibility to increase the network capacity and data transmission rate as a consequence. One of the main issues in these types of networks is bandwidth fragmentation due to dynamic spectrum allocation and releasing. Management of this problem would lead to optimal use of network bandwidth. As a result, more connection demands can be accommodated. In this paper, two novel algorithms of multi-path routing are presented to improve fragmentation, where both algorithms are fragmentation-aware. The first algorithm, named Fragmentation-aware Best Splits (FABS), exploits fragmentation criteria as well as a new split technique called Best Split. In addition to these features, the second algorithm called Crosstalk-aware Best Split (CABS) considers inter-core crosstalk (XT) which is a significant issue in SDM. By employing these two algorithms and considering distance-adaptive modulation, our performance evaluation results illustrate a remarkable improvement in reducing blocking rate and enhancement of spectrum utilization.

High-Speed Performance Evaluation of Graded-Index Multicore Fiber Compatible With Multimode and Quasi-single Mode Operation

A comprehensive study on the optical characteristics and high-speed transmission performance of a hexagonally arrayed seven-core fiber supporting both multimode transmission at 850 nm and quasi-single mode transmission at 1310 and 1550 nm are presented. We investigated its optical and transmission characteristics such as inter-crosstalk, modal bandwidth, chromatic dispersion, and misalignment tolerance. The results show that the multicore fiber (MCF) has a high bandwidth at 850 nm while remaining a comparable performance with standard single-mode fiber (SSMF) at 1310 and 1550 nm. The worst inter-core crosstalk is less than -43 dB over a 10 km-long MCF connected with a pairwise fanin/fan-out devices. The minimum effective modal bandwidth of 8.44 GHz·km is achieved with the MCF for multimode operation, while the chromatic dispersion of the MCF at 1550 nm only increases by 7% compared with that of SSMF. In experiment, we successfully performed the 7 × 25 Gb/s multimode transmission over a 300 meter-long MCF at 850 nm and quasi-single mode transmission over a 12.4 and 10 km-long fiber at 1310 and 1550 nm, respectively, with an OOK modulation format. There was no obvious degradation observed when all cores operated simultaneously. Moreover, we carried out 7 × 50 Gb/s PAM4 transmission over a 300 meter-long and 10 km-long MCF at 850 and 1310 nm, respectively, as well as 7 × 40 Gb/s PAM4 transmission over a 10 km-long MCF at 1550 nm. A BER which is well below the forward error correction (FEC) threshold (5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> ) was reached for different MCF links.

Intercore spontaneous Raman scattering impact on quantum key distribution in multicore fiber

We propose a theoretical model to quantitatively estimate the intercore spontaneous Raman scattering (ICSRS) in multicore fiber based on intercore crosstalk. We analyze the properties of ICSRS through numerical simulations. Then, the impact of ICSRS on quantum key distribution (QKD) is evaluated with a classical dense wavelength division multiplexing system in a seven-core fiber. We evaluate the impact in two scenarios, long-distance transmission and metropolitan area transmission. For long-distance transmission, it is revealed that both the forward-ICSRS and backward-ICSRS will reduce the maximum transmission distance of QKD and backward-ICSRS has more impairment to QKD. However, over the range of metropolitan area networks, QKD is not impaired seriously by ICSRS with typical power of classical signal. Finally, we compare the properties of SRS generated in single-core fiber and the properties of ICSRS.

Joint impact of fibre dispersion and frequency chirp on MCF‐based radio‐over‐fibre mobile fronthaul

Multicore fibre (MCF) has attracted considerable attention in mobile fronthaul networks as a promising solution to face the ever-increasing mobile traffic demand and to support network capacity scaling in a cost-effective manner. However, MCF-based systems suffer from the inter-core crosstalk (IC-XT) noise, which could incur severe performance degradation. In this study, we investigate for the first time, to our knowledge, the performance of an analogue radio-over-fibre mobile fronthaul link employing MCF under the joint impact of fibre dispersion and frequency chirp. Herein, the authors consider the radio frequency (RF) power fading induced by fibre dispersion and frequency chirp. In addition, the frequency-dependent IC-XT power fluctuations induced by dispersion and frequency chirp have been discussed. Closed-form equations for the carrier-to-noise ratio penalty due to power fading and IC-XT are derived. Moreover, new expressions for the average and variance of the crosstalk transfer function are obtained. Also, the analysis accounts for inter-core skew, laser relative intensity noise, shot noise and thermal noise. It has been shown that both the desired signal and IC-XT signals show the same behaviour in the presence of fibre dispersion and frequency chirp. Moreover, results show that RF power fading is the key issue.

Impact analysis of a dense hole-assisted structure on crosstalk and bending loss in homogeneous few-mode multi-core fibers.

We present a novel dense hole-assisted structure (DHAS) in homogeneous few-mode multi-core fibers (FM-MCFs) for significantly suppressing inter-core crosstalk (XT) and bending loss while realizing a high spatial density in limited cladding diameter. The fabrication methods of DHAS FM-MCFs are illustrated and the equivalent model of DHAS is proposed. To point out the superiority, the XT of a DHAS 7-core 4-LP-mode fiber is investigated by an average power-coupling coefficients analytical expression in DHAS model and the equivalent model, respectively. A simple derived analytical expression for XT estimation in the trench-assisted homogeneous MCFs is introduced to verify the change of XT with fiber parameters in the equivalent model. The results imply that the XT obtained by the equivalent model is in good agreement with the one through DHAS model. Furthermore, the bending loss and chromatic dispersion dependences on DHAS are calculated by the finite element method (FEM). Through numerical simulations, we show that the DHAS has great contribution to meet the XT value requirement of lower than -30 dB/100km and make the bending loss values satisfy the ITU-T recommendations of G. 654 in a 7-core 4-LP-mode fiber with a 125-μm cladding diameter. The designed structure targets applications in space division multiplexing (SDM) fibers with independent transmission channels.

True-Time Delay Line Based on Dispersion-Flattened 19-Core Photonic Crystal Fiber

A novel design of a tunable True-Time delay line (TTDL) based on a multicore photonic crystal fiber (PCF) is proposed. It enables simultaneous transport and processing of microwave photonic signals over a broad radiofrequency processing range. Independent group delay behavior in 19 different cores characterized by a constant differential group delay between cores provides TTDL operation on 19 signal samples. The 19-core PCF structure allows tailoring the chromatic dispersion range between 1.5 and 31.2 ps/nm·km, which translates into a very broad microwave signal processing range from a few up to tens of GHz. A near-zero dispersion slope reduces the time delay errors in the TTDL's operation to less than 5% in a 40-nm optical wavelength range, thus ensuring its satisfactory performance. Its performance as a TTDL is evaluated in terms of higher-order dispersion as well as other degradation effects such as crosstalk and nonlinear fiber response. A high index contrast between core and cladding, between 0.3 to 1.5%, enables low intercore crosstalk and confinement losses as well as greater robustness against fiber bends and twists. Fabrication of this type of MCF might be possible although it will prove challenging. This work advances the state-of-the-art of a TTDL based on SDM technology by increasing the number of samples and microwave processing range.

Crosstalk Characteristics of WDM Channels in Quasi-Homogeneous Multicore Fibers

The relative average inter-core crosstalk (IC-XT) among wavelength-division multiplexed (WDM) optical channels in a quasi-homogeneous two-core fiber system exhibiting slight differences on the propagation constant, inverse group-velocity, and the group-velocity dispersion (GVD) between cores is numerically investigated. This analysis is performed using a theoretical formalism that includes fiber nonlinearities in the presence of random perturbations. The results show oscillations on the average IC-XT of WDM channels that mainly depend on the walk-off between cores. These oscillations arise superimposed to the linear wavelength-dependence of the IC-XT caused by the core-coupling coefficient. As transmission power increases, the nonlinearities do not reduce the presence of these oscillations, but slightly modify their amplitudes and periods. These results provide valuable information to fully understand the wavelength dependence of the average IC-XT of real multi-channel MCF systems, especially those of very short reach.

High-Capacity Super-Channel-Enabled Multi-Core Fiber Optical Switching System for Converged Inter/Intra Data Center and Edge Optical Networks

The introduction of the space domain in optical communications, collectively referred as space division multiplexing (SDM), have recently gained a lot of research attention motivated by the capacity exhaustion of conventional wavelength division multiplexing (WDM) systems. SDM not only increases the capacity, but also introduces new challenges and opportunities owing to the properties of the spatial channels in the form of fiber cores, modes or a combination of both. One promising application of SDM systems could be in future edge optical networks, where multi-core fibers (MCFs) with low inter-core crosstalk may be used with packet spatial super-channels (pSSCs) and core-joint SDM optical switches in a time-slotted network. We first briefly review the state-of-the-art of SDM transmission systems and other important technologies for edge networks such as burst-mode transmission. Finally, we focus on two recent experimental demonstration of such a switching system, achieving a capacity of 53.3 Tb/s for 7 spatial channels and QPSK modulation formant and an upgraded version with 8 spatial channels and 8PSK modulation format that achieves 83.33 Tb/s.

Minimizing Inter-Core Crosstalk Jointly in Spatial, Frequency, and Time Domains for Scheduled Lightpath Demands in Multi-Core Fiber-based Elastic Optical Network

Elastic optical networks (EON) technology in combination with space division multiplexing (SDM) is considered as having the potential to expand the transmission capacity of optical transport networks. However, inter-core crosstalk may cause serious signal impairment in a multi-core fiber (MCF) links. At the same time, scheduled lightpath demands, for which the expected setup and teardown times are known in advance, are considered as an important type of traffic demand for future networks. In this article, we develop approaches to schedule simultaneous lightpaths onto non-adjacent MCF cores so as to reduce inter-core crosstalk between these lightpaths. To this end, we first define a new metric to estimate the inter-core crosstalk jointly considering the spatial, frequency, and time domains. We then tackle the routing, spectrum, core, and time assignment (RSCTA) problem for the MCF-based EON by developing an integer linear programming (ILP) model, as well as an auxiliary graph (AG) based heuristic algorithm, which jointly optimize spectrum resource utilization and reduce the lightpath inter-core crosstalk. Simulation studies show the effectiveness of the proposed approach in terms of both performance aspects. In addition, the performance of the proposed heuristic algorithm is shown to be close to that of the ILP model in small networks.

Low-DMD and low-crosstalk few-mode multi-core fiber with air-trench/holes assisted graded-index profile

We propose a novel scheme of air-trench (AT) assisted graded-index profile to design a heterogeneous few-mode multi-core fiber (hetero-FM-MCF) with high density of cores and low inter-core crosstalk (XT). By the using of finite element method (FEM), it is revealed that the AT structure can remarkably reduce XT and bending loss, and leaving no significant impact on other fiber properties. The numerical analyses show that the AT hetero-FM-MCF achieves a low XT of about -66 dB/km (15.7% lower than that of trench-assisted (TA) hetero-FM-MCF), the max differential model delay (DMD) over C+L band of 70 ps/km and relative core multiplicity factor (RCMF) of 41.7. To further improve the light confinement ability of individual cores, air-holes (AH) are introduced to the AT structure. The air-trench/hole assisted (AT-AH) hetero-FM-MCF generates a lower XT of -68 dB/km and 6% bending loss values optimization than AT hetero-FM-MCF.