Inter-core Crosstalk Research Articles

Multi-core fiber channel equalization algorithm based on K nearest neighbor method

Aiming at the channel damage problems of multi-core fiber (Multi-core Fiber, MCF), the AKNN (Adapted K-Nearest Neighbor) algorithm is proposed. The MATLAB software was used to simulate the channel of the seven-core fiber, both the inter-core crosstalk (Crosstalk, XT) and nonlinear effects were considered. Then the KNN algorithm was adapted to equalize the damaged signal at the receiver. The simulation results show that the AKNN algorithm possesses better BER performance and lower OSNR requirements with the extension of transmission distance, compared to traditional KNN algorithm.

A low-crosstalk and high-density multi-core few-mode fiber based on heterogeneous core and trench-assisted air-holes isolation

The capacity of a traditional optical communication system based on single-mode fiber has approached to its theoretical limit. Multi-core few-mode fibers provide an effective way to break through the bottleneck of existing transmission capacity. In this paper, a 5-LP-mode weakly-coupled low-crosstalk 7-core fiber is designed by using a combination of trench assistance and air hole isolation structure. The fiber with a standard outer diameter achieves low crosstalk between cores and modes. The inter-core crosstalk area and the effective mode area of the core are calculated by the finite element method. After design optimization, there are 5 stable transmission LP modes in the C+L band of optical communication in this fiber. The effective refractive index difference between LP<sub>21</sub> mode and LP<sub>02</sub> mode is the smallest and is greater than 1.1 × 10<sup>–3</sup>. The LP<sub>31</sub> mode in the optical fiber has the largest inter-core crosstalk and the loss is lower than –50 dB/km. The fiber can achieve low crosstalk transmission between modes and cores at the same time. The mode areas of the 5 LP modes in the 7 cores are larger than 86 μm<sup>2</sup>, and the relative core multiplexing factor is 57.63 at a wavelength of 1550 nm. Therefore, this fiber can be used in a large-capacity high-speed fiber transmission system.

A kind of single trench 19-core single-mode heterogeneous fiber with low crosstalk and anti-bending performance

The rapid growth of the demand for optical communication capacity promotes optical fiber communication technology. As a method to break through the capacity limitation of conventional single-mode fiber, multi-core fiber based on space division multiplexing technology has attracted extensive attention. In order to respond to the capacity of traditional single-mode fiber positively, we design a scheme of single-mode multi-core fiber combining the arrangement of heterogeneous fiber cores with secondary structure of low refractive index trench. The scheme consists of nineteen fiber cores arranged in a hexagonal closed-packed structure. Heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF) has low inter-core crosstalk and excellent anti-bending performance. Compared with conventional single-mode fiber, the Hetero-TA-MCF has the large transmission capacity and average effective area of each core of about 80 μm<sup>2</sup>. The transmission capacity of 19 cores is equivalent to the sum of the transmission capacities of 19 single-core single-mode fibers. We use COMSOL Multiphysics to simulate the fiber structure, finding the parameters that affect the properties of the fiber, selecting parameters and structures for optimal performance. Then we calculate the transmission characteristics by the finite element method,and the results of substantive simulating compute are as follows. The Hetero-TA-MCF achieves a low inter-core crosstalk (XT) of about –39 dB/100 km so that each core can be transmitted as a separate channe. It meets the standard of multi-core fiber long distance transmission. The XT of the heterogeneous 19-core single-mode fiber is suitable for multi-core fiber long distance transmission standards. The bending loss of the outermost fiber core is –7.7×10<sup>–5</sup> dB/m when the bending radius is 10 cm, which reflects the low loss characteristics of the structure. The nonlinear coefficients of three kinds of core are 1.28 W<sup>–1</sup>·km<sup>–1</sup>, 1.31 W<sup>–1</sup>·km<sup>–1</sup>, and 1.30 W<sup>–1</sup>·km<sup>–1</sup> respectively, reducing the nonlinear effect of optical fiber effectively; the dispersions of three kinds of cores are less than 24 ps/(nm·km). In addition, the steady single-mode transmission is achieved in <i>C</i>+<i>L</i> band. Compared with traditional single-mode fiber and single-trench homogeneous fiber, the proposed fiber in this work has low crosstalk, good bending resistance and large mode field area, which is suitable for long distance and large capacity transmission in space division multiplexing system.

Distribution Characteristics of Inter-Core Crosstalk in Weakly Coupled Multicore Fibers

Distribution Characteristics of Inter-Core Crosstalk in Weakly Coupled Multicore Fibers

Design of 21-core trench and air-hole assisted multi-core fiber for high speed optical communication

We present a novel 21-core homogeneous multi-core fiber (MCF) structure with three different types of core placement layouts (1-Ring, 2-Ring, and Square Lattice) and densely pack the cores with a minimum cladding diameter of 200 μm to support high-speed applications. In the first phase, three MCF structures (1-Ring, 2-Ring, and Square Lattice) were designed with a step refractive index (RI) profile under the limited cladding thickness and diameter of 35 and 200 μm, respectively, and their crosstalk was calculated. In the second phase, all three structures were designed with a low-RI trench-assisted (TA) profile to significantly reduce the crosstalk. To further suppress the crosstalk, air holes were placed between the TA cores, and it was found that the TA Square Lattice core arrangement with air holes achieved the minimum crosstalk value of approximately −70 dB / 100 km. The impact of important design parameters, such as transmission distance, bending radius, operating wavelength, and core diameter on inter-core crosstalk for all three design structures for both RI profiles and air-hole arrangement were numerically analyzed. Furthermore, other transmission characteristics, such as group delay and dispersion concerning wavelength, were analyzed for the above-stated design structures.

Low cross talk homogeneous seven-core five-LP mode fiber based on a high and low refractive index double ring

We propose a what we believe is a novel, to the best of our knowledge, high and low refractive index double-ring (HL-DR) structure in few-mode multicore fibers (FM-MCFs) to simultaneously suppress intercore cross talk (XT) and intermodal cross talk. The fabrication methods of HL-DR seven-core fiber are introduced. A series of intercore XT values are calculated by the average power coupling coefficient, which shows that the HL-DR structure has a great contribution to meet the intercore XT target of less than − 30 d B / 100 k m in FM-MCFs. Intermodal XT is mainly measured by the effective refractive index difference ( Δ n e f f ) between modes, and the Δ n e f f between any two adjacent modes is larger than 2 × 1 0 − 3 . Other main fiber properties, including bending loss and dispersion, are also simulated by the finite-element method. The results imply that the bending loss can satisfy the requirement of five-LP mode operation over the C + L band. In addition, the dispersion comparison of L P 01 mode illustrates that the fiber dispersion performance is not significantly influenced by the addition of the HL-DR structure. Through numerical analyses, an optimal HL-DR seven-core five-LP mode fiber with low XT is obtained, and it has a relative core multiplicity factor (RCMF) of 62.17. The proposed structure targets the design and application of space division multiplexing fibers with high capacity and independent channels.

Design of Four-Core Uncoupled Multicore Fiber for Next-Generation Inter-Data Center Networks

The increasing demands within and between the data centers used for data traffic has required. Efficient links are important to data center applications for supporting the unlimited demand. Transmission capacity of single-mode fiber (SMF) is limited by fiber nonlinearity which prevents the increasing transmission power and finite amplifier bandwidth. Single-mode multi-core fibers (SM-MCFs) that are expected to overcome the current limitation of optical communication capacity. However, the inter-core crosstalk still has an effect on SM-MCF, which can limit the transmission of the inter-data center. In this paper, the design of four-core uncoupled multicore fiber is discussed for next generation inter-data center networks in order to support the unlimited use of data traffic in the future. The objective of this paper is to determine the appropriate range of core radius and core pitch, which are taken into consideration to reduce the inter-core crosstalk inside the optical fiber. These parameters can be able to improve various constraints to achieve the best multi-core fibers design. From the simulation concerned with the inter-core crosstalk, the experiment results show that the range of core pitch is at 47.5 μm to 50 μm and the range of core radius starts from 4.5 μm to 5.5 μm, that can achieve with crosstalk lower than – 30 dB/100 km for the future inter-data center networks.

Superposition-assisted 125-μm cladding multi-core fibers with ultra-low inter-core crosstalk and high relative core multiplicity factor

We propose a novel superposition-assisted structure (SAS), composed of trench and air holes, in few-mode multi-core fibers (FM-MCFs) to effectively suppress the inter-core crosstalk (XT). The proposed SAS can minimize the inter-core XT to −70.58 dB/100 km in the 7-core fibers, which is 60 dB/100 km lower than the typical trench-assisted structure and 29 dB/100 km lower than the typical hole-assisted structure, respectively. In addition, the SAS does not introduce extra fiber loss and dispersion. In this paper, the key properties of SAS MCFs are numerically simulated using the finite element method (FEM). The large effective area (Aeff) over 90 μm2 enables the suppression of nonlinear effects. The values of dispersion coefficient and differential mode group delay (DMGD) are in the range of 24–42 [ps/(nm·km)] and 5–23 ps/m over C + L band, respectively, which are favorable for fiber communication systems. Such SAS 7-core 5-LP-mode fibers can be further optimized to obtain the cladding diameter of 125 μm, where five propagation modes achieve inter-core XT target below −50 dB/100 km with the bending radius less than 500 mm. Besides, the optimized relative core multiplicity factor (RCMF) can be up to 97.55. The proposed SAS provides a promising solution for the design of FM-MCFs with ultra-low inter-core XT and high capacity for space division multiplexing (SDM).

Tbit/s Multi-Dimensional Multiplexing THz-Over-Fiber for 6G Wireless Communication

Photonics-aided terahertz (THz) wireless systems have been progressively developed to accommodate the forthcoming wireless communication with extremely high data rates in recent years. However, restrained by the obtainable signal-to-noise ratio (SNR) and the dimensions explored in THz photonic wireless systems, achieving data rates of Tbit/s and beyond is still challenging. In this paper, we present a multi-dimensional multiplexing Tbit/s THz-over-Fiber wireless communication system, by efficiently benefiting from the multiplexing gain in both optical wavelength and space domains. Enabled by a combined routine of an optical frequency comb, a low inter-core crosstalk (IC-XT) multi-core fiber and advanced digital signal processing, a line rate of up to 1176 Gbit/s over a wireless distance of 10 m in the 350 GHz band is experimentally demonstrated without any THz amplifiers, resulting in a net data rate of up to 1059 Gbit/s. To the best of our knowledge, this is the first time that beyond Tbit/s wireless data rate is successfully achieved in the THz region above 300 GHz, making a significant contribution to the development of THz-over-Fiber systems for the sixth generation (6G) wireless communication.

Manufacturable low-crosstalk high-RCMF 13-core 5-LP mode fiber with graded-index core and stairway-index trench.

We propose a novel scheme of high doped core and stairway-index trench structure to design a manufacturable graded-index 13-core 5-LP mode fiber with low inter-core crosstalk (ICXT) and large mode differential group delay (MDGD). By using the couple power theory and the finite element method (FEM), the change ICXT of with fiber parameters are investigated. The design of core graded profile and trench structure are optimized to achieve better performance and to meet the fabrication conditions. The numerical result demonstrate that this fiber achieves a low ICXT of lower than -30dB/km (Rb≤500 mm). The bending loss values satisfy the ITU-T recommendation G.655 in 195 µm cladding diameter. Furthermore, the dispersion and the MDGD dependences on wavelength are calculated. The relative core multiplicity factor (RCMF) is obtained as 75.17, which realizes the high density multiplexing. The fabrication methods of this fiber are briefly introduced. The designed fiber may be used for Space-division multiplexing (SDM) system to solve the problem of fiber capacity limitation.

Analysis of intercore crosstalk of WDM channels around zero-dispersion wavelength in homogeneous multicore fibers

We present a numerical analysis of the average intercore crosstalk (IC-XT) of wavelength-division multiplexed (WDM) optical channels in a homogeneous two-core fiber system. This analysis is performed considering cores with zero-dispersion wavelengths at 1550 nm. In the analysis, we consider 11 WDM channels spaced 100 GHz apart transmitted in three different schemes, one centered at 1510 nm with negative dispersion D = − 3.5 ps / nm · km, one centered at 1550 nm with D = 0, and one centered at 1590 nm with D = + 3.5 ps / nm · km. This selection allows for the observation of how the IC-XT of WDM channels is modified using positive, zero, and negative dispersion parameters. To analyze more realistic scenarios of IC-XT in multicore fibers, we considered random bending and twisting perturbations along the fiber. In addition, we considered fiber nonlinearities such as four-wave mixing (FWM) among WDM channels. The results show that FWM produces a power transfer among the transmitted WDM channels that depends on the dispersion parameter D at core 1, and this effect is transferred to the average crosstalk of the WDM channels at core 2. Therefore, the average IC-XT of WDM channels can be modified in a controlled way by selecting an adequate dispersion parameter D in combination with FWM nonlinearity. These results provide valuable information for understanding the wavelength dependence of the average IC-XT of homogeneous multichannel MCF systems working around a zero-dispersion wavelength.

Dynamic service provisioning of hybrid advanced and immediate requests in SDM-EONs

Hybrid immediate request (IR) and advanced reservation (AR) is an inevitable traffic model for future SDM-EONs to accommodate uncertainty and abundance of services. Different from the traditional IR handling instant services like chat messaging, AR can balance the traffic load by postponing allocation to delay-tolerant requests such as cloud computing, e-science, IPTV etc. Although IR/AR mixed traffics are essential in future SDM-EONs, several problems arise especially when solving dynamic RSCA problems. Most importantly, inter-core crosstalk (XT) and resource fairness need to be considered to satisfy high-quality transmission and to avoid unnecessary resource occupation of formerly arriving AR requests.We propose a dynamic HTA-RSCA algorithm that combines time recycling and reuse mechanisms in the time domain with routing, spectrum and core assignment (RSCA) in the spatial-spectrum domain. Specially, pretreatment techniques include super-channel (Sp-Ch) order pre-definition and time-spectrum partition. These appropriate operations are ready for efficient resource searching and requests assignment. Taking transmission quality into consideration, XT-avoid principle is involved in Sp-Ch order pre-definition and dynamic time domain mechanisms to realize the fair and flexible provisioning. XT-awareness is included in the final allocation step to rank available resources options. Simulation results confirm that the dynamic HTA-RSCA can reduce network blocking probability (BP) than comparison algorithms. To the best of our knowledge, this is the first attempt applying time mechanisms to address dynamic provisioning of hybrid IR and AR requests in SDM-EONs.

Analysis of an optical SDM ring network model for applying MIMO processing

Optical multiple-input multiple-output (MIMO) technology is introduced into optical space division multiplexing (SDM) communication as an effective solution for the crosstalk issue. Unfortunately, this application is limited because all the channel information over the transmission link must be transmitted to MIMO receivers for complete crosstalk compensation, which indicates difficulty in applying MIMO to the transmission path via optical switches. We newly propose that one of the solutions to this limitation is a ring network because all channel information can be maintained in the ring. In particular, a typical ring network model comprising optical add-drop multiplexers (OADMs) and multicore fibers (MCFs) is introduced, and the ability of MIMO to compensate for capacity degradation because of the inter-core crosstalk is investigated. Moreover, the protection function, which is essential for the ring network, is also evaluated under the fiber and fiber core failures. As per the simulation results, although the protection operation affects the transmission capacity, the capacity loss is suppressed in the event of a failure to < 5%. We believe that these results provide useful information for designing SDM-based optical networks.

Performance study of randomly coupled erbium-doped fiber amplifier using machine learning

This study describes the performance of a randomly coupled erbium-doped optical fiber amplifier (EOFA) using machine learning. A reinforcement learning-based artificial neural network (RL-ANN) is used to study various parameters of EOFA, such as gain, noise figure, and inter-core crosstalk for the 12-core EOFA. In addition to this, the relationship between mode-dependent loss, a penalty of the EOFA, and the average bending loss (BL) for three operating wavelengths, such as 1550, 1580, and 1600 nm, has been studied. To predict the performance of RL-ANN, three different machine learning algorithms, namely, K-nearest neighbor (KNN), ANN, and support vector machine (SVM) are used. The performance of the proposed RL-ANN is evaluated using five statistical error criteria: mean absolute error, root mean square error, scatter index, coefficient of variation, and overall index measurement. The statistical test result shows that the proposed RL-ANN provides the best performance compared to KNN, ANN, and SVM. The performance of the RL-ANN-based erbium-doped fiber amplifier (RA-EOFA) is studied and the results show that the gain and estimated NF based on a wavelength range from 1555 to 1565 nm provides a maximum gain. A comparison study was carried out for the proposed RA-EOFA with other learning methods. The results show that the proposed RA-EOFA provides the best performance in terms of less BL (0.25 dB), mode-dependent loss (0.5 dB), computational time (36.8 s), and a success rate of 98.39%.

Impact of Classical Modulation Signals on Quantum Key Distribution Over Multicore Fiber

In this paper, the influence of noise induced by classical signals on independent core transmitting quantum key distribution (QKD) is studied over multicore fiber (MCF). We propose a theoretical model of the cascaded noise of spontaneous Raman scattering and inter-core crosstalk (SpRS-ICXT), and the cascaded noise of four-wave mixing and inter-core crosstalk (FWM-ICXT) considering modulation frequency over MCF. To analyze the effect of modulation frequency on QKD, we define and simulate the average power difference and power fluctuation difference of SpRS-ICXT and FWM-ICXT. Then, SpRS-ICXT and FWM-ICXT noises are simulated in two cases: (1) 1 or 3 classical channels and 1 km MCF; (2) 8 or 16 classical channels and 45 km MCF. For the latter case, SpRS-ICXT and FWM-ICXT noises have a serious impact on QKD. Therefore, the impact of noise on QKD is studied. A higher modulation frequency can improve QKD performance, and the secure transmission distance can be further extended by 15.2% when the modulation frequency is 10 GHz relative to ignoring the modulation frequency. However, inter-core crosstalk (ICXT), SpRS-ICXT and FWM-ICXT noises will shorten 52.2% secure transmission distance compared to only transmitting QKD. Finally, the noise of quantum channel is experimentally measured at different symbol rates and modulation formats of classical signals, and it is shown that increasing the symbol rate of data signals at the same optical power degrades the secure key rate of QKD system.

A low crosstalk multi-core few-mode fiber with composite refractive index profile and air-hole embedded trench assistance

In order to reduce the fiber crosstalk for the space division multiplexing (SDM), this paper proposes a novel multi-core few-mode fiber of composite refractive index profile and dual assistance structure. On the one hand, by adopt dual assistance of air-hole embedment and trench wrapping around all the seven cores, inter-core crosstalk is greatly reduced. On the other hand, a non-circularly symmetrical refractive index (RI) profile is innovatively proposed for the six outer-ring cores, which not only weakens the inter-mode coupling but also has a suppression effect on inter-core crosstalk. The finite element method (FEM) is used to analyze the fiber structure. In the case of transmitting six spatial modes, the minimum inter-mode effective RI difference (Δneff) between the linear polarization (LP) modes of the center core and between the spatial modes of the outer-ring core are greater than 0.5 × 10−3, the inter-core crosstalk of the sixth mode LP02 at 1550 nm is less than -66dB/100km, and the relative core multiplexing factor reaches 29.24. The characteristics of this structure are significantly better than that of current commercial optical fibers, promising broad application prospects in large-capacity SDM optical communication system.

Correlation and Power Distribution of Intercore Crosstalk Field Components of Polarization-Coupled Weakly Coupled Single-Mode Multicore Fibres

The correlation and power distribution of intercore crosstalk (ICXT) field components of weakly coupled multicore fibers (WC-MCFs) are important properties that determine the statistics of the ICXT and ultimately impact the performance of WC-MCF optical communication systems. Using intensive numerical simulation of the coupled mode equations describing ICXT of a single-mode WC-MCF with intracore birefringence and linear propagation, we assess the mean, correlation, and power distribution of the four ICXT field components of unmodulated polarization-coupled homogeneous and quasi-homogeneous WC-MCFs with a single interfering core in a wide range of birefringence conditions and power distribution among the field components at the interfering core input. It is shown that, for homogeneous and quasi-homogeneous WC-MCFs, zero mean uncorrelated ICXT field components with similar power levels are observed for birefringence correlation length and birefringence beat length in the ranges of 0.5m,10m and 0.1m,10m, respectively, regardless of the distribution of power between the four field components at the interfering core input.

Effect of pitch on gain-noise figure and crosstalk performance of different core structure Multicore Erbium-Doped Fiber Amplifier: A comparative analysis

A detailed analysis of 7-cores Erbium-Doped Fiber Amplifier has been done to assess the effect of different core and pitch arrangement on the fiber amplifier's performance. The investigation is performed within the C-band communication wavelengths to compare the performance of Y-structure, T-structure, circular and linear structure of the core. The comparison is also extended further by including the performance of a widely used hexagonal structure. A thorough analysis is conducted by examining the profiles of gain, noise figure, and mode of transmission. The inter-core crosstalk (IC-XT) is also calculated for all the core structures and pitch, making this research effort conclusive. Maximum gain has been observed for the Y-shape/structure core arrangement, whereas minimum IC-XT has been observed for the hexagonal structure. However, an encouraging observation is that the percentage decrease in crosstalk due to an increase in pitch is significantly higher for Y-structure than hexagonal. Therefore, the authors believe that this detailed investigation gives significant insight to design efficient multicore EDFA in current research and development.

Polarization-Maintaining Multi-Core Few-Mode Fiber With a Cladding Diameter of 125 μm

We present the theoretical study of polarization-maintaining multi-core few-mode fiber (PM MC-FMF) with a cladding diameter of 125 μm, in order to secure the maximum number of independent spatial channels arising in the single fiber. By taking both the inter-core crosstalk (XT) and the intra-core mode coupling into account, we identify that the PM five-core three-mode fiber is able to provide 19 parallel channels without the need of MIMO signal processing. After the numerical optimizations, the inter-core XT of MC-FMF is less than −30 dB/100km, due to the use of trench-assisted heterogeneous core distribution. Meanwhile, the effective refractive index (RI) differences among both spatial modes and polarization modes are more than 8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> and 3.26 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> over the C+L band, respectively, due to the introduction of stress and geometric birefringence. Moreover, the cladding diameter of such MC-FMF is the same as that of the standard single-mode fiber (SSMF), leading to a convenient upgrading from the SSMF to the MC-FMF.

Stochastic Crosstalk Analyses for Real Weakly Coupled Multicore Fibers Using a Universal Semi-Analytical Model

A novel universal semi-analytical model (USAM) of intercore crosstalk (ICXT) for real weakly coupled multicore fibers (MCFs) is derived based on the coupled mode theory (CMT) with bending and twisting perturbations. An approximated closed-form expression of longitudinal sectional approach is introduced to solve the modified coupled-mode equations (CME) with the random phase shift. Numerical simulations are carried out based on various parametric configurations for the MCF transmission system considered. Our studies indicate that the USAM can be used in the phase-matching region and non-phase-matching region for homogeneous and heterogeneous MCFs. In addition, in the range of tested segment length from 0.0025 m to 0.0380 m, the proposed model can provide a quite accurate and fast simulation results for real weakly coupled MCFs. Therefore, compared with other models based on the CMT, the USAM is more useful for crosstalk estimation in practical MCF and has a wider scope of application.