Heterogeneous Multicore Fiber Research Articles

Impact analysis of core layout and crosstalk on the coexistence of quantum key distribution and classical communications in heterogeneous multicore fiber

Heterogeneous multicore fibers (Hetero-MCFs) offer relatively lower inter-core crosstalk (XT) and improved bend resistance, making them feasible for the coexistence of quantum key distribution (QKD) and classical communications. However, detailed analyses of their impact on QKD performance remain limited. This study comprehensively investigates the significant effects of core layouts and XT on QKD performance in Hetero-MCFs under varying QKD service demands in theory. When allocating classical and quantum cores based on the maximum XT levels, hexagonal Hetero-MCFs perform the worst under low QKD service demand (LD), but comparably and superior to circular layouts under high QKD service demand (HD). The challenge of determining optimal parameters for the central core in hexagonal Hetero-MCFs critically influences overall QKD performance in both HD and LD scenarios. To address this, we propose a novel trench-assisted Hetero-MCF structure featuring an interleaved hexagonal core layout. This structure achieves at least 29-fold improvement in transmission distance for LD and 32-fold for HD scenarios compared to the benchmark, by utilizing two types of heterogeneous cores and providing greater flexibility. Our results also show that deploying homogeneous cores as quantum channels in Hetero-MCFs enhances QKD performance. Even with strict fabrication tolerances, the proposed structure maintains stable QKD performance with a maximum fluctuation of only 12%. These innovations provide valuable insights for analyzing the performance of Hetero-MCFs under varying QKD demands and further developing their co-fiber transmission applications.

Random Polarization-Mode Coupling Effects in Heterogeneous Multi-Core Fibers with Different Cladding Diameters

Random Polarization-Mode Coupling Effects in Heterogeneous Multi-Core Fibers with Different Cladding Diameters

Photonic-Assisted Analog-to-Digital Conversion Based on a Dispersion-Diversity Multicore Fiber

Multigigabit-per-second photonic-assisted timeinterleaved analog-to-digital conversion (ADC) built upon a dispersion-diversity multicore fiber (MCF) is proposed and experimentally demonstrated for the first time to our knowledge. Tunable true-time delay line operation for the analog signal replicas is provided by the different cores of a heterogeneous MCF, which feature different radial dimensions and GeO2 dopant concentrations. Continuous real-time ADC tunability with the optical wavelength is experimentally demonstrated by time interleaving 5 digitized channels achieving equivalent sampling rates of 25, 50 and 125 GS/s for optical wavelengths of 1538, 1534 and 1531.6 nm, respectively. These values are beyond the 10.25-GS/s sampling rates offered by commercial electronic ADCs. One of the advantages of using a MCF for the parallel channels relies on the fact that all the cavities are subject to similar environmental conditions. The introduction of space and dispersion diversities provided by MCFs in photonic-assisted ADCs brings benefits in terms of size, weight, and power consumption reduction along with system flexibility and stability improvement. This is of particular relevance in those application scenarios where an optical fiber link is required to distribute the signal, such as fiber distributed sensing networks, as well as access networks for radar systems or next-generation wireless communications.

Crosstalk-Avoid Virtual Optical Network Embedding Over Elastic Optical Networks With Heterogeneous Multi-Core Fibers

With the advent of the era of the 5th generation fixed networks (F5G), some bandwidth-hungry services such as 4K/8K video, telemedicine, and virtual reality (VR) gaming are emerging, which will bring more challenges to the optical network. Typically, both flexible bandwidth allocation and dynamic services deployment will encounter a bottleneck when huge volumes of access data are flooded. To break it, virtual optical network embedding (VONE) over the space division multiplexing (SDM) based elastic optical network (EON) using multi-core fiber (MCF) is adopted. However, the problem of inter-core crosstalk (IC-XT) between two adjacent cores in an MCF link has not been solved. In this paper, we first design a heterogeneous MCF (HMCF) structure. The IC-XT can be significantly reduced when virtual networks are embedded over EONs with HMCF. Then some integer linear programming (ILP) models are established to formulate the process of VONE over EONs with HMCF. Next, to avoid the high time complexity of ILP, we propose a novel VONE algorithm supporting joint IC-XT-avoid cores and frequency slots assignment in EONs with HMCF. Finally, simulation results demonstrate that the proposed algorithm can effectively improve 72% of the available frequency ratio (AFR) and reduce 35% of the fragmentation ratio (FR) compared with conventional VONE over MCF-based networks without any IC-XT reducing strategy, which squeezes more bandwidth resources.

Dispersion-Diversity Multicore Fiber Signal Processing.

Beyond playing a primary role in high-capacity communication networks, multicore optical fibers can bring many advantages to optical and microwave signal processing, as not only space but also chromatic dispersion are introduced as new degrees of freedom. The key lies in developing radically new multicore fibers where the refractive index profile of each individual core is tailored properly to provide parallel dispersion-diversity signal processing with application in a variety of scenarios such as parallel channel equalization, analogue-to-digital conversion, optical computing, pulse generation and shaping, multiparameter fiber sensing, medical imaging, optical coherence tomography, broadband measurement instrumentation, and next-generation fiber-wireless communications. Here, we experimentally prove, for the first time to our knowledge, reconfigurable two-dimensional dispersion-managed signal processing performed by a novel dispersion-diversity heterogeneous multicore fiber. The fiber comprises seven different trench-assisted cores featuring a different refractive index profile in terms of both radial geometry and core dopant concentration. As a representative application case, we demonstrate reconfigurable microwave signal filtering with increased compactness as well as performance flexibility and versatility as compared to previous technologies.

Heterogeneous multicore fiber-based microwave frequency measurement.

A novel microwave frequency measurement scheme using a heterogeneous multicore fiber (MCF) is experimentally demonstrated. The inherently different relative group delays among the cores of a heterogeneous 7-core MCF are used to realize two individual 2-tap microwave filters with different free spectral ranges (FSRs). The ratio of the frequency response traces of these two filters is used to establish an amplitude comparison function (ACF). Furthermore, by varying the operational wavelength, the relative group delays between the cores and consequently the FSRs of the filters are tuned and different ACF curves are obtained. The complementary information provided by these different ACFs allows us to estimate the unknown frequency with an improved accuracy, over a broad measurement range. In our experiments, a measurement error of ±71 MHz is achieved over a frequency range of 0.5-40 GHz. The proposed scheme offers flexibility and compactness, thanks to the parallelism provided by the MCF.

A Novel Core Allocation in Heterogeneous Step-Index Multi-Core Fibers With Standard Cladding Diameter

We numerically reveal the feasibility of designing multi-core fibers (MCFs) with the simple step-index (SI) profile within the standard 125- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m cladding diameter. We show that the standard cladding diameter enables us to incorporate five identical cores for C-band operation, and the number of cores can be further increased with the heterogeneous core arrangement. We then propose a novel method of allocating the non-identical cores in heterogeneous MCFs (Hetero-MCFs), where the crosstalk (XT) and critical bending radius ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pk</sub> ) are improved comparing to that of Hetero-MCFs designed by the conventional method. These new types of step-index MCFs (SI-MCFs) with standard cladding diameter have potential application in the space-division multiplexing (SDM) systems.

Design analysis for ultra-low crosstalk in high core count heterogeneous multicore fiber

Design analysis for ultra-low crosstalk in high core count heterogeneous multicore fiber

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.

Design and analysis of uncoupled heterogeneous trench-assisted multi-core fiber (MCF)

Abstract In this paper, author has presented design strategies for two core uncoupled heterogeneous multi-core fiber (MCF) to achieve minimum crosstalk. Authors have studied the variation of crosstalk between the cores in respect to the MCF parameters like bending radius, wavelength, core position and fiber length, so that single mode propagation can be made through designed uncoupled heterogeneous cores. The other important performance parameters of MCF i.e. group delay and dispersion have also been discussed in the paper.

Comparative crosstalk performance analysis of different configurations of heterogeneous multicore fiber

Abstract In order to select the heterogeneous multicore fiber (MCF) configuration with ultra-low crosstalk and low peak bending radius, comparative crosstalk analysis have been done for the three possible core configurations, namely, Configuration 1 - different refractive index (R.I.) and different radius, Configuration 2 - different R.I., and Configuration 3 - different radius. Using the coupled mode equation and the simplified expressions of mode coupling coefficient (MCC) for different configurations of heterogeneous cores, the crosstalk performance of all the heterogeneous MCF configurations along with the homogeneous MCF have been investigated analytically with respect to core pitch (D) and fiber bending radius ( R b ${R}_{b}$ ). Further, these expressions of MCC have been extended to obtain the simplified expressions of MCC for the estimation of crosstalk levels in respective trench-assisted (TA) heterogeneous MCF configurations. It is observed from the analysis that in Configuration 1, crosstalk level is lowest and the rate of decrease in the crosstalk with respect to the core pitch is highest compared to the other configurations of heterogeneous MCF. The values of crosstalk obtained analytically have been validated by comparing it with the values obtained from finite element method (FEM) based numerical simulation results. Further, we have investigated the impact of a fixed percent change (5%) in the core parameters (radius and/or R.I.) of one of the core of a homogeneous MCF, to realize the different heterogeneous MCF configurations, on the variations in crosstalk levels, difference in the mode effective refractive index of the core 1 and core 2 ( Δ n e f f = n e f f 1 − n e f f 2 $\Delta {n}_{eff}={n}_{eff1}-{n}_{eff2}$ ), and the peak bending radius ( R p k ${R}_{pk}$ ). For the same percent variations (5%) in the core parameters (radius and/or R.I.) of different configurations of cores (Config. 1-Config. 3), Config. 1 MCF has highest variation in Δ n e f f $\Delta {n}_{eff}$ value compared to other configurations of MCF. Further, this highest variation in Δ n e f f $\Delta {n}_{eff}$ value of Config. 1 MCF results in smallest peak bending radius. The smaller value of peak bending radius allows MCF to bend into smaller radius. Therefore, Configuration 1 is the potential choice for the design of MCF with smaller peak bending radius and ultra-low crosstalk level compared to the other configurations of SI-heterogeneous MCF.

Telecommunication Compatibility Evaluation for Co-existing Quantum Key Distribution in Homogenous Multicore Fiber

Quantum key distribution (QKD) is regarded as an alternative to traditional cryptography methods for securing data communication by quantum mechanics rather than computational complexity. Towards the massive deployment of QKD, embedding it with the telecommunication system is crucially important. Homogenous optical multi-core fibers (MCFs) compatible with spatial division multiplexing (SDM) are essential components for the next-generation optical communication infrastructure, which provides a big potential for co-existence of optical telecommunication systems and QKD. However, the QKD channel is extremely vulnerable due to the fact that the quantum states can be annihilated by noise during signal propagation. Thus, investigation of telecom compatibility for QKD co-existing with high-speed classical communication in SDM transmission media is needed. In this paper, we present analytical models of the noise sources in QKD links over heterogeneous MCFs. Spontaneous Raman scattering and inter-core crosstalk are experimentally characterized over spans of MCFs with different refractive index profiles, emulating shared telecom traffic conditions. Lower bounds for the secret key rates and quantum bit error rate (QBER) due to different core/wavelength allocation are obtained to validate intra- and inter-core co-existence of QKD and classical telecommunication.

Worst crosstalk analysis in heterogeneous and trench assisted heterogeneous multicore fiber for different core counts and layouts

In order to obtain appreciably low worst crosstalk level in high core count multicore fiber, heterogeneous MCF with different core configurations and layouts have been investigated analytically using coupled mode equation. The variations in worst crosstalk behavior with respect to the bending radius, core configurations, core layouts, and outer cladding thickness have been analyzed. Further, the core count dependent variations in worst crosstalk level, core pitch, and peak bending radius in different core layouts have been presented. It has been observed that the selection of core configuration, core layout, and outer cladding thickness have significant impact on the reduction in worst crosstalk level. For example, in 12-core normal-index heterogeneous MCF with SLS structure, the core Configuration 1 can provide the worst crosstalk level reduced by 7.09 dB and 16.92 dB in comparison to the other two core configurations, i.e., Configuration 2, and Configuration 3, respectively. Further, for the same core count with TA Configuration 1, SLS core layout can achieve the reduced worst crosstalk level in comparison with circular ORS and DRS, hexagonal ORS and DRS respectively by 24.62 dB, 25.86 dB, 28.87 dB and 30.64 dB.

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.

Microwave Signal Processing over Multicore Fiber

We review the introduction of the space dimension into fiber-based technologies to implement compact and versatile signal processing solutions for microwave and millimeter wave signals. Built upon multicore fiber links and devices, this approach allows the realization of fiber-distributed signal processing in the context of fiber-wireless communications, providing both radiofrequency access distribution and signal processing in the same fiber medium. We present different space-division multiplexing architectures to implement tunable true time delay lines that can be applied to a variety of microwave photonics functionalities, such as signal filtering, radio beamsteering in phased array antennas or optoelectronic oscillation. In particular, this paper gathers our latest work on the following multicore fiber technologies: dispersion-engineered heterogeneous multicore fiber links for distributed tunable true time delay line operation; multicavity devices built upon the selective inscription of gratings in homogenous multicore fibers for compact true time delay line operation; and multicavity optoelectronic oscillation over both homogeneous and heterogeneous multicore fibers.

Crosstalk Analysis of Heterogeneous Multicore Fibers Using Coupled-Mode Theory

Intercore crosstalk of heterogeneous multicore fiber is investigated based on coupled-mode theory. Random twisting model is used for estimating the crosstalk. The crosstalk of two kinds of fibers: triangular lattice 30-core fiber with four kinds of cores and square lattice 32-core fiber with two kinds of cores is investigated both theoretically and experimentally. Unlike previous study, measured crosstalk for all the combinations of cores for both fibers is in good agreement with calculated values with single correlation length, showing the validity of the theoretical model used here.

Highly sensitive strain sensor based on helical structure combined with Mach-Zehnder interferometer in multicore fiber

Optical fiber sensors for strain measurement have been playing important roles in structural health monitoring for buildings, tunnels, pipelines, aircrafts, and so on. A highly sensitive strain sensor based on helical structures (HSs) assisted Mach-Zehnder interference in an all-solid heterogeneous multicore fiber (MCF) is proposed and experimentally demonstrated. Due to the HSs, a maximum strain sensitivity as high as −61.8 pm/με was experimentally achieved. This is the highest sensitivity among interferometer-based strain sensors reported so far, to the best of our knowledge. Moreover, the proposed sensor has the ability to discriminate axial strain and temperature, and offers several advantages such as repeatability of fabrication, robust structure and compact size, which further benefits its practical sensing applications.

Simulation of Heterogeneous Few Mode Multi-Core Fiber for Capacity Enhancement

Simulation of Heterogeneous Few Mode Multi-Core Fiber for Capacity Enhancement

Spatial-division multiplexed Brillouin distributed sensing based on a heterogeneous multicore fiber

We have experimentally investigated spatial-division multiplexed (SDM) Brillouin optical time-domain analysis in a heterogeneous multicore fiber whose central core and six outer cores are made from different preforms, showing a ∼70 MHz Brillouin frequency shift (BFS) difference between them. It reveals that the heterogeneous central core and the outer cores have different temperature sensitivities, but their strain sensitivities are almost the same. By making use of the distinct temperature coefficients of these two kinds of cores, simultaneous and discriminative temperature and strain measurements are achieved. The bending-induced Brillouin gain spectrum (BGS) broadening issue in off-center cores has been clarified, and a solution has been proposed to eliminate the uncertainty caused by a bending-induced BFS shift, by averaging the BFS variations of two symmetrical outer cores. We show a new perspective for discriminative measurement in Brillouin distributed sensors based on SDM solutions.

Crosstalk in rectangular cross-section heterogeneous multicore fiber

Using neighboring cores with different mode propagation constants (indexes) is a well-known way to reduce crosstalk in multicore fiber (MCF). However, in actual field-deployed fiber, random bends can cause a reduction in the difference between the mode indexes of neighboring cores, which consequently increases crosstalk. The level of crosstalk induced by bending in both rectangular cross-section and circular cross-section heterogeneous MCF with cores arranged in a line was investigated. The experimental results obtained indicate that in contrast to circular cross-section MCF, no bending-induced crosstalk occurs in rectangular cross-section MCF wound on the mandrel without special control of cross-section orientation. Thus, to eliminate undesirable bending-induced crosstalk in heterogeneous MCF a rectangular cross-section should be employed.