Multicore Fiber Design Research Articles

Multicore fiber design housing a fluorine-doped low-latency core and cutoff shifted cores

Design of heterogeneous 4-core fiber housing a low-latency core and conventional cutoff shifted cores in a standard 125-μm cladding diameter is investigated to enable signal processing delay reduction using common mode impairment. For the low-latency core, 1-μs propagation delay reduction and an optical signal-to-noise ratio (OSNR) comparable to or greater than that of cutoff shifted cores are required. In this paper, we consider an F-doped core and depressed cladding structure with a large effective area as the low-latency core. It can be expected to achieve sufficient group delay reduction of the low-latency core, the OSNR unification among heterogeneous cores, and low inter-core crosstalk in a standard 125-μm cladding diameter. Consequently, we revealed the optimized low-latency core achieving the group delay reduction of 1 μs and the OSNR as same level as the cutoff shifted cores as we expected. The designed heterogeneous 4-core fiber with the standard 125-μm cladding diameter suppressed the inter-core crosstalk to be low enough to support a 1,000-km long transmission. We expect the figure-of-merit (FoM) of the 4-core fiber to be as high as previously reported 4-core fibers, and the FoM improvement are found when the low latency core and cutoff shifted core are placed alternately thanks to the core heterogeneity.

Low crosstalk heterogeneous sixteen-core four-mode fiber with concave-convex double-type refractive index profiles.

Multi-core few-mode fiber based on space division multiplexing technology is widely regarded as the primary solution to the optical communication capacity issue. However, the quality of the communication signal of the multi-core few-mode fiber depends on the degree of energy coupling between the cores. Thus, we propose a heterogeneous sixteen-core four-mode fiber that achieves low inter-core crosstalk, which first employs a combination of concave and convex double-type refractive index profiles. The advantage of this double-type refractive index profile is that the fiber can more conveniently regulate the phase differences of the same modes in adjacent cores. At the wavelength of 1550 nm and the optimal fiber parameters, all cores of the proposed fiber support four LP modes, and the inter-core crosstalk of all LP modes is less than -72 dB/km. This research demonstrates the efficacy of the proposed fiber in enhancing signal quality and presents innovative ideas and solutions for the future design of various low crosstalk multi-core few-mode fiber.

Design and Analysis of Multicore Fiber with High Bend Tolerance and Uniform Effective Area

Design and Analysis of Multicore Fiber with High Bend Tolerance and Uniform Effective Area

35 Core Polarization-Maintaining Multi-core Fiber for High Power Operation

This work presents a novel rod-type 35 core multi-core fiber design that is capable of overcoming the inherent lack of polarization maintenance in such structures. A polarization extinction ratio of 10.5dB is achieved.

Design and characteristics study of bend-resistant and low-crosstalk few-mode multi-core fiber

This study introduces a novel 9-core, 4-mode fiber characterized by its bend resistance and minimal crosstalk. Utilizing a hetero-core scheme, the cores are effectively isolated through trenches and air holes, significantly reducing inter-core crosstalk. The cores are strategically arranged in a square array to facilitate easier splicing. Through comprehensive simulation calculations, it is demonstrated that four LP modes can be consistently transmitted within the C+L band. The effective refractive index difference between adjacent modes exceeds 10−3, ensuring negligible intra-core crosstalk. Concurrently, the inter-core crosstalk of the LP02 mode is maintained below − 40 dB/100 km. The fiber's bending radius can be minimized to 60 mm without compromising performance, with the corresponding bending loss for the LP02 mode in the two outer cores measured at 1.6 × 10−5 dB/m and 3.95 × 10−6 dB/m, respectively. Each of the nine cores exhibits an effective mode area exceeding 95 µm2, and the fiber's relative core multiplexing factor at 1550 nm is 27.62. The proposed fiber's superior performance positions it as a promising solution for practical applications in optical fiber communication networks, potentially addressing the burgeoning capacity crisis in communication systems.

Investigation of double-cladding heterogeneous step-index 2LP-mode multicore fiber with a two-ring layout

This paper investigates the design of C-band double-cladding 2LP mode multicore fiber (2LP-MCF) with 125-µm cladding diameter by numerical simulation, in which a two-ring layout is used to improve crosstalk and critical bend radius. The numerical results show that the crosstalk can be less than −60 and −35dB/km for 2LP mode eight-core and 10-core fibers, respectively, and it is also shown that the crosstalk can be suppressed compared with that in heterogeneous 2LP-MCFs without a double-cladding layer.

Feasibility studies of multimodal nonlinear endoscopy using multicore fiber bundles for remote scanning from tissue sections to bulk organs

Here, we report on the development and application of a compact multi-core fiber optical probe for multimodal non-linear imaging, combining the label-free modalities of Coherent Anti-Stokes Raman Scattering, Second Harmonic Generation, and Two-Photon Excited Fluorescence. Probes of this multi-core fiber design avoid moving and voltage-carrying parts at the distal end, thus providing promising improved compatibility with clinical requirements over competing implementations. The performance characteristics of the probe are established using thin cryo-sections and artificial targets before the applicability to clinically relevant samples is evaluated using ex vivo bulk human and porcine intestine tissues. After image reconstruction to counteract the data’s inherently pixelated nature, the recorded images show high image quality and morpho-chemical conformity on the tissue level compared to multimodal non-linear images obtained with a laser-scanning microscope using a standard microscope objective. Furthermore, a simple yet effective reconstruction procedure is presented and demonstrated to yield satisfactory results. Finally, a clear pathway for further developments to facilitate a translation of the multimodal fiber probe into real-world clinical evaluation and application is outlined.

Capacity Expansion in Submarine Cable Systems With Bidirectional Transmission Using 4-Core-or-More MCF and MC-EDFA

Bidirectional transmission in a multicore fiber (MCF) reduces core-to-core crosstalk, changing optimal MCF design and multicore submarine cable systems, compared with conventional unidirectional transmission systems. We first demonstrate analytically the increase in capacity with bidirectional transmission in submarine systems. From the perspective of the capacity of one MCF, we show that bidirectional transmission enables an optimum design based on a 6-core MCF with a standard cladding diameter, taking advantage of the reduction of XT. For multicore submarine cable systems, we demonstrate that 6-core MCF systems with bidirectional transmission offer the highest cable capacity for regional Asian systems in the range of 3,000 km. We further discuss a multicore erbium doped fiber amplifier (MC-EDFA) system even for bidirectional transmission to improve the energy efficiency in submarine systems, and we show that 6-core MCF systems offer the highest capacity up to 6,000 km because of MC-EDFA. We also show a 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> larger capacity for a 3,000 km 6-core MCF system, and 43 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> larger capacity for a 9,000 km 4-core MCF system in bidirectional transmission with MC-EDFA, compared with the conventional repeater amplifier in FIFO devices. In addition, we investigated the potential increase in the cable capacity with the future cable system assumption, especially concerning cable supply voltage.

Impact of multicore fiber (MCF) opticals, cross-talk, radiative leakage loss, splice loss and propagation configuration on the system transmission performance

MCF design strongly impacts the fiber optical properties, cross-talk and radiative leakage-loss contributing to increased transmission-loss. A systematic study is presented of the impact of MCF optical properties, cross-talk, transmission-loss and splice-loss on transmission system SNR. Different trench/ moat assisted core-designs having effective area of 80μm2 and 112μm2 are evaluated in 1x2 MCF design for different inter-core distance, with the impact of cross-talk, radiative leakage loss and propagation configuration (bidirectional (counterpropagating)/ copropagating) studied on transmission SNR. Sensitivity of any increased average splice-loss because of multiple cores on the SNR has also been calculated and reported. It is predicted that the maximum SNR for the case of copropagating and bidirectional propagation occurs at an inter-core distance of 48–50μm and 44μm respectively. Even though the absolute maximum SNR is higher for designs with larger effective area, the overall capacity at 80μm2 for copropagating transmission can be larger as it may accommodate 4 uncoupled cores compared to only 2 uncoupled cores for Aeff=112μm2. © 2022 Author(s)

Power-Over-Fiber in Support of 5G NR Fronthaul: Space Division Multiplexing Versus Wavelength Division Multiplexing

We investigatethe potential of space division multiplexing (SDM) for the power-over-fiber (PWoF) technique, in terms of weakly-coupled multicore fiber (WC-MCF) design and experimental verification. Our simulation results indicate that, the SDM-enabled PWoF scheme is superior to the wavelength division multiplexing (WDM) enabled counterpart, due to the scalability of the energy light delivery and the save of wavelength division multiplexers. Then, we experimentally demonstrate the co-transmission of optically carried fifth generation new radio (5G NR) signals at 1550-nm and the 60-W energy light at 1064-nm over the 1-km weakly-coupled seven-core fiber (WC-7CF) fronthaul link. Since the optical power delivery characteristic per the WC-MCF core is similar to that of the standard single-mode fiber (SSMF), the SDM-enabled PWoF scheme can evenly distribute the high-power energy light into multiple spatial channels, to maximize the efficiency of the optical power transition efficiency (OPTE). Consequently, the total collected optical power of the energy light over the 1-km WC-7CF is 4.6 times higher than that of the 1-km SSMF, mainly due to the insertion loss of the fan-in and fan-out (FIFO) devices. Meanwhile, the error-vector magnitude (EVM) of 1.5-Gbit/s 5G NR signal is 0.28%, when the received electrical signal power is −25 dBm. The EVM value of the co-transmitted 5G NR signal with the 60-W energy light only has a fluctuation of 0.01% over six hours. The SDM-enabled PWoF is useful for the optically powered 5G pico-cells, with the capability of high-speed access and centralized management.

Mitigation of thermally-induced performance limitations in coherently-combined multicore fiber amplifiers.

Multicore fiber (MCF) amplifiers have gained increasing interest over the past years and shown their huge potential in first experiments. However, high thermal loads can be expected when operating such an amplifier at its limit. Especially in short MCF amplifiers that are pumped in counter-propagation, this leads to non-uniform mode-shrinking in the cores and, consequently, to a degradation of the system performance. In this work we show different ways to counteract the performance limitations induced by thermal effects in coherently-combined, multicore fiber amplifiers. First, we will show that pumping MCFs in co-propagation will significantly improve the combinable average power since the thermal load at the fiber end is reduced. However, this approach might not be favorable for high energy extraction. Therefore, we will introduce a new MCF design pumped in counter-propagation that leads to a reduction of the thermal load at the fiber end, which will allow for both high combined output power and pulse energy.

Design of M-type core trench-assisted multi-core fiber with a cladding diameter of 200 µm for low-crosstalk long-haul transmission

The design of homogenous trench-assisted multi-core fiber with M-type cores and a cladding diameter of 200 µm is proposed by numerical simulations. The M-type core is introduced to reduce the inter-core crosstalk (XT) of a 19-core fiber to below −30 dB/100 km with a low cost of cutoff wavelength. At the same time, the trade-off between the XT suppression and the effective area of the fundamental mode is mitigated to ensure the effective area is larger than 70 μm2 to suppress the non-linear effects. The study of the effects of the high-index ring of the M-typed core on the XT, effective area, and power fraction shows the potential in improving the fiber performance by a well-designed micron structure of the core. The bending performance shows this design can operate with low loss and crosstalk, and an effective area of over 70 μm2 at a bending radius from 15 mm to 500 mm. The proposed design provides the possibility to improve the MCF performance for long-haul transmission.

Design of ring-core few-mode multi-core fiber with low crosstalk and low bending loss

Aiming at solving the problems of coupling between modes in a core and mode coupling between cores in few-mode multi-core fiber, a fiber with seven cores each with step index is proposed, and each core can support five modes. Each core has a central low refractive index region and a high refractive index ring to ensure that there is a large refractive index difference between modes in the core, so as to reduce the problem of mode coupling. The bending loss of central core and outer core, the crosstalk characteristics between modes and the influence of core parameters on crosstalk performance are simulated and analyzed by the finite element method. The simulation results show that at a wavelength of 1.55 μm and a bending radius of 50 mm, the bending loss of the proposed multi-core fiber is much lower than its attenuation loss, and the crosstalk between the adjacent cores of the five core modes are less than –20 dB/100 km. Therefore, this multi-core fiber can realize independent transmission of the core modes with long-distance under small bending radius.

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.

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.

Analytical expression for mode-coupling coefficient between non-identical step-index cores and its application to multi-core fiber design within 125-[formula omitted]m cladding diameter

Analytical expression is derived for the mode-coupling coefficient between non-identical step-index (SI) cores, which enables a quick estimate of the crosstalk in the heterogeneous SI multi-core fibers (Hetero-SI-MCFs). The analytical results are in good agreement with the results obtained by numerical simulations by finite-element method (FEM). Using the derived analytical expression, the feasibility of Hetero-SI-MCF design within standard 125-μm cladding diameter is discussed. The designed MCFs have potential applications in space division multiplexing (SDM) systems.

Design and analysis of trench-assisted dual-mode multi-core fiber with large-mode-field-area.

A novel trench-assisted dual-mode multi-core fiber with large-mode-field-area is proposed. The structure consists of 17 conventional cores and two air holes according to a regular hexagon, which can realize strict dual-mode transmission. The structural parameters' effect on mode transmission characteristics, mode-field-area, and bending loss are analyzed systematically. By optimizing the structural parameters, the mode-field-area of the fundamental mode can reach ${2100.619}\;{{\unicode{x00B5}{\rm m}}^2}$. The introduction of the trench with a lower refractive index than cladding can reduce the bending loss to ${9.88} \times {{10}^{- 4}}\;{\rm dB}/{\rm m}$ when the bending radius is 2.3 cm. Besides, the structural design is flexible, and the manufacturing process is simple, which has broad application prospects.

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.

Design and Applicability of Multi-Core Fibers With Standard Cladding Diameter

We show the design and applicability of multi-core fiber (MCF) with the standard 125 μm cladding diameter to the telecommunication systems. It offers easier and practical use of the space-division multiplexing (SDM) technology given its excellent fiber productivity and utilization of existing standard technologies. We numerically and experimentally reveal that the simple step-index (SI) profile enables us to allocate four cores in the standard 125 μm cladding and to realize full compliance with G.657.A1 fiber. We then expand the design and concept of the standard cladding MCF and show the index profile and single-mode bandwidth offer control of XT characteristics. We elucidate the application scope of the standard cladding MCF for not only the full-band (O-L band) application to short-reach and terrestrial transmission consistent with ITU-T Recommendations G.652 and G.657, but also ultra-long haul transmission, such as submarine systems compliant with ITU-T Recommendation G.654.