Multicore Fiber Research Articles

Design of a multi-core photonic crystal fiber supporting 518 OAM modes

A multi-ring core photonic crystal fiber (PCF) with seven high-index ring regions is designed for the transmission of multiple orbital angular momentum (OAM) modes. The new structure which has an optimized air hole arrangement shows improved transmission quantity and number of OAM modes. Numerical analysis reveals that the PCF can transmit 518 OAM modes in the range of 1.4–1.75 µm stably while avoiding cross talk from the adjacent ring core resulting in a low cross talk of merely −208.5dB, confinement loss less than 10−9dB/m, as well as mode quality greater than 0.925. The results disclose a new method to increase the communication capacity and suggest that the PCF has great application potential.

Incremental shape measurement with fiber Bragg gratings in a multi-core fiber for three-dimensional paths.

A multi-core fiber (MCF) provides a compact solution for three-dimensional (3D) shape measurement. In this Letter, an incremental shape measurement method for 3D paths is proposed, using an MCF based on fiber Bragg gratings (FBGs). A few FBG sets can iteratively provide plenty of strain information about the 3D path during navigation. The overall continuities of the curvature and torsion are improved based on intensive strain calculations. Meanwhile, the transformation matrix algorithm is used to reconstruct the shape of 3D paths. Dynamic measurement experiments of a seven-core fiber with two FBG sets are carried out to verify the incremental shape measurement method. This method shows a great performance of the different paths, with a maximum incremental position error of 4.68%.

In-line open-cavity Mach-Zehnder interferometric refractive-index sensors based on inter-mode and inter-core-mode interferences

We experimentally demonstrated in-line Mach-Zehnder interferometric sensors based on inter-mode interference (IMI) and inter-core-mode interference (ICMI) for refractive index (RI) measurements. The structure of the IMI-based sensor consisted of a C-shaped fiber spliced between two multi-mode fibers (MMF) with the opposite ends spliced to lead-in and lead-out single-mode optical fibers. The ICMI-based sensor has a similar structure as the IMI-based sensor but with an additional in-house fabricated three-core multi-core fiber (MCF) spliced between the MMF and C-shaped fiber. The performance of the two RI sensors was evaluated and compared using water-glycerol solutions with RI values ranging from 1.333 to 1.338. The RI sensitivity, as well as other characteristic properties such as free spectral range and extinction ratio in relation to three lengths of the C-shaped fiber of 2, 2.5 and 3 mm, were investigated in both structures. The open-cavity characteristic of the C-shaped fiber allows the solution to interact with the optical path of the sensors, hence improving their RI measurement sensitivities. Both types of sensors exhibit linear wavelength responses with variation in RI. The RI sensitivity of the IMI-based sensor exhibits minor variations when the length of the C-shaped fiber is altered, with a maximum sensitivity of -7999.76 nm/RIU. In contrast, the RI sensitivity of ICMI-based sensor exhibits variability when changing the C-shaped fiber length, potentially due to variations in the RI distribution among the cores within the MCF, with its maximum sensitivity being -6211.98 nm/RIU. The temperature sensitivity of all the tested sensors was around 0.7 nm/ °C, denoting a low temperature cross-sensitivity of -7.427 × 10−5 RIU/ °C. This work experimentally exhibits a comparison of two sensing effects in the same configuration. The long length (longer than 2 mm) of the sensing element facilitates the fabrication process, and the open-cavity present in the proposed sensors yields a high sensitivity, which is in the same order of magnitude as the IMI-based sensors and is one order of magnitude higher than the ICMI-based sensors, emphasizing their promising candidacy for label-free optical sensing of chemical and biological samples.

Randomly coupled trench-assisted multicore fibers with different arrangements for high tolerance of manufacturing errors

Randomly coupled trench-assisted multicore fibers with different arrangements for high tolerance of manufacturing errors

End-coupled cladding pumping using a dual helical lens for a 32-core doped fiber amplifier with two annular cores arrangement

We have designed a dual helical lens (DHL) to produce two rings of light intensities for end-coupled cladding pumping of a multi-core doped fiber (MCDF) amplifier. The MCDF is a 32-core doped fiber, and the cores are arranged in two annular forms of 20 and 12 cores. The DHL has been designed by combining two helical lenses with different topological charges. The angular spectrum (AS) method has been used to calculate the propagation of optical waves behind the DHL. The results show that the power efficiency and uniformity have improved compared to the direct cladding pumping conditions.

Design of Single-Mode Single-Polarization Large-Mode-Area Multicore Fibers.

In laser science and industry, considerable effort is directed toward designing fibers for fiber laser and fiber amplifier applications, each of which offers a particular advantage over the others. Evanescently coupled multicore fibers, however, have been studied less extensively due to the relatively small mode area in the single-mode regime. Here, by proposing a new structure with stress-applying parts in a 37-core fiber and optimizing this structure through a comprehensive framework, we present 21 solutions characterized by large-mode-area and high beam quality in the single-mode, single-polarization regime. Different fiber designs are optimal for different output parameters. In one design, the mode area can significantly increase to above 880 μm2, which is comparable with that of photonic-crystal fibers. Moreover, besides the single-mode operation, the beam quality factor (M2 factor) of the fundamental mode is considered an output parameter in the bent state and is improved up to 1.05 in another design. A comprehensive tolerance analysis is then performed to assess the performance of the designs under deviations from normal conditions. Moreover, in spite of the shifts in the loss of modes, the proposed high beam quality LMA fibers maintain single-polarization, single-mode operation across a wide range of core pitches, bending orientation angles, and bending radius deviations. Our results highlight the potential of multicore fibers for the efficient operation of fiber lasers and amplifiers.

Migration of elastic optical networks to the C + L-bands subject to a partial upgrade of the number of erbium-doped fiber amplifiers

Space division multiplexing (SDM) and band division multiplexing (BDM) are considered promising technologies to increase the capacity of optical transport networks. The progressive shortage of available dark fibers and the immaturity of multicore and multimode fibers for multichannel transmission induce network operators to postpone the process of capacity enhancement through SDM. Therefore, capacity increase revolves around BDM by lighting up at least the L-band of the already installed optical fiber infrastructure, which is a practical solution in the short to middle term. However, L-band activation requires the upgrade of network components such as erbium-doped fiber amplifiers (EDFAs). To manage the imposed cost while leveraging the L-band, a network can be partially rather than fully migrated in a single step by upgrading just a subset of the fibers and thus a subset of EDFAs to operate in the C+L-bands. In this paper, the focus is set on determining which fibers in the network should be upgraded to exploit the L-band, subject to a constraint on the maximum number of EDFAs to be upgraded, and analyzing its impact on network performance when facing dynamic traffic in terms of the blocking ratio. To this end, three heuristic algorithms, each pursuing a different objective, and two of them based on an integer linear programming (ILP) formulation, are proposed for the network planning to identify which fibers to upgrade. Simulation results demonstrate that, thanks to the use of these heuristics, the upgrade of a partial set of links to the C+L line system is a viable solution for network operators to circumvent the huge cost associated with migrating the full network. For instance, we demonstrate that a strategic partial upgrade using the proposed methods, subject to upgrading a maximum of 60% of the EDFAs, can significantly boost the supported traffic load in the examined topologies, ranging from 175% to 322%, when compared to the non-upgraded network.

Effect of in-source beam shaping and laser beam oscillation on the electromechanical properties of Ni-plated steel joints for e-vehicle battery manufacturing

Laser welding is a key enabling technology that transitions toward electric mobility, producing joints with elevated electrical and mechanical properties. In the production of battery packs, cells to busbar connections are challenging due to strict tolerances and zero-fault policy. Hence, it is of great interest to investigate how beam shaping techniques may be exploited to enhance the electromechanical properties as well as to improve material processability. Industrial laser systems often provide the possibility to oscillate dynamically the beam or redistribute the power in multicore fibers. Although contemporary equipment enables elevated flexibility in terms of power redistribution, further studies are required to indicate the most adequate solution for the production of high performance batteries. Within the present investigation, both in-source beam shaping and beam oscillation techniques have been exploited to perform 0.2–0.2 mm Ni-plated steel welds in lap joint configuration, representative of typical cell to busbar connections. An experimental campaign allowed us to define process feasibility conditions where partial penetration welds could be achieved by means of in-source beam shaping. Hence, beam oscillation was explored to perform the connections. In the subset of feasible conditions, the mechanical strength was determined via tensile tests alongside electrical resistance measurements. Linear welds with a Gaussian beam profile enabled joints with the highest productivity at constant electromechanical properties. Spatter formation due to keyhole instabilities could be avoided by redistributing the emission power via multicore fibers, while dynamic oscillation did not provide significant benefits.

Research on the Low-loss Low-crosstalk Bending-insensitive Seven-core Fiber

Multicore fiber is regarded as the most efficient way to realize SDM networks. Fabrication of low-loss and low-crosstalk multi-core fiber is the key to achieving high-capacity communication systems. This paper designed and fabricated low transmission loss seven-core single-mode fiber with a standard 125μm cladding diameter based on modified chemical vapor deposition. The preform of MCF is fabricated using drilling method. The seven-core fiber has achieved a low transmission loss of 0.19~0.28dB/km throughout the 1250-1600nm wavelength range. The transmission loss is below 0.19dB/km at 1550nm wavelength. The crosstalk is below -54dB/100km from 1200nm to 1600nm wavelength by utilizing a trench-assisted structure. At 1550nm wavelength, the bending loss (BL) is lower than 0.45dB/m with a bending radius of 5 mm, and the BL is 0.05dB/m with a bending radius of 15 mm. The fabricated multi-core fiber can be used for long-haul transmission and space division multiplexing communication systems.

Impact of Zero-Dispersion Wavelength Fluctuations in a Coupled Dual-Core Fiber Optical Parametric Amplifier

We have studied the impact of zero dispersion wavelength (ZDW) fluctuations in coupled dual-core fiber optical parametric amplifiers (FOPAs) and compared it to the single-core single-pump FOPA. We have designed a Monte Carlo model, that simulates a multidimensional Ornstein-Uhlenbeck process, describing accurately ZDW fluctuations in single and multicore fibers. In order to validate our results we have developed an analytical model, based on Taylor expansion of the average gain and successfully compared both models, with experimental results published in the literature. We have found that a significant reduction of the impact of ZDW fluctuations can be obtained in dual-core FOPAs compared to single-core ones.

Dynamics of the Self-Action of Wave Fields in Multicore Fibers

Dynamics of the Self-Action of Wave Fields in Multicore Fibers

Design and analysis of ultra-large capacity heterogeneous nineteen-core seven-mode fiber

The multi-core few-mode fiber based on space division multiplexing technology is one of the critical ways to address the issue that the current single-mode fiber communication capacity has reached its maximum. However, the energy crosstalk between the cores is an important reason for the deterioration of signal quality. Consequently, this paper designs a nineteen-core seven-mode fiber with more transmission channels, and the auxiliary structures such as trench, high and low refractive index double rings, and air holes can effectively reduce intra-core crosstalk and inter-core crosstalk. After optimization, each core of the nineteen-core fiber can transmit seven LP modes, and the effective refractive index difference between adjacent modes is greater than or equal to 10−3. The optical fiber provides 228 transmission channels for the entire transmission system, the inter-core crosstalk of the highest order LP41 mode is lower than −30 dB/100 km at 1550 nm. Based on the above, the proposed optical fiber has excellent optical properties and can be applied to data centers, optical sensors, high-speed computer communication networks, and other applications.

Loss Imbalance Effect on Inter-Core Crosstalk in Bidirectional Uncoupled Multi-Core Fiber Transmission

The effect of core loss imbalance on inter-core crosstalk (XT) is studied theoretically and numerically in long-haul bidirectional uncoupled multi-core fiber transmission. The coupled power equations are solved for two cores with imbalance of loss coefficients, and the solution is used to obtain expressions for direct, indirect, backscattered and bidirectional XT powers. These expressions generalize expressions reported by other authors to arbitrary loss coefficients. It is shown that, in the range of the loss coefficient imbalance from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-0.04$</tex-math></inline-formula> dB/km to 0.04 dB/km, the XT powers in decibel show approximately a linear dependence on the loss coefficient imbalance. In that range, differences of the indirect XT power of nearly 1.2 dB relative to the indirect XT power with equal loss coefficients can occur for the larger loss coefficient imbalance; for the direct and backscattered XT powers, the differences are even larger, reaching around 2 dB. These values of 1.2 and 2 dB of difference translate directly to the bidirectional XT power of long-haul 2-core fiber links, in case of dominance of indirect and backscattered XT, respectively.

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.

Evaluation of Crosstalk in Nonlinear Regime for Weakly Coupled Multi-Core Fiber With Random Perturbations Using a Split-Step Numerical Model

Evaluation of Crosstalk in Nonlinear Regime for Weakly Coupled Multi-Core Fiber With Random Perturbations Using a Split-Step Numerical Model

Multi-core fiber rotated optical switch

We propose an optical switch technique that enables unused optical fiber to be activated without on-site work for the purpose of efficiently supporting future optical communication services. The proposed technique performs fiber switching by rotating a multi-core fiber (MCF) by a single motor. The MCF is inserted into a cylindrical ferrule to avoid fiber damage, when rotated against another fiber. The two ferrules are inserted into a sleeve so that their endfaces oppose each other, and the cores are linked across the gap. We optimize the sleeve size not only to reduce the friction between the ferrule and the sleeve, which can create excessive motor power consumption, but also to reduce the loss. We fabricate a 2.5-mm diameter cylindrical ferrule attached a homogeneous MCF with a hexagonal seven-core structure and experimentally evaluate the loss characteristics during rotation of the MCF for core switching. We clarify the potential of the resulting optical switch.

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.

A Novel Catheter Shape-Sensing Method Based on Deep Learning with a Multi-Core Optical Fiber.

In this paper, we propose a novel shape-sensing method based on deep learning with a multi-core optical fiber for the accurate shape-sensing of catheters and guidewires. Firstly, we designed a catheter with embedded multi-core fiber containing three sensing outer cores and one temperature compensation middle core. Then, we analyzed the relationship between the central wavelength shift, the curvature of the multi-core Fiber Bragg Grating (FBG), and temperature compensation methods to establish a Particle Swarm Optimization (PSO) BP neural network-based catheter shape sensing method. Finally, experiments were conducted in both constant and variable temperature environments to validate the method. The average and maximum distance errors of the PSO-BP neural network were 0.57 and 1.33 mm, respectively, under constant temperature conditions, and 0.36 and 0.96 mm, respectively, under variable temperature conditions. This well-sensed catheter shape demonstrates the effectiveness of the shape-sensing method proposed in this paper and its potential applications in real surgical catheters and guidewire.

High-speed chaos-based secure optical communications over 130-km multi-core fiber.

Chaotic optical communication is of great significance for secure data transmission. Despite rapid development over the decades, high-speed (>100 Gbps) and long-distance (>100 km) chaotic optical communication in a single fiber is still full of challenges. Here, we propose and experimentally demonstrate high-speed and long-distance chaos-based secure optical communications using mutual injection of semiconductor lasers and space-division multiplexing (SDM) techniques. The encrypted signals are transmitted through all seven core channels of the multi-core fiber (MCF), which effectively expands the aggregate transmission capacity of a single fiber. A pair of source and synchronization devices based on mutual injection of semiconductor lasers are employed to effectively encrypt and decrypt signals. Chaos-based secure optical communications with 70-Gbps on-off keying (OOK) and 140-Gbps quadrature phase-shift keying (QPSK) signals over a 130-km MCF are successfully demonstrated in the experiment with favorable performance. The demonstration may pave the way for future ultrahigh capacity and ultra-long distance chaotic optical communications by fully exploiting multi-dimensional resources of light waves, including the spatial dimension.

Analysis of optical core-to-core coupling: challenges and opportunities in multicore fiber amplifiers.

In this work we study in detail core-to-core coupling effects in multicore fibers (MCFs) using a simulation tool based on supermodal interference. We pay particular attention to the impact of core area scaling, which plays an important role in prospective amplifier systems. We consider geometrical and optical properties of the MCF structure, including the ability for dense packaging of the cores but also the influence on the core guidance (V-parameter). In general, this study is important to unlock the power and energy scaling potential of the next-generation MCF amplifiers.