Few-mode Multi-core Fiber Research Articles

Inter-Mode Crosstalk Estimation between Cores for LPmn Modes in Weakly Coupled Few-Mode Multicore Fiber with Perturbations.

A novel inter-mode crosstalk (IMXT) model of LPmn mode for weakly coupled few-mode multicore fiber is proposed based on the coupled mode theory (CMT) with bending and twisting perturbations. A universal expression of the mode coupling coefficient (MCC) between LPmn modes is derived. By employing this MCC, the universal semi-analytical model (USAM) of inter-core crosstalk (ICXT) can be applied to calculate the IMXT. Simulation results show that our model is generally consistent with previous theories when stochastic perturbations are absent. Moreover, our model can work effectively when stochastic perturbations are present, where former theories are not able to work properly. It has been theoretically found that the MCC has an intimate relationship with core pitch. Our model, based on the CMT, can provide physical characteristics in detail, which has not been reported clearly by former theories. In addition, our model is applicable to phase-matching and non-phase-matching regions of both real homogeneous and heterogeneous few-mode multicore fibers (FM-MCFs) with a wider range of applications.

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.

A novel heterogeneous sixteen-core four-mode fiber with two types of refractive index profiles

To this day, the inter-core crosstalk and intra-core crosstalk are potential factors limiting optical fiber transmission over long distances and serious degradation of signal quality in communication systems of multi-core few-mode fibers. To this end, this work proposes a heterogeneous sixteen-core four-mode fiber with two types of refractive index profiles is proposed in this paper, which consists of a graded-index core and a step-index core. After optimization, the four modes can transmit independently and steadily at 1550 nm, and the effective refractive index difference between adjacent modes is more than 1 × 10−3. Under the optimal fiber parameters, the inter-core crosstalk results can be obtained from the research results and are less than −69 dB/km, which reflects the significant effect of the proposed fiber in reducing the inter-core crosstalk. The proposed fiber is the first to use the combination of step-index core and graded-index core so that the inter-core crosstalk of heterogeneous fibers is successfully suppressed at a low level, while providing new ideas and solutions for future research on novel low-crosstalk multi-core few-mode fibers that can be applied to terrestrial and undersea communication systems.

Torsion and bending sensing based on the specklegrams from a coupled few-mode multi-core fiber

A novel optical torsion and bending dual-parameter sensor based on the detection of a coupled few-mode multi-core fiber (FM-MCF) specklegrams is proposed and experimentally demonstrated. A 3-mm-long multimode fiber (MMF) is used to couple the light from the injection single mode fiber into all cores of the FM-MCF, and thus the fiber structure is formed by the ultra-short MMF and FM-MCF. The specklegrams of the FM-MCF can be detected and segmented to infer the change of the FM-MCF. The torsion angle of 0-360° of the FM-MCF can be determined accurately by detecting the overall rotation angles of the specklegrams. And since the modes in each core of the FM-MCF are strongly coupled and the mode interference is highly sensitive to external disturbance, bending sensing based on the analysis of sub-specklegrams from each outer core is verified within the curvature range of 2.57–12.83 m−1 with the help of deep learning. Furthermore, based on a dual-output layer classification network, the capability of the sensor for torsion and bending sensing simultaneously is tested. The recognition accuracy for 36 combinations of 4 torsion angles and 9 bending states are 100% and 97.12%, respectively. The sensor here proposed has the advantages of low cost, simple structure, ease of interrogation and good accuracy.

O-Band 3D Optical Waveguide Fan-in/Fan-Out Devices for Few-Mode Multi-Core Fibers

O-Band 3D Optical Waveguide Fan-in/Fan-Out Devices for Few-Mode Multi-Core Fibers

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.

Distributed multi-parameter sensor based on Brillouin scattering in an etched few-mode multi-core fiber

We theoretically propose a unique distributed multi-parameter sensor based on an etched few-mode multi-core fiber (FM-MCF) for simultaneous sensing of temperature, strain, and sample refractive index (SRI). These three parameters can be sensed simultaneously by combining space division multiplexing (SDM) and stimulated Brillouin scattering (SBS). FM-MCF naturally contains an inner core and an outer core, and it is etched until the outer core is exposed. Therefore, the inner core can be employed to calculate the Brillouin gain spectrum (BGS) of LP01 and LP11 through distributed temperature and strain, and the outer core can be used to sense SRI through the BGS of LP01. Note that due to the high corrosion of the outer core, the higher order mode is cut off, so only the LP01 mode can be observed in the outer core. By demodulating the Brillouin frequency shift (BFS) of the BGS in the outer core and the inner core, the proposed sensor has a maximum temperature sensitivity of 1.7220 MHz/°C, a maximum strain sensitivity of 0.0783 MHz/με, and an SRI sensitivity of 274.3245 MHz/RIU around 1.440 RI unit (RIU). Therefore, we show a new perspective for discriminative sensing of multi-parameter in Brillouin distributed sensors based on SDM. The developed multifunctional fiber sensor could find application in the field of environmental sensing, materials science, biomedical applications, and structural health monitoring.

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.

A novel few-mode multi-core fiber with large effective mode area and low inter-core crosstalk

This paper proposes a novel 7-core 5-mode fiber with large mode area and low inter-core crosstalk(XT). The proposed optical fiber, by adopting a combination of air-trench structure and high-index ring, realizes large mode area and low XT. The numerical analyses reveal that the proposed fiber can stably transmit 5 LP modes in the C+L band; the effective mode area of each of the 5 LP modes is larger than 110 µm2; the XT of in each of the 5 LP modes is smaller than −79 dB/km; each mode has an effective refractive index difference bigger than 1.1 × 10−3. Therefore, a low XT result can still be achieved when the mode area is large. The proposed optical fiber may be well utilized to construct a high-capacity fiber optic transmission system.

3D waveguide device for few-mode multi-core fiber optical communications

Space-division multiplexing based on few-mode multi-core fiber (FM-MCF) technology is expected to break the Shannon limit of a single-mode fiber. However, an FM-MCF is compact, and it is difficult to couple the beam to each fiber core. 3D waveguide devices have the advantages of low insertion loss and low cross talk in separating various spatial paths of multi-core fibers. Designing a 3D waveguide device for an FM-MCF requires considering not only higher-order modes transmission, but also waveguide bending. We propose and demonstrate a 3D waveguide device fabricated by femtosecond laser direct writing for various spatial path separations in an FM-MCF. The 3D waveguide device couples the LP01 and LP11a modes to the FM-MCF with an insertion loss below 3 dB and cross talk between waveguides below − 36 dB . To test the performance of the 3D waveguide device, we demonstrate four-channel multiplexing communication with two LP modes and two cores in a 1-km few-mode seven-core fiber. The bit error rate curves show that the different degrees of bending of the waveguides result in a difference of approximately 1 dB in the power penalty. Femtosecond laser direct writing fabrication enables 3D waveguide devices to support high-order LP modes transmission and further improves FM-MCF communication.

Supermode Characteristics of Nested Multiple Hollow-Core Anti-Resonant Fibers

Mode-division multiplexing (MDM) can achieve ultra-high data capacity in optical fiber communication. Several impressive works on multicore fiber (MCF), multi-mode fiber, and few-mode multicore fiber have made significant achievements in MDM. However, none of the previous works can simultaneously maintain the transmission loss, chromatic dispersion (CD), and differential group delay (DGD) at a relatively low level. A nested multiple hollow-core anti-resonant fiber (NMH-ARF) has significant potential for applications in MDM. This study proposes a novel NMH-ARF with its structural design based on the traditional single-core nested anti-resonant fiber. We increased the number of nodes between capillaries. By changing the position of the nested tubes, several interconnected areas form when a single core is separated. We investigated the mode-coupling theory and transmission characteristics of this fiber. This fiber structure showed a low sensitivity to bending and achieved a super-low DGD and a super-low confinement loss (CL) at a wavelength of 1.55 µm while keeping CD relatively low.

Low cross talk homogeneous few-mode multi-core fiber with a suspended air-trench and V-type index core

We propose a homogeneous 13-core 4 linear-polarized modes fiber based on a suspended air-trench V-type index core (SA-VC) structure that can suppress inter-core cross talk (ICXT) and inter-mode cross talk. The SA structure is used to control ICXT, and the VC structure is used to control inter-mode cross talk. Cross talk and other properties of the fiber are calculated by coupled power theory and the finite element method. The numerical results demonstrate that the SA-VC structure has stronger cross talk suppression ability compared with traditional trench structure, the ICXT meets the requirement of lower than − 30 d B / 100 k m , the effective refractive index difference ( Δ n e f f ) between modes is greater than 1.52 × 10 − 3 , the maximum intrinsic loss is 0.189 dB/km, and the relative core multiplicity factor reaches 49.09. Based on the current optical fiber fabrication technology, a manufacturing scheme for this fiber preform is proposed. This design structure aims to be applied to communication systems to increase the transmission capacity of optical fibers.

1-Pbps orbital angular momentum fibre-optic transmission

Space-division multiplexing (SDM), as a main candidate for future ultra-high capacity fibre-optic communications, needs to address limitations to its scalability imposed by computation-intensive multi-input multi-output (MIMO) digital signal processing (DSP) required to eliminate the crosstalk caused by optical coupling between multiplexed spatial channels. By exploiting the unique propagation characteristics of orbital angular momentum (OAM) modes in ring core fibres (RCFs), a system that combines SDM and C + L band dense wavelength-division multiplexing (DWDM) in a 34 km 7-core RCF is demonstrated to transport a total of 24960 channels with a raw (net) capacity of 1.223 (1.02) Peta-bit s−1 (Pbps) and a spectral efficiency of 156.8 (130.7) bit s−1 Hz−1. Remarkably for such a high channel count, the system only uses fixed-size 4 × 4 MIMO DSP modules with no more than 25 time-domain taps. Such ultra-low MIMO complexity is enabled by the simultaneous weak coupling among fibre cores and amongst non-degenerate OAM mode groups within each core that have a fixed number of 4 modes. These results take the capacity of OAM-based fibre-optic communications links over the 1 Pbps milestone for the first time. They also simultaneously represent the lowest MIMO complexity and the 2nd smallest fibre cladding diameter amongst reported few-mode multicore-fibre (FM-MCF) SDM systems of >1 Pbps capacity. We believe these results represent a major step forward in SDM transmission, as they manifest the significant potentials for further up-scaling the capacity per optical fibre whilst keeping MIMO processing to an ultra-low complexity level and in a modularly expandable fashion.

Low cross talk crescent-shaped pore-assisted few-mode multi-core optical fiber for optical communication

In this paper, we propose a crescent-shaped pore-assisted structure to realize a few-mode multi-core (four-mode six-core) optical fiber with low inter-core cross talk for large-capacity long-distance communication. Using the finite element method, a theoretical analysis is conducted at a working wavelength of 1550 nm, where the proposed optical fiber achieves a total loss of less than 0.2 dB/km and an effective mode field area of greater than 108 µ m 2 . The inter-core cross talk of the fiber is less than − 56 d B / 100 k m , and the relative multiplexing factor reaches up to 28.3, which significantly increases the core density. The maximum dispersion of the four supported linear polarization modes is less than 5.85 p s / ( n m × k m ) , which is greatly advantageous in communication systems. In addition, the fabrication scheme of the proposed optical fiber is practical, and its standard-compatible 42 µm core pitch promises the feasibility of integration with commercial optical networks.

Genetic algorithm assisted bridge fiber design and fabrication for few-mode multi-core fiber Fan-in/Fan-out device.

We present a rapid and precise method to design the multiple step-index bridge fiber for ultra-low insertion loss few-mode multi-core fiber Fan-in/Fan-out device. The genetic algorithm is applied to optimize the structural parameters to support multi-mode operation. Based on the proposed intelligent iteration platform, core-based multiplex/demultiplex optimization can be achieved with less than 1.0 dB insertion loss for the first 6 LP modes in space division multiplexing system consisting of few-mode multi-core fibers. Besides, we have successfully drew the designed bridge fiber and fabricated the corresponding Fan-in/Fan-out device. When connecting it with the single-core 6-mode fiber and 7-core 6-mode fiber, the average insertion losses of mode LP01, LP11a, LP11b, LP21a, LP21b, and LP02 are 0.88 dB, 1.11 dB, 1.07 dB, 1.42 dB, 1.33 dB, and 1.04 dB, respectively.

Low-cross-talk and multi-channel heterogeneous 13-core four-LP mode fiber with a high refractive index ring and a trench

We propose a heterogeneous 13-core few-mode fiber, which is characterized by the combination of a high refractive index ring and a trench. All cores of the optical fiber can stably transmit L P 01 , L P 11 , L P 21 , and L P 02 modes in the C + L band through structural optimization and numerical analysis. The effective refractive index difference of the L P 21 and L P 02 modes reaches 2 × 10 − 3 , which is conducive to the independent transmission of modes. The trench can effectively suppress the inter-core cross talk below − 50 d B / k m at 1550 nm. The proposed few-mode multi-core fiber is conducive to solving the long-distance signal transmission problem of large capacity and low cross talk in future optical communication systems.

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.

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.

Multicore raised cosine fibers for next generation space division multiplexing systems

Space division multiplexing (SDM) is a promising solution for improving the transmission capacity and increasing the spectral efficiency. SDM over few mode multicore fibers (FM-MCFs) is an attractive approach that observes increasing interest. In this work, after reviewing the basic guidelines for FM-MCFs design, we address, for the first time, the Raised Cosine (RC) profile to multicore fibers context. We optimize the single core RC profile in order to support exactly 6 linearly polarized (LP) modes with large effective mode area (min Aeff = 96 μm2), with low differential mode delay (DMD) (max DMD = 33 ps⧹km) and high intermodal separation (⩾2×10−4). Once achieved, we investigate the induced crosstalk between twin RC cores and twin trenched RC (TRC) cores. We then propose four 6 modes 7 cores RC⧹TRC fibers. Their crosstalk and their relative core multiplicity are analysed and compared with those reported in recently graded index-FM-MCFs. The selected designs could be promising candidates for short and medium haul interconnects.

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.