Core Pitch Research Articles

Unified Model of Linear and Nonlinear Crosstalk in Multi-Core Fiber

In this paper, the theoretical unification of linear and nonlinear inter-core crosstalk (IC-XT) in step-index single-mode multicore fiber (MCF) media is reported and validated experimentally. In order to estimate the IC-XT when operating in linear and nonlinear regimes, the coupled-mode theory (CMT) and the coupled-power theory (CPT) have been unified in both power regimes. The theoretical analysis of the CMT indicates that in coupled MCFs with reduced core-to-core distance (core pitch) the nonlinear self-coupling and cross-coupling effects should be considered when operating with high optical powers. However, considering a core pitch value higher than three times the core radius only the self-coupling effect should be taken into account for estimating the nonlinear IC-XT. Considering these results, the CPT is also extended to nonlinear regime including the dominant nonlinear coupling effect. Using both CMT and CPT, the statistical model of nonlinear IC-XT is completed with the closed-form expressions for estimating the cumulative distribution function, the probability density function and the crosstalk mean and variance as a function of the power level launched into a single-core of the MCF. The crosstalk model presented is additionally extended when multiple cores are simultaneously excited. Finally, the theoretical model is experimentally validated in a homogeneous four-core fiber considering different bending radius configurations.

Actuarial calculation of the squareness errors for ultra-precision micro v-groove machine tools

A v-groove structure is extensively used for planar optical-fiber alignment because of its unique features. To obtain low connection loss, the core pitch tolerance of a v-groove structure should be within submicrons. The squareness errors have great influences on the machining precision of micro v-grooves. Therefore, the actuarial calculation of squareness errors is of great significance for ultra-precision micro v-groove machine tools. In view of the disadvantages of the squareness error analysis method basing on small angle error assumption, the actuarial calculation of squareness error is implemented. A more precise geometric error model basing on the actuarial calculation of squareness error is established. The transformation between actuarial calculation of squareness error and improvement of machining precision is achieved through the geometric error modeling. The results show that the actuarial calculation of squareness error is effective and practical.

Fiber Twisting- and Bending-Induced Adiabatic/Nonadiabatic Super-Mode Transition in Coupled Multicore Fiber

The characteristics of power transfer between super-modes in coupled multicore fiber are investigated numerically and experimentally. We reveal that fiber twisting and macrobending cause mode coupling through an adiabatic/nonadiabatic process, which is different from the mode coupling caused by structural perturbation, namely microbending. We also investigate the impulse response characteristics of coupled two-core fibers, and find that there is an optimum core pitch for minimizing the complexity of the MIMO processing. Finally, we conduct a 4 × 4 MIMO transmission experiment using two-core fibers to evaluate their transmission characteristics. We show that strongly coupled multicore fiber can reduce the quality difference between the received signals even when the transmission line has large core-dependent gain/loss values.

Analysis and characterization of high-resolution and high-aspect-ratio imaging fiber bundles.

High-contrast imaging fiber bundles (FBs) are characterized and modeled for wide-angle and high-resolution imaging applications. Scanning electron microscope images of FB cross sections are taken to measure physical parameters and verify the variations of irregular fibers due to the fabrication process. Modal analysis tools are developed that include irregularities in the fiber core shapes and provide results in agreement with experimental measurements. The modeling demonstrates that the irregular fibers significantly outperform a perfectly regular "ideal" array. Using this method, FBs are designed that can provide high contrast with core pitches of only a few wavelengths of the guided light. Structural modifications of the commercially available FB can reduce the core pitch by 60% for higher resolution image relay.

Large Spatial Channel (36-Core × 3 mode) Heterogeneous Few-Mode Multicore Fiber

We describe the design and characterization of a heterogeneous 36-core, three-mode fiber with three core types. Intercore crosstalk for LP11 modes is estimated to be below −31 dB for 5.5 km propagation at a core pitch of 34 μm. Feasibility of 108 space/mode division multiplexed transmission is investigated using free-space multiplexing/demultiplexing technologies, 40-wavelength division multiplexed, 25 GBd, 93.4-Gb/s dual-polarization QPSK signals, and coherent detection with a sparse 6 × 6 MIMO equalizer. The total transmission capacity amount to 403.7 Tb/s.

Effect of waist diameter and twist on tapered asymmetrical dual-core fiber MZI filter.

A compact in-fiber Mach-Zehnder interferometer (MZI) filter fabricated from custom-designed asymmetrical dual-core fiber is numerically analyzed in detail and experimentally verified. The asymmetrical dual-core fiber has core diameters and a core pitch of 6.9, 6, and 19.9 μm, respectively. The fiber tapering technique is introduced to fuse the originally uncoupled cores into strong coupling tapered regions. The length and diameter of the waist region have a close impact on the splitting ratio, which further affects the spectral properties of the MZI filter. The field evolution with varied waist parameters is characterized by the finite element method and beam propagation method. Repeatable comb filters with ∼15 dB extinction ratio are successfully achieved under the guidance of simulated optimum conditions. The twist-induced circular birefringence gives rise to a retardance that causes the spectral shifts of the MZI filter. The theoretical and experimental results confirm that the relative wavelength shift is proportional to the retardance, which follows a sinc function in the limit of a large twist rate.

Supermode fiber for orbital angular momentum (OAM) transmission

We present a multi-orbital-angular-momentum (OAM) multi-core supermode fiber (MOMCSF) to transmit OAM modes. The MOMCSF consists of equally-spaced and circularly-arranged multiple cores, in which the core pitch is small enough to support strong coupling OAM supermodes. The characteristics of OAM modes in MOMCSFs with different core pitches and core numbers are analyzed. The performances of mode coupling and nonlinearity are optimized by designing multiple degrees of freedom of the supermode fiber. The obtained results show that the designed MOMCSF can transmit multiple OAM modes with favorable performance of low mode coupling, low nonlinearity, and low modal dependent loss.

Heterogeneous trench-assisted few-mode multi-core fiber with graded-index profile and square-lattice layout for low differential mode delay.

We propose a kind of heterogeneous trench-assisted graded-index few-mode multi-core fiber with square-lattice layout. For each core in the fiber, effective area (A(eff)) of LP(01) mode and LP(11) mode can achieve about 110 μm(2) and 220 μm(2). Absolute value of differential mode delay (|DMD|) is smaller than 100 ps/km over C + L bands, which can decrease the complexity of digital signal processing at the receiver end. Considering the upper limit of cladding diameter (D(cl)) and cable cutoff wavelength of LP(21) mode in the cores located at the inner layer, we set core pitch (Λ) as 43 μm. In this case, D(cl) is about 220.4 μm, inter-core crosstalk (XT) is lower than -40 dB/500 km and the relative core multiplicity factor (RCMF) reaches 15.93.

Development of a simulation analysis environment for ventilated slab systems

In this paper, a new simulation environment is presented to evaluate the energy performance of ventilated slab systems in multi-floor buildings. The simulation environment combines a transient two-dimensional finite difference solution of a ventilated slab system comprising slab-wall joints with thermal network model for indoor spaces and associated exterior walls. The developed simulation environment can assess the impact of thermal bridging effects on both heating and cooling building thermal loads. First, the predictions of the developed simulation environment are verified against those obtained from a detailed whole-building energy simulation tool when thermal bridging effects are neglected. Then, a series of parametric analyses are performed to determine the performance of ventilated slab systems under various design and operating conditions considering the thermal bridging effects. It is found that the energy performance of ventilated slab systems and thermal bridging impact depend on a wide range of factors including size of the slab, supply air inlet temperature, air mass flow rate, core diameter, core pitch, and depth of hollow cores. In particular, it is found that the thermal bridge affects significantly the energy performance of ventilated slab systems and can increase both heating and cooling energy consumptions by 17% and 11%, respectively.

Crosstalk analysis in homogeneous multi-core two-mode fiber under bent condition.

We analyze the inter-core crosstalk in homogeneous multi-core two-mode fibers (MC-TMFs) under bent condition by using the coupled-mode equations. In particular, we investigate the effects of the intra-core mode coupling on the inter-core crosstalk for two different types of MC-TMFs at various bending radii. The results show that the inter-core homo-mode crosstalk of LP(11) mode is dominant under the gentle fiber bending condition due to its large effective area. However, as the fiber bending becomes tight, the intra-core mode coupling is significantly enhanced and consequently makes all the inter-core crosstalk levels comparable to each other regardless of the mode. A similar tendency is observed at a reduced bending radius when the difference in the propagation constants between modes is large and core pitch is small.

A study of single kinematic errors accurate fitting for ultra-precision micro v-groove machine tools

V-groove structure is extensively used for planar optical-fiber alignment, and its quality is critical to the functionality of the fiber connector. The core pitch of v-groove structure is the most important machining parameter, which determines the v-groove quality. And its machining precision is dominated by the machine tool motion precision in the z-axis direction. Six single kinematic errors derived from the multi-body system theory are the main factors affecting the accuracy of the machine tool motion in the z-axis direction. Therefore, this paper presents a single kinematic errors accurate fitting methodology for the micro v-groove machine tools. A function is fitted with a set of data using the existing fitting method and verified by another set of data measured at other testing points. Each set of test points have the same intervals, but their starting points are different from each other. The fitted function is constantly upgraded in the process of the verification. And the accurate fitted function is finally obtained by the recycling verification process. The results show that the accurate fitting method is effective and practical.

Simple analytical expression for crosstalk estimation in homogeneous trench-assisted multi-core fibers.

An analytical expression for the mode coupling coefficient in homogeneous trench-assisted multi-core fibers is derived, which has a simple relationship with the one in normal step-index structures. The amount of inter-core crosstalk reduction (in dB) with trench-assisted structures compared to the one with normal step-index structures can then be written by a simple expression. Comparison with numerical simulations confirms that the obtained analytical expression has very good accuracy for crosstalk estimation. The crosstalk properties in trench-assisted multi-core fibers, such as crosstalk dependence on core pitch and wavelength-dependent crosstalk, can be obtained by this simple analytical expression.

Design of low crosstalk and bend insensitive optical interconnect using rectangular array multicore fiber

A design strategy for achieving minimum crosstalk in a space division multiplexed 2×4 multicore fiber (MCF) for application in next generation exa-bandwidth optical interconnects is proposed. To evaluate crosstalk in adjacent cores of MCF, a semi-analytical approach based on finite element method and coupled power theory is applied. The dependence of crosstalk on various MCF parameters – such as index profile parameter, core to core pitch and bending radius – is investigated. It is shown that optical interconnects based on heterogeneous core MCF with triangular index profile and optimum core to core pitch, can be configured to have minimum crosstalk per unit length. Moreover, such interconnects show high degree of bend tolerance for both small and large bending radii.

Role of wavelength dependent sensitivity in affecting the crosstalk mitigation of homogeneous multicore fiber: an analytical estimation approach.

The mitigation of both crosstalk and its wavelength dependent sensitivity for homogeneous multicore fiber (MCF) is theoretically investigated using an analytical evaluation approach. It is found there exists a performance trade-off between the crosstalk mitigation and its wavelength dependent sensitivity suppression. After characterizing the fabricated homogeneous MCFs, we verify that although the increasing core pitch can mitigate the crosstalk, the wavelength dependent sensitivity is drastically degraded from 0.07dB/nm to 0.11dB/nm, which is harmful to the dense wavelength division multiplexing (DWDM) transmission over C + L band using MCF.

A study of the fabrication of v-groove structure in ultra-precision milling

V-groove structure is the key component for the alignment of fibres in the fibre array connector. This paper presents an investigation on the application of ultra-precision multi-axis machining technology in the fabrication of v-groove component. Ultra-precision raster milling (UPRM) is one of the multi-axis machining technologies which provide a new solution and more freedom to produce v-groove structure as compared with the precision grinding technology. In this paper, the cutting strategy is developed to schedule the material removal process, select cutting parameters and plan tool paths for v-groove machining in UPRM. Based on the study of the factors affecting machining efficiency, a novel model for machining time prediction is built for the fabrication of v-groove taking into consideration the acceleration and deceleration time of the ultra-precision machine. An integrated system is developed for the machining of v-groove structure in UPRM. Experiment is conducted to produce two different types of v-groove structures in UPRM. The experimental results show that the increase of feed rate decreases the surface quality and the core pitch tolerance of v-groove structure produced by raster milling process reaches to sub-micrometre. The developed model provides a precise machining time prediction for the actual machining process.

Large-effective-area uncoupled few-mode multi-core fiber

Characteristics of few-mode multi-core fiber (FM-MCF) were numerically analyzed and experimentally confirmed. The cores of FM-MCF were designed to support transmission of LP(01) and LP(11) modes from the point of bending loss of LP(11) and LP(21) modes. Inter-core crosstalk between LP(11) mode was calculated to determine core pitch of fibers. It was confirmed that the fabricated fibers was two-mode transmission over C-band and L-band with the effective area of LP(01) mode of about 110 μm(2) at 1550 nm. The crosstalk of the fibers was estimated to be smaller than -30 dB at 1550 nm after 100-km propagation. The crosstalk dependence on wavelength was also measured and matched well with the simulated results.

Feasibility of lead cooled breed and burn reactors

The objective of the present work is to assess the feasibility and achievable performance of a large Breed-and-Burn (B&B) reactor core that is cooled with heavy-liquid-metal (HLM), such as lead or lead-bismuth eutectic (LBE), relative to a sodium-cooled B&B core. With the exception for the initial critical fissile fuel loading the B&B reactor systems are to be fuelled only with fertile material. When the fuel reaches its radiation damage limit it is reconditioned and recycled without separating the actinides from most of the fission products. It was found that in order to get a total power of 3000 MWth as of the reference sodium-cooled B&B core, core pitch to diameter ratio needs to be increased from the 1.112 value of the reference sodium-cooled core to 1.3 for Pb and LBE coolants and to 1.24 for PbLi coolant. As a consequence, the minimum burnup required to sustain the B&B mode of operation is approximately 29% FIMA for the LBE and PbLi cooled cores – larger than the 21% FIMA required for the reference sodium cooled B&B reactor. Had the Pb been enriched to nearly 100% 208Pb, the minimum required burnup would be 22.5%; less than 10% higher than for Na-cooled core. The maximum possible accumulated burnup in the LBE-cooled B&B core is ∼45% FIMA – smaller than the 55% FIMA possible to achieve in the reference sodium cooled core. For the same core volume HLM-cooled cores require 26% smaller mass of fissile material to establish initial criticality.

Carbonaceous mesophase spherule/activated carbon composite as anode materials for super lithium ion capacitors

A series of carbonaceous mesophase spherule/activated carbon composites were prepared as anode materials for super lithium ion capacitors using carbonaceous mesophase spherules as the core materials and pitch as the active carbon shell precursor. The structures of the composites were examined by scanning electron microscopy and X-ray diffractometry. The electrochemical performance was investigated in electric double layer capacitor and half-cell. The results show that, the composite exhibits good performance in both capacitor and battery with a high reversible capacity of 306.6 mA·h/g (0.2C) in the half-cell, along with a capacitance of 25.8 F/g in the capacitor when an optimum ratio of carbonaceous mesophase spherules to active carbon is adopted. The composite also shows a favorable rate performance and good cycle ability. A working model of this anode in super lithium ion capacitors was established.

Materials and Fabrication Issues of Micro V-Groove for Optoelectronics Packaging

Micro V-groove is an important optical component in the packaging of optoelectronic devices, which holds the position of the optical fibers. The micro V-groove can be fabricated by three main techniques on different materials: wet etching, ultra precision machining and molding press. The main parameters, which determine the micro V-groove quality, include core pitch, surface roughness and cured adhesive height. In this paper, three fabrication techniques are discussed to address the future needs of the next-generation optical fiber communication technology. The advantages and disadvantages of each fabrication technique are discussed with the quality issues. The findings can be served as guides for the materials and fabrication in micro V-groove in optoelectronic packaging.

A Study of Micro V-Groove Fabrication in Ultra-Precision Freeform Machining

Optical communication is gaining popularity in telecommunications; most obviously evident is the increase in demand for a large variety of fiber optic components. V-groove structure quality is critical to the functionality of the fiber connector as it governs the position of the optic fibers. The main parameters, which determine the V-groove quality, include core pitch, surface roughness and the position of centerline. In this paper, a series of cutting conditions were applied to investigate the effect of different machining parameters on the V-groove quality. In the experiment, the V-groove structure is machined by a 40° single crystal diamond tool on aluminium alloy with different cutting conditions. Based on the findings, the relationship between the cutting conditions on the surface roughness was explored. The implications of these findings on the improvement of the surface finish of V-groove profile are discussed.