Double-core Structure Research Articles

General relativistic moving-mesh hydrodynamic simulations with arepo and applications to neutron star mergers

ABSTRACT We implement general relativistic hydrodynamics in the moving-mesh code arepo. We also couple a solver for the Einstein field equations employing the conformal flatness approximation. The implementation is validated by evolving isolated static neutron stars using a fixed metric or a dynamical space–time. In both tests, the frequencies of the radial oscillation mode match those of independent calculations. We run the first moving-mesh simulation of a neutron star merger. The simulation includes a scheme to adaptively refine or derefine cells and thereby adjusting the local resolution dynamically. The general dynamics are in agreement with independent smoothed particle hydrodynamics and static-mesh simulations of neutron star mergers. Coarsely comparing, we find that dynamical features like the post-merger double-core structure or the quasi-radial oscillation mode persist on longer time scales, possibly reflecting a low numerical diffusivity of our method. Similarly, the post-merger gravitational wave emission shows the same features as observed in simulations with other codes. In particular, the main frequency of the post-merger phase is found to be in good agreement with independent results for the same binary system, while, in comparison, the amplitude of the post-merger gravitational wave signal falls off slower, i.e. the post-merger oscillations are less damped. The successful implementation of general relativistic hydrodynamics in the moving-mesh arepo code, including a dynamical space–time evolution, provides a fundamentally new tool to simulate general relativistic problems in astrophysics.

Design and fabrication of conversion fiber with double-core structure for photonic integrated circuits high-density interconnect

To relax photonic integrated circuits (PIC) and its optical interconnect densification limitations from the standard fiber, we proposed a special fiber with a double core structure, named conversion fiber, which can convert both its spot size and its outer diameter, simultaneously. The simulation in this study clarified that the proposed conversion fiber had more capability of spot size and the outer diameter conversion with a large conversion ratio, as compared with a conventional fiber with a single core structure, and had large tolerance for the refractive index fluctuation. After that, we fabricated a conversion fiber based on the design parameters. The insertion loss of fabricated conversion fiber, including the coupling loss and the excess loss due to the mode conversion was 0.2 dB, which well agrees with the simulation results. From these results, it is expected that the proposed fiber can achieve higher density optical interconnect for PIC.

Using a Double-Core Structure to Reduce the LUT Count in FPGA-Based Mealy FSMs

A method is proposed which aims at reducing the numbers of look-up table (LUT) elements in logic circuits of Mealy finite state machines (FSMs). The FSMs with twofold state assignment are discussed. The reduction is achieved due to using two cores of LUTs for generating partial Boolean functions. One core is based on maximum binary state codes. The second core uses extended state codes. Such an approach allows reducing the number of LUTs in the block of state codes’ transformation. The proposed approach leads to LUT-based Mealy FSM circuits having three levels of logic blocks. Each partial function for any core is represented by a single-LUT circuit. A formal method is proposed for redistribution of states between these cores. An example of synthesis is shown to explain peculiarities of the proposed method. An example of state redistribution is given. The results of experiments conducted with standard benchmarks show that the double-core approach produces LUT-based FSM circuits with better area-temporal characteristics than they are for circuits produced by other investigated methods (Auto and One-hot of Vivado, JEDI, and twofold state assignment). Both the LUT counts and maximum operating frequencies are improved. The gain in LUT counts varies from 5.74% to 36.92%, and the gain in frequency varies from 5.42% to 12.4%. These improvements are connected with a very small growth of the power consumption (less than 1%). The advantages of the proposed approach increase as the number of FSM inputs and states increases.

Comparative Study of Superconducting DC Energy Pipelines With Different Bipolar Structural Layouts

Superconducting DC energy pipeline combines long-distance electricity and liquified natural gas transmission, which could improve the overall efficiency of the pipeline system and save construction space. The characteristics of superconducting DC energy pipelines with bipolar coaxial, double-core and partitioned structural layouts are analyzed and compared in this paper, which are significant for the selection of pipeline structure. The voltage levels are set as ±10 kV/±100 kV for electricity distribution and transmission applications, and the rated current is set as 1 kA. The simulation models of energy pipelines with each structural layout are established in order to evaluate the respective operation characteristics. The operation conditions of the steady state, the quench transient process under a short-circuited fault and the quench recovery transient process afterwards are analyzed in order to evaluate the stability and safety of the pipeline system. The comparison results illustrate the distinguishing features of each structural layout from various aspects. It is found that the double-core structure and the partitioned structure are the most suitable options at the voltage levels of ±10 kV and ±100 kV respectively.

The Coordinated Development of “Transport-Industry” in Lanzhou-Xining Urban Agglomeration from the Perspective of flow space

Under the perspective of “flow space”, the text of this paper constructs an interactive spatial connection network within 30 county towns units of the Lanzhou-Xining urban agglomeration. Further, with the help of social network and ArcGIS (which is the spatial analysis method), the quantitative analysis is used to analyze the structural characteristics of multi-dimensional spatial network of the Lan-Xi urban agglomeration and their mutual relations. Moreover, the synergetic degree model is used to measure the synergetic development level of the “traffic-industry” compound system in urban agglomeration at present. The results indicate that the Lanzhou-Xining urban agglomeration forms a double-core structure that Lanzhou and Xining as the center, and the space of county towns presents the correlated characteristic of “dumbbell-like” expansion. Furthermore, the central power of county towns which are restricted by the traffic condition and industrial structure indicates some outstanding differences. Further, it shows that the center develops to the periphery in a low-level but balanced way. In addition, the random disequilibrium of order degree of traffic and industry in urban agglomeration restricts the synergetic development of county towns, and the compound system of urban agglomeration is in the low-medium stage of synergetic development. The results of this study can provide some references for improving the traffic infrastructure construction and industrial functional structure of Lan-Xi urban agglomeration. This study also can shed some lights on the construction of the Silk Road Economic Belt.

Three-dimensional thermohaline anomaly structures of rings in the Kuroshio Extension region

Using AVISO satellite altimeter observations during 1993–2015 and a manual eddy detection method, a total of 276 anticyclonic rings and 242 cyclonic rings shed from the Kuroshio Extension (KE) were identified, and their three-dimensional (3D) anomaly structures were further reconstructd based on the Argo float data and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) cruise and buoy data through an interpolation method. It is found that the cyclonic (anticyclonic) rings presented consistent negative (positive) anomalies of potential temperature; meanwhile the relevant maximum anomaly center became increasingly shallow for the cyclonic rings whereas it went deeper for the anticyclonic rings as the potential temperature anomaly decreased from the west to the east. The above deepening or shoaling trend is associated with the zonal change of the depth of the main thermocline. Moreover, the composite cold ring between 140° and 150°E was found to exhibit a double-core vertical structure due to the existence of mode water with low potential vorticity. Specifically, a relatively large negative (positive) salinity anomaly and a small positive (negative) one appeared for the composite cyclonic (anticyclonic) ring at the depth above and below 600 m, respectively. The underlying driving force for the temperature and salinity anomaly of the composite rings was also attempted, which varies depending on the intensity of the background current and the temperature and salinity fields in different areas of the KE region, and the rings’ influences on the temperature and salinity could reach deeper than 1 000 m on average.

Conical Double-Core Structure LPmn Mode Converter

In this work, a LPmn mode converter is proposed based on a tapered double-core structure. With opposite changes of the two cores in radius dimension, matching the effective refractive index between the fundamental mode (LP01) in one core and one desired high-order mode in the other core is obtained, and the fundamental mode power is gradually converted to the high-order mode. The simulation results show that the mode conversion of LP01 to any high-order mode of LP11, LP21, LP31, LP12, LP41, LP22, LP32, LP42 and LP23 modes can be obtained with a conversion efficiency of ∼99%, ∼96%, ∼92%, ∼93%, ∼82%, ∼92%, ∼84%, ∼85% and ∼81% at the working wavelength of 1550 nm, respectively, and also conversion bandwidth (>80% conversion efficiency) is 265, 125, 130, 130, 100, 35 and 30 nm, respectively. Compared to the previously reported works, the proposed mode converter structure is more universal to support any of higher-order mode conversions with wider bandwidth.

The Emergence of the North Icelandic Jet and Its Evolution from Northeast Iceland to Denmark Strait

AbstractThe North Icelandic Jet (NIJ) is an important source of dense water to the overflow plume passing through Denmark Strait. The properties, structure, and transport of the NIJ are investigated for the first time along its entire pathway following the continental slope north of Iceland, using 13 hydrographic/velocity surveys of high spatial resolution conducted between 2004 and 2018. The comprehensive dataset reveals that the current originates northeast of Iceland and increases in volume transport by roughly 0.4 Sv (1 Sv ≡ 106 m3 s−1) per 100 km until 300 km upstream of Denmark Strait, at which point the highest transport is reached. The bulk of the NIJ transport is confined to a small area in Θ–S space centered near −0.29° ± 0.16°C in Conservative Temperature and 35.075 ± 0.006 g kg−1 in Absolute Salinity. While the hydrographic properties of this transport mode are not significantly modified along the NIJ’s pathway, the transport estimates vary considerably between and within the surveys. Neither a clear seasonal signal nor a consistent link to atmospheric forcing was found, but barotropic and/or baroclinic instability is likely active in the current. The NIJ displays a double-core structure in roughly 50% of the occupations, with the two cores centered at the 600- and 800-m isobaths, respectively. The transport of overflow water 300 km upstream of Denmark Strait exceeds 1.8 ± 0.3 Sv, which is substantially larger than estimates from a year-long mooring array and hydrographic/velocity surveys closer to the strait, where the NIJ merges with the separated East Greenland Current. This implies a more substantial contribution of the NIJ to the Denmark Strait overflow plume than previously envisaged.

Long-term postmerger simulations of relativistic star coalescence: Formation of toroidal remnants and gravitational wave afterglow

It has been estimated that a significant proportion of binary neutron star merger events produce long-lived massive remnants supported by differential rotation and subject to rotational instabilities. To examine formation and oscillation of rapidly rotating neutron stars (NS) after merger, we present an exploratory study of fully general-relativistic hydrodynamic simulations using the public code Einstein Toolkit. The attention is focused on qualitative aspects of long-term postmerger evolution under [Formula: see text]-rotational symmetry. As simplified test models, we use a moderately stiff [Formula: see text] ideal-fluid equation-of-state and unmagnetized irrotational equal-mass binaries with three masses well below the threshold for prompt collapse. Our high resolution simulations generate postmerger “ringdown” gravitational wave (GW) signals of 170 ms, sustained by rotating massive NS remnants without collapsing to black holes. We observe that the high-density double-core structure inside the remnants gradually turns into a quasi-axisymmetric toroidal shape. It oscillates in a quasi-periodic manner and shrinks in size due to gravitational radiation. In the GW spectrograms, dominant double peaks persist throughout the postmerger simulations and slowly drift to higher frequencies. A new low-frequency peak emerges at about 100 ms after merger, owing to the growth of GW-driven unstable oscillation modes. The long-term effect of grid resolution is also investigated using the same initial model. Moreover, we comment on physical conditions that are favorable for the transient toroidal configuration to form, and discuss implication of our findings on future GW observation targeting rapidly rotating NS remnants.

Unified picture of the post-merger dynamics and gravitational wave emission in neutron star mergers

We introduce a classification scheme of the postmerger dynamics and gravitational wave emission in binary neutron star mergers, after identifying a new mechanism by which a secondary peak in the gravitational wave spectrum is produced. It is caused by a spiral deformation, the pattern of which rotates slower with respect to the double-core structure in the center of the remnant. This secondary peak is typically well separated in frequency from the secondary peak produced by a nonlinear interaction between a quadrupole and a quasiradial oscillation. The new mechanism allows for an explanation of low-frequency modulations seen in a number of physical characteristics of the remnant, such as the central lapse function, the maximum density and the separation between the two cores, but also in the gravitational wave amplitude. We find empirical relations for both types of secondary peaks between their gravitational wave frequency and the compactness of nonrotating individual neutron stars, that exist for fixed total binary masses. These findings are derived for equal-mass binaries without intrinsic neutron star spin analyzing hydrodynamical simulations without magnetic field effects. Our classification scheme may form the basis for the construction of detailed gravitational wave templates of the postmerger phase. We find that the quasiradial oscillation frequency of the remnant decreases with the total binary mass. For a given merger event, our classification scheme may allow one to determine the proximity of the measured total binary mass to the threshold mass for prompt black hole formation, which can, in turn, yield an estimate of the maximum neutron star mass.

Double core of ozone valley over the Tibetan Plateau and its possible mechanisms

In this study, the three-dimensional structure of the ozone valley (e.g. ozone depletion) over the Tibetan Plateau (TP) in summer (June–August) has been investigated using Microwave Limb Sounder (MLS) ozone (O3) data from 2005 to 2013. A double core structure of the ozone valley over the TP is found with one depletion center in upper stratosphere (10–2hPa) named the upper core, and another depletion center, named the lower core, is observed in the upper troposphere/lower stratosphere (200–50hPa). The analysis indicates that the zonal deviation of ozone O3* (O3*=O3–[O3], here [O3] is the zonal mean of O3) at the upper core is nearly −1DU while its counterpart's deviation at the lower core is nearly –15DU. Large scale atmospheric circulation and terrain effects play an important role in the ozone valley at the lower core. In contrast, photochemistry reactions of odd chlorine including chlorine atoms (Cl) and chlorine monoxide (ClO) dominate the ozone valley at the upper core. Based on the MLS products, support for a chemical driver at the upper core is based on: (1) the significant diurnal variability of ozone suggests possible photochemistry reactions impacting the ozone valley; (2) the positive center of the ClO zonal deviation (CLO*) and hydrogen chloride (HCl) zonal deviation (HCl*) are found at the bottom of the upper core, which means the higher odd chlorine may accelerate the ozone loss.

Water Mass Structure of Warm and Cold Anticyclonic Eddies in the Western Boundary Region of the Subarctic North Pacific

Abstract Vertical profile data of temperature and salinity from various sources were analyzed together with satellite altimeter data to investigate the water mass characteristics of warm and cold anticyclonic eddies in the western boundary region of the subarctic North Pacific. A dense distribution of anticyclonic eddies with warm and saline core water occurred near the Kuroshio Extension, and the distribution extends northward–northeastward into the western subarctic gyre along the Japan and Kuril–Kamchatka trenches. Eddies with cold and fresh core water are found mainly around the Oyashio southward intrusions and farther north near the Kuril Islands. Based on the heat content anomaly integrated over 50–200 dbar, 85% of the anticyclonic eddies within the study area (35°–50°N, 140°–155°E) have a warm and saline core and 15% have a cold and fresh core. Warm and saline eddies around the Japan and Kuril–Kamchatka trenches have a double-core structure, with a cold and freshwater mass located below the warm core. The northward propagation of these eddies along the trench line results in a large northward heat (salinity) transport in the upper 400 dbar (250 dbar) and a negative salinity transport below 350 dbar. The lower core water is colder and fresher on isopycnal surfaces at around 26.70σθ compared with the climatology. Given that the 26.70σθ isopycnal surface does not outcrop in the open North Pacific, an alignment process is suggested to occur between the warm and saline and the cold and fresh anticyclonic eddies in the upper and intermediate layers, respectively.

TADPOLE GALAXIES IN THE HUBBLE ULTRA DEEP FIELD

Tadpole galaxies have a head-tail shape with a large clump of star formation at the head and a diffuse tail or streak of stars off to one side. We measured the head and tail masses, ages, surface brightnesses, and sizes for 66 tadpoles in the Hubble Ultra Deep Field (UDF), and we looked at the distribution of neighbor densities and tadpole orientations with respect to neighbors. The heads have masses of 10^7-10^8 Msun and photometric ages of ~0.1 Gyr for z~2. The tails have slightly larger masses than the heads, and comparable or slightly older ages. The most obvious interpretation of tadpoles as young merger remnants is difficult to verify. They have no enhanced proximity to other resolved galaxies as a class, and the heads, typically less than 0.2 kpc in diameter, usually have no obvious double-core structure. Another possibility is ram pressure interaction between a gas-rich galaxy and a diffuse cosmological flow. Ram pressure can trigger star formation on one side of a galaxy disk, giving the tadpole shape when viewed edge-on. Ram pressure can also strip away gas from a galaxy and put it into a tail, which then forms new stars and gravitationally drags along old stars with it. Such an effect might have been observed already in the Virgo cluster. Another possibility is that tadpoles are edge-on disks with large, off-center clumps. Analogous lop-sided star formation in UDF clump clusters are shown.

New high negative dispersion photonic crystal fiber

A new high negative dispersion photonic crystal fiber is proposed. It has double-core structure. The inner core has a circle germanium-doped region. The outer core is formed by removing the 3rd ring air-holes around the core. There are two ring air-holes between the two cores, Diameter of the 1st ring air holes is bigger than that of the 2nd ring air-holes, this can make mode coupling between inner mode and outer mode and showed that the high negative PCF is the result of this structure characteristics. There are honeycomb photonic lattice in the PCF's cladding. The influence of the structure parameters deviated from the design those on the chromatic dispersion are evaluated. When the structure parameters Λ=1.50 μm, d core=2.10 μm, d 1=0.90 μm, d 2=0.44 μm and d 3=1.04 μm, the dispersion coefficient D is −1320 ps/(nm·km) at 1550 nm. This is a new kind of chromatic dispersion compensation PCF.

Multi-core structure of the Kuroshio in the East China Sea from long-term transect observations

Long-term hydrographic temperature and salinity transect data in the East China Sea from June 1955 to November 2001 are analyzed in order to examine the geostrophic velocity and the structure of the Kuroshio current. The structure of the Kuroshio Current is divided into three basic forms, a single-core structure, a double-core structure and a multi-core structure; the appearance percentage of the three forms are 53.1%, 31.4%, and 15.4%, respectively. The analysis suggests that multi-core structures have significant seasonal and interannual variabilities that are not fully understood but may relate to variations in transport and associated flow instabilities. The Kuroshio's spatial character is also analyzed in detail by applying a simplified model of motion instability into this multi-core structure of the East China Sea Kuroshio. The theoretical results are found to be consistent with the observations, suggesting that the instability of the Kuroshio in the East China Sea may bring forth the formation of the observed multi-core structure.

Silicon photonic circuit with polarization diversity

We devised a silicon photonic circuit with polarization diversity that consists of polarization splitters and polarization rotators. The splitter is based on a simple directional coupler and the rotator has an off-axis double-core structure. Both devices can be made by using planar fabrication technology and require no complex proceses for the fabrication of three-dimensional structures. We fabricated a polarization-independent wavelength filter based on Si wire waveguides as an application of the polarization diversity. The filter consists of the polarization splitters, the rotators, and a ring resonator. The polarization-dependent loss of the filter is about 1 dB. A 10-Gbps data transmission with scrambled polarization is demonstrated.

Polarization rotator based on silicon wire waveguides

We describe a polarization rotator based on an off-axis double-core structure consisting of a silicon wire waveguide and a silicon-oxinitride waveguide. The rotator can be made by planar fabrication technology and do not require complex processes, such as three-dimensional structure formation. A rotator with 35-microm long provides the polarization rotation angle of 72 degrees and the polarization extinction ratio of 11 dB with the excess loss of about 1 dB. Our polarization rotator represents a significant step towards accomplishing an optical circuit with polarization diversity based on silicon wire waveguides.

Multi-core structure of the main part of the Kuroshio at G-PN section in the East China Sea

Sectional velocity distribution of the East China Sea Kuroshio is one of the basic points in the study of the Kuroshio. Hydrographic temperature and salinity data at G-PN section in the East China Sea from June 1955 to November 2001 are collected and properly processed to calculate the geostrophic current using dynamic height method at the transect of the Kuroshio. After analysis of calculation results, the basic current structure of the Kuroshio in its main part is examined together with scalar estimate and characters of multi- core structure, and spacial-temporal variations of current cores' position. Main result shows that (1) single-core structure, double-core structure and multi-core structure are basic forms in axial part of the Kuroshio; (2) abvious temporal variations exist in current structure of the Kuroshio; (3) the current of structure of the Kuroshio has distinctly seasonal association. The number of current cores is on the high side of core numbers in average and multi-core stucture appears in fall mostly.

Optimization design of an electroabsorption modulator integrated with spot-size converter

A novel 1.55 μ spot-size converter integrated with electroabsorption modulator is designed and fabricated with conventional photolithography and chemical process. For the modulator, we employed a deeply-etched ridge waveguide structure with polyimide on each side of its walls. For the spot-size converter, we employed a buried double-core structure in which a ridge-based passive wavegude is incorporated. The wide and thim passive waveguide is optically combined with a laterally tapered active waveguide to control the mode size. By simulation, the figure of merit is 4. 17 (dB/V/ 100 μ) and the beam divergence angles in teh horizontal and vertical directions are as small as 11. 2† x 13. 0† respectively.

1.55 µm spot-size converter with monolithically integrated laser diode by conventional process

A novel 1.55 µm laser diode (LD) with monolithically integrated spot-size converter (SSC) is designed and fabricated using conventional photolithography and the chemical wet etching process. For the laser diode, a ridge double-core structure is employed. For the spot-size converter, a buried double-waveguide structure is incorporated. The laterally tapered active core is designed and optically combined with the thin passive core to control the size of the mode. The threshold current was measured to be 40 mA together with high slope efficiency of 0.35 W A−1. The beam divergence angles in the horizontal and vertical directions were as small as 14.9° and 18.2°, respectively.