Effective Core Diameter Research Articles

Demonstration of constant-cladding tapered-core Yb-doped fiber for mitigating thermally-induced mode instability in high-power monolithic fiber amplifiers

In this work, a large-mode-area (LMA) step-index constant-cladding tapered-core (CCTC) Yb-doped fiber with a cladding diameter of ∼600 µm is successfully fabricated. The CCTC fiber has a small-core region (diameter of ∼20 µm) at both ends and a large-core region (diameter of ∼36 µm) in the middle. To prove the laser performance of the CCTC fiber, a detailed comparison experiment with conventional uniform fiber with the same effective core diameter is carried out in a multi-kW all-fiber MOPA configuration. The experimental results show that employing the CCTC fiber can effectively mitigate the thermally-induced transverse mode instability (TMI) in both co-pump and counter-pump schemes, and realize high slope efficiency and single-mode beam quality (M2∼1.30). Under the counter-pump scheme, the TMI threshold of the CCTC fiber is observed at ∼2.49 kW with a slope efficiency of 86.2%, while the uniform fiber amplifier exhibits a TMI threshold of ∼2.05 kW. The theoretical analysis based on a semi-analytical model indicates this CCTC fiber can effectively improve the TMI threshold owing to a stronger gain saturation. Our results verify the great potential of such an LMA CCTC fiber to mitigate thermal-induced TMI effect and achieve single-mode operation without sacrifice of laser efficiency in high power monolithic fiber lasers, and the further power scaling is expected by optimizing the fiber design.

Quantitation method of loss powers using commercial magnetic nanoparticles based on superparamagnetic behavior influenced by anisotropy for hyperthermia

Local hyperthermia using magnetic nanoparticles has attracted attention as a less invasive cancer therapy. Accurate estimation of heat dissipation of magnetic nanoparticles is important for hyperthermia treatment. In this study, the magnetization properties and heat dissipation of Feraheme®, Resovist®, and Synomag®-D commercial magnetic nanoparticles was measured. The effective core diameter and anisotropy constant were estimated by the Langevin function taking into consideration the effect of anisotropy. The required frequency and intensity of an applied magnetic field with respect to the weight and concentration of magnetic nanoparticles in an assumed spherical tumor were estimated based on measurement and theory of magnetic relaxation. A method to quantitate realistic heat dissipation of magnetic nanoparticles in tumor tissue by measurement of their magnetization properties is introduced. Because the physical rotation of magnetic nanoparticles is inhibited in tumor tissue, the realistic heat dissipation is estimated by solidifying a mass of nanoparticles in an experimental sample. The specific loss power and intrinsic loss power as the heat dissipations of magnetic nanoparticles were estimated from the area of the magnetization curves. The results indicated that only the linear component of magnetization was associated with the loss powers. Our method to quantitate loss powers and the index for applied magnetic field will facilitate the clinical application of hyperthermia treatment using magnetic nanoparticles.

Comparative study on transverse mode instability of fiber amplifiers based on long tapered fiber and conventional uniform fiber

Compared with conventional uniform fibers, long tapered fibers provide an effective way to maintain beam quality and suppress the generation of nonlinear effects in fiber lasers. In this work, the transverse mode instability (TMI) of two amplifiers based on ytterbium-doped fiber (YDF) with uniform core diameter and tapered core diameter has been investigated experimentally. For a reasonable comparison of TMI in these two fiber amplifiers, the same effective core diameters and design parameters are applied to both. In the uniform YDF amplifier, the TMI threshold is around 1046 W, and the beam quality M2 factor increases with the power and reaches 3.2 when the power is around 1500 W. In a tapered YDF (T-YDF) amplifier, the maximum output power is up to 2170 W with a beam quality of M2 ~ 2.2 and no sign of TMI is observed. To the best of our knowledge, this is the first reported all-fiber tapered fiber laser with a 2 kW-level average output power. The experimental results and theoretical analysis indicate that the TMI threshold of the T-YDF amplifier is higher than the uniform YDF amplifier in spite of sharing the same effective core diameter.

A six-strut suspended core fiber for cylindrical vector mode generation and propagation.

We report fabrication and simulation of an F2 glass six-strut suspended core fiber (SCF) with small effective core diameter of 2.5 μm for cylindrical vector (CV) modes generation and propagation. Simulation results show that the fiber has a large effective refractive index difference in the order of 10-4-10-3 between the first higher-order CV modes, including TE01, HE21even and odd and TM01 modes. TE01 and TM01 were experimentally generated and were evaluated as having high purity of 82 percent and 85 percent, respectively. The results demonstrate that the SCF is a competitive waveguide candidate for selectable CV mode generation and propagation.

Effects of size and anisotropy of magnetic nanoparticles associated with dynamics of easy axis for magnetic particle imaging

Abstract The structure of magnetic nanoparticles affects the signal intensity and resolution of magnetic particle imaging, which is derived from the harmonics caused by the nonlinear response of magnetization. To understand the key effects of particle structures on the magnetization harmonics, the dependence of the harmonics on the size and anisotropy of different structures was investigated. We measured the harmonic signals with respect to different magnetic nanoparticle structures by applying an AC field with a gradient field for magnetic particle imaging, which was compared with the numerically simulated magnetization properties. In addition, the dynamics of the easy axis of magnetic nanoparticles in the liquid state were evaluated. The difference between the harmonics in the solid and liquid states indicates the effective core size and anisotropy due to particle structures such as single-core, chainlike, and multicore particles. In the case of the chainlike structure, the difference between the harmonics in the solid and liquid states was larger than other structures. In the numerical simulations, core diameters and anisotropy constants were considered as the effective values, such as the increase in anisotropy in the chainlike structure due to dipole interaction. The multicore particles showed high harmonics owing to their large effective core diameters. The superparamagnetic regime in the multicore structure despite the large effective core diameter was derived from the small effective anisotropy. The effective core size and the effective anisotropy of each particle structure and their impacts on the harmonic signals were revealed.

Design and analysis of large-core single-mode windmill single crystal sapphire optical fiber

We present a large-core single-mode “windmill” single crystal sapphire optical fiber (SCSF) design, which exhibits single-mode operation by stripping off the higher-order modes (HOMs) while maintaining the fundamental mode. The “windmill” SCSF design was analyzed using the finite element analysis method, in which all the HOMs are leaky. The numerical simulation results show single-mode operation in the spectral range from 0.4 to 2 μm in the windmill SCSF, with an effective core diameter as large as 14 μm. Such fiber is expected to improve the performance of many of the current sapphire fiber optic sensor structures.

Modal reduction in single crystal sapphire optical fiber

A new type of single crystal sapphire optical fiber (SCSF) design is proposed to reduce the number of guided modes via a highly dispersive cladding with a periodic array of high and low index regions in the azimuthal direction. The structure retains a “core” region of pure single crystal (SC) sapphire in the center of the fiber and a “cladding” region of alternating layers of air and SC sapphire in the azimuthal direction that is uniform in the radial direction. The modal characteristics and confinement losses of the fundamental mode were analyzed via the finite element method by varying the effective core diameter and the dimensions of the “windmill” shaped cladding. The simulation results showed that the number of guided modes were significantly reduced in the “windmill” fiber design, as the radial dimension of the air and SC sapphire cladding regions increase with corresponding decrease in the azimuthal dimension. It is anticipated that the “windmill” SCSF will readily improve the performance of current fiber optic sensors in the harsh environment and potentially enable those that were limited by the extremely large modal volume of unclad SCSF.

Infrared photonic crystal fiber

An infrared photonic crystal fiber has been made by multiple extrusions of silver halide (AgClxBr1−x) crystalline materials. The core of the fiber consisted of pure AgBr, and the cladding area included fiberoptic elements made of AgCl (with lower refractive index). Fibers of outer diameter 1 mm and length of the order of 1 m were flexible and transparent in the spectral range 3–20 μm. Measurements of the power distribution and the transmission losses in the fibers indicated that they behave like core-clad structures, with an effective core diameter 200 μm. These fibers will be extremely useful for many applications in the middle- and far-infrared.

Optical Absorption Spectra of Nanocrystal Gold Molecules

The optical absorption spectra of a series of nanocrystal gold moleculeslarger, crystalline Au clusters that are passivated by a compact monolayer of n-alkylthiol(ate)shave been measured across the electronic range (1.1−4.0 eV) in dilute solution at ordinary temperature. Each of the ∼20 samples, ranging in effective core diameter from 1.4 to 3.2 nm (∼70 to ∼800 Au atoms), has been purified by fractional crystallization and has undergone a separate compositional and structural characterization by mass spectrometry and X-ray diffraction. With decreasing core mass (crystallite size) the spectra uniformly show a systematic evolution, specifically (i) a broadening of the so-called surface-plasmon band until it is essentially unidentifiable for crystallites of less than 2.0 nm effective diameter, (ii) the emergence of a distinct onset for strong absorption near the energy (∼1.7 eV) of the interbandgap (5d → 6sp), and (iii) the appearance in the smallest crystallites of a weak steplike structure above this onset...

Calculation of winding losses in high-frequency toroidal inductors using single strand conductors

The paper is concerned with the optimisation of inductor design to improve the design capability for these large, expensive and heat-producing magnetic components in power electronic circuits. The work focuses on toroidal inductors which consist of uniformly distributed, solid conductor windings on low permeability toroidal cores. It develops methods of calculating the eddy current losses in the windings. The basic analytical equations for losses in the windings are evolved and the problem of variable turn pitch that arises in the toroidal geometry is dealt with by means of an effective core diameter concept. Effective core diameters are presented graphically in terms of normalised parameters. The theoretical and graphical results given in the paper enable toroidal inductor designs to be optimised, and losses to be calculated in a rapid and convenient way. Experimental results are shown to agree well with the theoretical results presented in the paper.

Equilibrium Properties of a System of Charged Particles

Motivated by the formal identification that can be made between the electronic charge e and a range parameter γ that we have previously studied, we have developed high-temperature, high-density expansions of the Helmholtz free energy and potentials of average force for systems of ions with repulsive cores. It appears that the natural ordering parameter for our free energy expansion is κR, where κ is the inverse Debye length and R an effective core diameter, and as a result the expansion is useful for either sufficiently high temperatures at any density or sufficiently low densities at any temperature. We find the terms in this expansion through order (κR)7. We have also found effects that will serve to modify the shielding length at high densities so that it is no longer just κ−1. One such effect depends upon the disparity in the core sizes of the positive and negative ions, and it vanishes only when the disparity does. A higher-order modification of the shielding length that remains in this symmetric case is also discussed.