Wing-Ki Liu

Tunable Passively

An all-fiber tunable passively Q-switched erbium-doped fiber (EDF) laser is presented. Saturable absorbers are constructed by optically driven deposition of single-wall carbon nanotubes on fiber connectors. A low pump threshold of 11.1 mW is achieved. Self-mode-locking effect is also observed, and it could be suppressed by splicing an extra unpumped EDF into the laser ring cavity. The laser can be tuned by applying axial strain on the fiber Bragg grating which serves as a narrowband external mirror of the cavity.

Q

Various methods for calculating the energies and widths of quasibound levels (orbiting or shape resonances) for spherical potentials are critically compared. A derivation for the previously‐proposed Airy function boundary condition method is presented, and a Weber function boundary condition method for locating resonances which lie above the potential barrier maximum is derived, tested, and found wanting. It is shown that the Weyl m‐function method of Hehenberger et al. [J. Chem. Phys. 65, 4559 (1976)] yields results in essentially exact agreement with the time‐delay maximum method of Le Roy and Bernstein [J. Chem. Phys. 54, 5114 (1971)]. An improved semiclassical method of calculating these resonance widths, suggested by M.S.Child, is presented and shown to be reliable even for levels lying right at a potential barrier maximum.

-switched Erbium-Doped Fiber Laser With Carbon Nanotubes as a Saturable Absorber

An all-fiber sensor capable of simultaneous measurement of temperature and strain is newly presented. The sensing head is formed by a fiber Bragg grating combined with a section of multimode fiber that acts as a Mach-Zehnder interferometer for temperature and strain discrimination. The strain and temperature coefficients of multimode fibers vary with the core sizes and materials. This feature can be used to improve the strain and temperature resolution by suitably choosing the multimode fiber. For a 10 pm wavelength resolution, a resolution of 9.21 mu epsilon in strain and 0.26 degrees C in temperature can be achieved.

Energies and widths of quasibound levels (orbiting resonances) for spherical potentials

A novel lateral force sensor based on a core-offset multi-mode fiber (MMF) interferometer is reported. High extinction ratio can be obtained by misaligning a fused cross section between the single-mode fiber (SMF) and MMF. With the variation of the lateral force applied to a short section of the MMF, the extinction ratio changes while the interference phase remains almost constant. The change of the extinction ratio is independent of temperature variations. The proposed force sensor has the advantages of temperature- and phase-independency, high extinction ratio sensitivity, good repeatability, low cost, and simple structure. Moreover, the core-offset MMF interferometer is expected to have applications in fiber filters and tunable phase-independent attenuators.

Simultaneous measurement for strain and temperature using fiber Bragg gratings and multimode fibers.

We present detailed numerical results on the ground state structures of metallic clusters. The Gupta-type many-body potential is used to account for the interactions between atoms in the cluster. Both the genetic algorithm technique and the basin hopping method have been applied to search for the global energy minima of clusters. The excellent agreement found in both schemes for the global energy minima gives credence to the optimized energy values obtained. For four monovalent and one polyvalent metals studied in this work and within the accuracy of the energies presented here, we find that the global energy minima predicted by the basin hopping method are the same as those values obtained by the genetic algorithm. Our calculations for the ground state energies of alkali metallic clusters show regularities in the energy differences, and the cluster growth pattern manifested by this same group of clusters is generally icosahedral, which is quite different from the close-packed and decahedral preferentiall...

Temperature- and phase-independent lateral force sensor based on a core-offset multi-mode fiber interferometer.

Starting from kinetic theory collision integrals obtained from a generalized Boltzmann equation for a linear molecule in a bath of atomic perturbers and using Liouville (vector) space algebra, general expressions are derived for the three cross sections determining the shear viscosity Senftleben–Beenakker effects. These expressions are presented in terms of S‐matrix elements in the total‐J representation since this representation is especially useful for dynamical calculation and approximation procedures. Coupled‐states and infinite‐order–sudden dynamical approximations are then introduced and expressions obtained for the three cross sections in initial‐l, final‐l, and average‐l labeling schemes. All cross sections simplify greatly when initial or final‐l labeling is employed but little or not at all when average‐l labeling is used. Nonetheless, even when the latter choice is made, less work will be involved than would be required for the corresponding full close‐coupled or coupled‐states calculation.

Structures of metallic clusters: Mono- and polyvalent metals

A new all-fiber sensor capable of simultaneous measurement of strain and temperature is presented. The sensor system is formed by a fiber Bragg grating and two sections of multimode fibers (MMFs). One section of the MMF is isolated from strain and acts as a temperature-dependent edge filter, while the other is isolated from both strain and temperature changes. By monitoring the optical power changes, it is feasible to obtain information that permits simultaneous measurement of strain and temperature with a low-cost and simple structure.

Production and relaxation cross sections for the shear viscosity Senftleben–Beenakker effect. I. Formal expressions and their coupled‐states and infinite‐order–sudden approximations for atom–diatom systems

Close‐coupled (CC) and coupled‐state (CS) calculations of kinetic theory relaxation and production cross sections are presented for the system H2 at infinite dilution in a bath of He. Performing these calculations for the same potential has allowed a test to be made of the efficacy of the CS procedure for use in calculating kinetic theory cross sections, especially those determining the Senftleben–Beenakker effects (SBE) in transport phenomena. The CS procedure is found to work extremely well (better than 1% agreement with CC results) for those cross sections that are predominantly elastic, to work rather well for relaxation cross sections determined either by reorientation of energetically inelastic collisonal events (about 10% agreement with CC results), and to work relatively poorly for production cross sections. In fact, the CS procedure for light molecules fares only marginally better than the IOS procedure for heavier molecules in the calculation of production cross sections. Finally, using only CC ...

Simultaneous Strain and Temperature Measurement With Fiber Bragg Grating and Multimode Fibers Using an Intensity-Based Interrogation Method

A simple and accurate method is proposed for transforming the Legendre expansion of an atom–diatom potential about the diatom center of mass into a new Legendre expansion about a shifted (by isotope substitution) diatom center of mass. It is found that a simple quadrature procedure yields accurate results throughout the range of intermolecular separation of physical interest, while a previously proposed Taylor series expansion procedure gives comparable accuracy only for intermolecular separations greater than the equilibrium separation. Numerical tests of the method are performed for three model atom‐plus‐rigid diatom systems H2(HD) –He, HCl(DCl) –Ar, and 35ClF(37ClF) –Kr. adequate to determine the residual energy and entropy as well as their volume derivatives. The temperature dependence of the derivatives of the energy and entropy supports the contention that the equation of state is that for segments having a steep repulsive potential of the form e (σ/r)n with a mean field containing the attractive po...

Close‐coupled and coupled‐states calculation of shear viscosity SBE cross sections for the H2–He system

The multicanonical basin hopping (MUBH) method, which uses a multicanonical weight in the basin hopping (BH) Monte Carlo method, was found to be very efficient for global optimization of large-scale systems such as Lennard-Jones clusters containing more than 150 atoms. We have implemented an asynchronous parallel version of the MUBH method using the message passing interface (MPI) to take advantage of the full usage of multiprocessors in either a homogeneous or heterogeneous computational environment. Based on the intrinsic properties of the Monte Carlo method, this MPI implementation used the task parallelism to minimize interthread data communication. For a Co nanocluster consisting of N atoms, we have applied the asynchronous multicanonical basin hopping (AMUBH) method (for 181 < N < or = 200), together with BH (for 2 < or = N < 150) and MUBH (for 150 < or = N < or = 180), to search for the molecular configuration of the global energy minimum. AMUBH becomes the only practical computational scheme for locating the energy minimum within realistic computational time for a relatively large cluster.