Characterization of longitudinal acoustic waves in a fiber using an extrinsic Fabry-Perot interferometer

Longitudinal acoustical modes propagating in a fiber are characterized using an extrinsic Fabry-Perot interferometer. The method allows the detection of the fiber axial deflection peaks which correspond to the acoustic resonances. The method can be further used for estimating properties of acoustic excited fibers, such as acoustic frequency and wavelength, which are essential in the design of acousto-optic devices employing Bragg gratings

Fiber Fabry-Perot interferometer sensor for measuring resonances of piezoelectric elements

The development of a fiber extrinsic Fabry-Perot interferometer for measuring vibration amplitude and resonances of piezoelectric elements is reported. The signal demodulation method based on the use of an optical spectrum analyzer allows the measurement of displacements and resonances with high resolution. The technique consists basically in monitoring changes in the intensity or the wavelength of a single interferometric fringe at a point of high sensitivity in the sensor response curve. For sensor calibration, three signal processing techniques were employed. Vibration amplitude measurement with 0.84 nm/V sensitivity and the characterization of the piezo resonance is demonstrated

Vortex configurations and metastability in mesoscopic superconductors

The vortex dynamics in mesoscopic superconducting cylinders with rectangular cross section under an axially applied magnetic field is investigated in the multivortex London regime. The rectangles considered range from a square up to an infinite slab. The flux distribution and total flux carried by a vortex placed in an arbitrary position of the sample is calculated analytically by assuming Clem's solution for the vortex core. The Bean-Livingston energy barrier is also analytically calculated in this framework. A Langevin algorithm simulates the flux penetration and dynamical evolution of the vortices as the external field is slowly cycled. The simulated magnetization process is governed by metastable states. The magnetization curves are hysteretic, with paramagnetic response in part of the downward branch, and present a series of peaks corresponding to the entry or expulsion of a single vortex. For elongated rectangles, the vortices arrange themselves into parallel vortex chains and an additional modulation of the magnetization, corresponding to creation or destruction of a vortex chain, comes out.Comment: 7 pages, 4 figures. Presented on the III European Conference on Vortex Matter in Superconductors, Crete, 2003. To appear in Physica

Vortices in superconducting strips: interplay between surface effects and the pinning landscape

Vortices in a narrow superconducting strip with a square array of pinning sites are studied. The interactions of vortices with other vortices and with external sources (applied magnetic field and transport current) are calculated via a screened Coulomb model. The edge barrier is taken into account and shown to have an important role on the system dynamics. Numerical simulations in this approach show that the field dependent magnetic moment presents peaks corresponding to history dependent configurations of the vortex lattice. Some effects of the edge barrier on the I-V characteristics are also reported.Comment: 4 pages, 3 figure

Strumigenys Fairchildi Brown, 1961 (formicidae, Myrmicinae): First Record Of This Rarely Collected Ant From Brazil

Strumigenys fairchildi Brown, 1961 is recorded for the first time in Brazil. This ant species was previously known only from a few specimens collected in Costa Rica, Panama and Ecuador. The worker S. fairchildi was collected at the Parque Estadual do Cristalino, a continuous area of Amazon tropical rain forest protected for biodiversity conservation in the municipality of Novo Mundo, Mato Grosso state. In addition, we present a distribution map and high-resolution images of the worker. © 2016 Check List and Authors.12

Atomic Charge Transfer-counter Polarization Effects Determine Infrared Ch Intensities Of Hydrocarbons: A Quantum Theory Of Atoms In Molecules Model

Atomic charge transfer-counter polarization effects determine most of the infrared fundamental CH intensities of simple hydrocarbons, methane, ethylene, ethane, propyne, cyclopropane and allene. The quantum theory of atoms in molecules/charge-charge flux-dipole flux model predicted the values of 30 CH intensities ranging from 0 to 123 km mol-1 with a root mean square (rms) error of only 4.2 km mol-1 without including a specific equilibrium atomic charge term. Sums of the contributions from terms involving charge flux and/or dipole flux averaged 20.3 km mol-1, about ten times larger than the average charge contribution of 2.0 km mol-1. The only notable exceptions are the CH stretching and bending intensities of acetylene and two of the propyne vibrations for hydrogens bound to sp hybridized carbon atoms. Calculations were carried out at four quantum levels, MP2/6-311++G(3d,3p), MP2/cc-pVTZ, QCISD/6-311++G(3d,3p) and QCISD/cc-pVTZ. 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Multi-wavelength reflection spectra from an acousto-optic modulated fiber Bragg grating in a highly birefringent suspended core fiber

The interaction of a fiber Bragg grating and longitudinal acoustic waves in a highly birefringent suspended-core fiber is investigated for the realization of a multi-wavelength reflection property. The modulated grating couples power from the fast and slow polarization modes to shifted superposed modes supported by the grating. The grating reflectivity of the superposed modes are tuned by the voltage of an electrical signal. Up to five different wavelength reflection peaks have been generated indicating new possibilities for compact and fast fiber-integrated multi-wavelength dynamic filters

Electrically Tunable Multiwavelength Bragg Grating Filter Acoustically Induced in a Highly Birefringent Suspended Core Fiber

Multiwavelength reflection spectra induced by an acoustically modulated fiber Bragg grating (FBG) in a highly birefringent suspended core fiber are experimentally investigated. Longitudinal acoustic waves interacting with a grating generate side lobes in the reflectivity spectrum and produce a superposed reflection band. The reflectivity of up to five wavelength peaks can be actively tuned by the voltage of the electrical signal inducing the acoustic waves. This indicates new possibilities for compact and fast multiwavelength dynamic and fiber-integrated reflection filters

Reflectivity and Bandwidth Modulation of Fiber Bragg Gratings in a Suspended Core Fiber by Tunable Acoustic Waves

The acousto-optic modulation of fiber Bragg gratings in a four-hole suspended core fiber is experimentally demonstrated. Strong modulations with a reflectivity amplitude decrease by up to 67% and a 57% bandwidth increase in the Bragg resonance are obtained for gratings of 0.26- and 1-nm 3-dB bandwidths, respectively. The reduction of the required acoustic power for achieving the acousto-optic modulation compared to conventional solid-core single-mode fibers points to more efficient modulator devices in suspended core fibers

Proton exchange membrane fuel cell operation and degradation in short-circuit.

International audienceThis paper presents an experimental study dealing with operation and degradation during an electrical short circuit of a proton exchange membrane fuel cell stack. The physical quantities in the fuel cell (electrical voltage and current, gas stoichiometry, pressures, temperatures and gas humidity) are studied before, during and after the failure. After a short circuit occurs, a high peak of current appears but decreases to stabilize in a much lower value. The voltage drops in all the cells and even some cells presents reversal potentials. The degradation is quantified by using electrochemical impedance spectroscopy