Stochastic phenomena in a fiber Raman amplifier

The interplay of such cornerstones of modern nonlinear fiber optics as a nonlinearity, stochasticity and polarization leads to variety of the noise induced instabilities including polarization attraction and escape phenomena harnessing of which is a key to unlocking the fiber optic systems specifications required in high resolution spectroscopy, metrology, biomedicine and telecommunications. Here, by using direct stochastic modeling, the mapping of interplay of the Raman scattering‐based nonlinearity, the random birefringence of a fiber, and the pump‐to‐signal intensity noise transfer has been done in terms of the fiber Raman amplifier parameters, namely polarization mode dispersion, the relative intensity noise of the pump laser, fiber length, and the signal power. The obtained results reveal conditions for emergence of the random birefringence‐induced resonance‐like enhancement of the gain fluctuations (stochastic anti‐resonance) accompanied by pulse broadening and rare events in the form of low power output signals having probability heavily deviated from the Gaussian distribution.

     

The effect of loading and particle size on the oxygen reaction in CGO impregnated Pt electrodes

Porous platinum electrodes impregnated with Gd _x Ce_1−x O_2−δ (CGO) are investigated to characterise how nano-sized CGO grains affect the oxygen reaction. Impedance measurements were performed at temperatures between 450 and 750 °C and at oxygen partial pressures of 0.2 and 5 × 10⁻⁵ bar for electrodes with various CGO loadings and electrodes annealed at various temperatures. The morphology was characterised by scanning electron microscopy and the CGO grain size was determined from X-ray diffraction peak broadening. The results showed that the polarisation resistance decreased with increasing CGO loading and increasing annealing temperature. CGO facilitates transport of oxygen ions thereby increasing the effective triple-phase boundary.     

Temperature dependence of the magnetization of canted spin structures

Numerous studies of the low-temperature saturation magnetization of ferrimagnetic nanoparticles and diamagnetically substituted ferrites have shown an anomalous temperature dependence. It has been suggested that this is related to freezing of canted magnetic structures. We present models for the temperature dependence of the magnetization of a simple canted spin structure in which relaxation can take place at finite temperatures between spin configurations with different canting angles. We show that the saturation magnetization may either decrease or increase with decreasing temperature, depending on the ratio of the exchange coupling constants. This is in agreement with experimental observations.     

An acousto-optic beamformer

There is a great variety of beamforming techniques that can be used for localization of sound sources. The differences among them usually lie in the array layout or in the specific signal processing algorithm used to compute the beamforming output. Any beamforming system consists of a finite number of transducers, which makes beamforming methods vulnerable to spatial aliasing above a certain frequency. The present work uses the acousto-optic effect, i.e., the interaction between sound and light, to localize sound sources in a plane. The use of a beam of light as the sensing element is equivalent to a continuous line aperture with an infinite number of microphones. This makes the proposed acousto-optic beamformer immune to spatial aliasing. This unique feature is illustrated by means of simulations and experimental results within the entire audible frequency range. For ease of comparison, the study is supplemented with measurements carried out with a line array of microphones.     

Subspace Preconditioned LSQR for Discrete Ill-Posed Problems

We present a novel implementation of a two-level iterative method for the solution of discrete linear ill-posed problems. The algorithm is algebraically equivalent to the two-level Schur complement CG algorithm of Hanke and Vogel, but involves less work per iteration. We review the algorithm, discuss our implementation, and show promising results from numerical experiments that give insight into the proper use of the algorithm.This revised version was published online in October 2005 with corrections to the Cover Date.     

SOFT X-RAY SCANNING MICROSCOPY: ITS PRACTICAL USE FOR ELEMENTAL MAPPING AT THE NSLS U15 BEAMLINE

Scanning soft X-ray microscopy offers several advantages over other forms of X-ray microscopy. It subjects the specimen to the lower doses of radiation and by digitally recording the image, gives direct quantitative information on the absorption of the specimen as a function of position. Elemental maps can be produced easily by comparing images taken at different wavelengths, chosen to exploit X-ray absorption edges or resonances. This technique was used to explore the distribution of calcium in 0.2 μm-thick bone specimens, detecting concentrations of 5% by weight, with a spatial resolution of 0.2 μm. Progress in X-ray sources in the form of undulators and in X-ray optics indicates that soon the range of elements that can be mapped will be extended and the spatial resolution and elemental sensitivity will be improved by an order of magnitude.     

Convergence acceleration of limit periodic continued fractions under asymptotic side conditions

Summary We present a method of convergence acceleration for limitk-periodic continued fractionsK(a _n /1) orK(1/b _n ) satisfying certain asymptotic side conditions. The method represents an improvement of the fixed point modification considered by Thron and Waadeland [8], under these conditions. The regularC-fraction expansions of hypergeometric functions₂ F ₁(a, 1;c; z) and₂ F ₁(a, b; c; z)/₂ F ₁(a, b+1;c+1;z) are examples of continued fractions satisfying these conditions.