Theory of the frequency comb output from a femtosecond fiber laser

The output of a femtosecond fiber laser will form a frequency comb that can be phase-locked through feedback to the cavity length and pump power. A perturbative theory is developed to describe this frequency comb output, in particular for a solitonic fiber laser. The effects of resonant dispersion, saturation of the self-amplitude modulation, cavity loss, third-order dispersion, Raman scattering, self-phase modulation, and self-steepening on the spacing and offset of the fiber-laser frequency comb are given. The mechanisms by which the pump power, cavity length and cavity loss control the frequency comb spacing and offset are identified. Transfer functions are derived for the comb response to change in cavity length, pump power or cavity loss. This theory can potentially be applied to other frequency comb sources as well.     

Tunable near-infrared femtosecond soliton generation in photoniccrystal fibres

Femosecond Raman soliton generation, tunable from 850-1050 nm, has been demonstrated by pumping untapered photonic crystal fibre with 110 fs pulses from a modelocked Ti:sapphire laser. Experimental results are in excellent agreement with the results of numerical simulations using an extended nonlinear Schrodinger equation     

Verification/validation of performance simulation using ANSI X3.102measurements

An approach to the simulation of computer networks, using SLAM II, version 3.2, is described. The implementation of a version of ANSI X3.102 standard, which allows for the validation/verification of simulation results using actual measurements, is discussed. An example of using ANSI X3.102 to verify simulation predictions is given     

Effect of the Structure of Dislocation Boundaries On Yield Strength

Two simple types of dislocation distribution were introduced into zinc crystal specimens and the effect of each on the stress-strain curve was investigated. Quantitative results were obtained for simple edge dislocation arrays and for an array of screw dislocations lying in the slip plane.     

MR angiography of the portal venous system: techniques, interpretation, and clinical applications

Magnetic resonance (MR) angiography is a noninvasive means of assessing the portal venous system that has potential advantages over currently used modalities. Time-of-flight and phase-contrast MR angiography are useful techniques that differ fundamentally in their means of data acquisition but are comparable in their ability to demonstrate normal anatomy as well as abnormalities of the portal venous system. Occasionally, artifacts caused by respiratory motion, implanted metallic devices or surgical clips, in-plane saturation, or areas of complex flow are seen at MR angiography of the portal venous system. However, most artifacts can easily be identified as such and either remedied or ignored. In addition, the suppression of signal from surrounding soft tissues may result in poor detection of parenchymal lesions. The utility of standard projection angiograms and source images can be increased through the use of intravenously administered contrast material and postprocessing techniques such as partial-volume maximum intensity projection reconstructions and shaded surface renderings. In addition to providing information on portal venous anatomy and portosystemic collateral vessels, MR angiography of the portal vein has clinical application in portal venous thrombosis and stenosis, liver transplantation, and the evaluation and planning of surgical and transjugular intrahepatic portosystemic shunts.     

Uncertain responses by humans and Rhesus monkeys (Macaca mulatta) in a psychophysical same–different task.

The authors asked whether animals, like humans, use an uncertain response adaptively to escape indeterminate stimulus relations. Humans and monkeys were placed in a same-different task, known to be challenging for animals. Its difficulty was increased further by reducing the size of the stimulus differences, thereby making many same and different trials difficult to tell apart. Monkeys do escape selectively from these threshold trials, even while coping with 7 absolute stimulus levels concurrently. Monkeys even adjust their response strategies on short time scales according to the local task conditions. Signal-detection and optimality analyses confirm the similarity of humans' and animals' performances. Whereas associative interpretations account poorly for these results, an intuitive uncertainty construct does so easily. The authors discuss the cognitive processes that allow uncertainty's adaptive use and recommend further comparative studies of metacognition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Deformation behaviour and shape memory effect of near equi-atomic NiTi alloy

The mechanical shape memory effect associated with the martensitic-type transformation which occurs in polycrystalline Ti-50.3 at. % Ni alloy has been investigated using the techniques of transmission and optical microscopy. Deformation of initially partially transformed material within the recoverable strain range was found to occur by: (1) stress-induced transformation of the most favourably oriented existing martensite variants at the expense of adjacent unfavourably oriented variants and retained high temperature phase (2) stress-induced re-orientation of favourably oriented martensite by utilizing the most favourably oriented twin system, and (3) stress-induced twin-boundary migration within the martensite. The reverse transformation during heating restores the original grain structure of the high-temperature phase in a highly coherent manner. It was concluded that deformation modes limited to those involved in the transformation process and the reversibility of the transformation give rise to the memory effect.     

Multilayer model of indium arsenide epilayers

The charge carrier transport coefficients of an inhomogeneous thin semiconductor in a MOS structure were obtained using a multilayer model. The expression for the Hall coefficient of a three-layer system extended to arbitrary strength magnetic fields was used to separate the bulk transport parameters from the parameters describing transport at the two surfaces. Experimentally, a gate voltage was used to vary the surface under the oxide from depletion to accumulation, and the Hall coefficient was measured as a function of magnetic field. The characteristics of the back surface were obtained with the front surface held at the flat-band condition. The variation of the front surface parameters with gate voltage was obtained with the front surface in accumulation. The measurements were made on a MOS structure consisting of an InAs epilayer deposited by vapor phase epitaxy procedures on a semi-insulating GaAs substrate covered by a pyrolytic silicon dioxide insulating layer and an aluminum gate.     

Electromechanical response of single-walled carbon nanotubes to torsional strain in a self-contained device

Nanoscale electronics seeks to decrease the critical dimension of devices in order to improve performance while reducing power consumption. Single-walled carbon nanotubes fit well with this strategy because, in addition to their molecular size, they demonstrate a number of unique electronic, mechanical and electromechanical properties. In particular, theory predicts that strain can have a large effect on the band structure of a nanotube, which, in turn, has an influence on its electron transport properties. This has been demonstrated in experiments where axial strain was applied by a scanning probe. Theory also predicts that torsional strain can influence transport properties, which was observed recently in multiwalled nanotubes. Here we present the first experimental evidence of an electromechanical effect from torsional strain in single-walled nanotubes, and also the first measurements of piezoresistive response in a self-contained nanotube-based nanoelectromechanical structure.