Filter assisted wavelength conversion with quantum-dot SOAs

The nonlinear operation regimes of quantum-dot semiconductor optical amplifiers (QD-SOAs) are investigated and the ideal filter providing the best all-optical wavelength conversion efficiency is derived theoretically. Results are confirmed by experiments with Q2-factors amounting to 16 dB.

Matrix-assisted diffusion-ordered spectroscopy:application of surfactant solutions to the resolution of isomer spectra

The component spectra of a mixture of isomers with nearly identical diffusion coefficients cannot normally be distinguished in a standard diffusion-ordered spectroscopy (DOSY) experiment but can often be easily resolved using matrix-assisted DOSY, in which diffusion behaviour is manipulated by the addition of a co-solute such as a surfactant. Relatively little is currently known about the conditions required for such a separation, for example, how the choice between normal and reverse micelles affects separation or how the isomer structures themselves affect the resolution. The aim of this study was to explore the application of sodium dodecyl sulfate (SDS) normal micelles in aqueous solution and sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) aggregates in chloroform, at a range of concentrations, to the diffusion resolution of some simple model sets of isomers such as monomethoxyphenols and short chain alcohols. It is shown that SDS micelles offer better resolution where these isomers differ in the position of a hydroxyl group, whereas AOT aggregates are more effective for isomers differing in the position of a methyl group. For both the normal SDS micelles and the less well-defined AOT aggregates, differences in the resolution of the isomers can in part be rationalised in terms of differing degrees of hydrophobicity, amphiphilicity and steric effects. Copyright © 2012 John Wiley & Sons, Ltd.

Matrix-assisted diffusion-ordered spectroscopy:mixture resolution by NMR using SDSmicelles

Diffusion-ordered spectroscopy (DOSY) is a powerful technique for mixture analysis, but in its basic form it cannot separate the component spectra for species with very similar diffusion coefficients. It has been recently demonstrated that the component spectra of a mixture of isomers with nearly identical diffusion coefficients (the three dihydroxybenzenes) can be resolved using matrix-assisted DOSY (MAD), in which diffusion is perturbed by the addition of a co-solute such as a surfactant [R. Evans, S. Haiber, M. Nilsson, G. A. Morris, Anal. Chem. 2009, 81, 4548-4550]. However, little is known about the conditions required for such a separation, for example, the concentrations and concentration ratios of surfactant and solutes. The aim of this study was to explore the concentration range over whichmatrix-assisted DOSY using the surfactant SDS can achieve diffusion resolution of a simple model set of isomers, the monomethoxyphenols. The results show that the separation is remarkably robust with respect to both the concentrations and the concentration ratios of surfactant and solutes, supporting the idea that MAD may become a valuable tool formixture analysis. © 2010 John Wiley & Sons, Ltd.

Isomer resolution by micelle-assisted diffusion-ordered spectroscopy

Diffusion-ordered NMR spectroscopy ("DOSY") is a useful tool for the identification of mixture components. In its basic form it relies on simple differences in hydrodynamic radius to distinguish between different species. This can be very effective where species have significantly different molecular sizes, but generally fails for isomeric species. The use of surfactant co-solutes can allow isomeric species to be distinguished by virtue of their different degrees of interaction with micelles or reversed micelles. The use of micelle-assisted DOSY to resolve the NMR spectra of isomers is illustrated for the case of the three dihydroxybenzenes (catechol, resorcinol, and hydroquinone) in aqueous solution containing sodium dodecyl sulfate micelles, and in chloroform solution containing AOT reversed micelles. © 2009 American Chemical Society.

Optimal designs of collaborative relay-assisted multiuser beamforming for cellular systems

With careful calculation of signal forwarding weights, relay nodes can be used to work collaboratively to enhance downlink transmission performance by forming a virtual multiple-input multiple-output beamforming system. Although collaborative relay beamforming schemes for single user have been widely investigated for cellular systems in previous literatures, there are few studies on the relay beamforming for multiusers. In this paper, we study the collaborative downlink signal transmission with multiple amplify-and-forward relay nodes for multiusers in cellular systems. We propose two new algorithms to determine the beamforming weights with the same objective of minimizing power consumption of the relay nodes. In the first algorithm, we aim to guarantee the received signal-to-noise ratio at multiusers for the relay beamforming with orthogonal channels. We prove that the solution obtained by a semidefinite relaxation technology is optimal. In the second algorithm, we propose an iterative algorithm that jointly selects the base station antennas and optimizes the relay beamforming weights to reach the target signal-to-interference-and-noise ratio at multiusers with nonorthogonal channels. Numerical results validate our theoretical analysis and demonstrate that the proposed optimal schemes can effectively reduce the relay power consumption compared with several other beamforming approaches. © 2012 John Wiley & Sons, Ltd.

Anterior segment anomalies and effects on visual quality

The tear film, cornea and lens dictate the refractive power of the eye and the retinal image quality is principally defined by diffraction, whole eye wavefront error, scatter, and chromatic aberration. Diffraction and wave aberration are fundamentally pupil diameter dependent; however scatter can be induced by refractive surgery and in the normal ageing eye becomes an increasingly important factor defining retinal image quality. The component of visual quality most affected by the tear film, refractive surgery and multifocal contact and intraocular lenses is the wave aberration of the eye. This body of work demonstrates the effects of each of these anomalies on the visual quality of the eye. When assessing normal or borderline self-diagnosed dry eye subjects using aberrometry, combining lubricating eye drops and spray does not offer any benefit over individual products. However, subjects perceive a difference in comfort for all interventions after one hour. Total higher order aberrations increase after laser assisted sub-epithelial keratectomy performed using a solid-state laser on myopes, but this causes no significant decrease in contrast sensitivity or increase in glare disability. Mean sensitivity and reliability indices for perimetry were comparable to pre-surgery results. Multifocal contact lenses and intraocular lenses are designed to maximise vision when the patient is binocular, so any evaluation of the eyes individually is confounded by reduced individual visual acuity and visual quality. Different designs of aspheric multifocal contact lenses do not provide the same level of visual quality. Multifocal contact lenses adversely affect mean deviation values for perimetry and this should be considered when screening individuals with multifocal contact or intraocular lenses. Photographic image quality obtained through a multifocal contact or intraocular lens appears to be unchanged. Future work should evaluate the effect of these anomalies in combination; with the aim of providing the best visual quality possible and supplying normative data for screening purposes.

The design and implementation of a computer-assisted instruction system in a university environment

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Co-propagating cladding modes coupling assisted by tilted Bragg grating with large tilting angle

We report for the first time forward propagating cladding modes coupling by using tilted gratings. The spectral responses of these gratings were investigated and their thermal characteristics and sensitivity to environmental refractive index were evaluated.

Students' perceptions of computer assisted learning:an empirical study

Research indicates that although students are the ultimate 'beneficiaries of Information and Communication Technology (ICT)-based' higher education learning their voices have been neglected in its development. This paper attempts to redress this imbalance by illuminating students' perceptions of the use of Computer Assisted Learning (CAL) in an undergraduate accounting module. The findings suggest that students are in favour of using EQL in a supportive role only. Interviewees rejected the idea of replacing human tutors with machine tutors and they believed that most of their learning occurs in tutorials and ranked these as the most important component of the module.

Effect of mean stress on hydrogen assisted fatique crack propagation in duplex stainless steel

Hydrogen assisted subcritical cleavage of the ferrite matrix occurs during fatigue of a duplex stainless steel in gaseous hydrogen. The ferrite fails by a cyclic cleavage mechanism and fatigue crack growth rates are independent of frequency between 0.1 and 5 Hz. Macroscopic crack growth rates are controlled by the fraction of ferrite grains cleaving along the crack front, which can be related to the maximum stress intensity, Kmax. A superposition model is developed to predict simultaneously the effects of stress intensity range (ΔK) and K ratio (Kmin/Kmax). The effect of Kmax is rationalised by a local cleavage criterion which requires a critical tensile stress, normal to the {001} cleavage plane, acting over a critical distance within an embrittled zone at the crack tip. © 1991.

Temperature effects on the mechanism of time independent hydrogen assisted fatigue crack propagation in steels

The effects of temperature on hydrogen assisted fatigue crack propagation are investigated in three steels in the low-to-medium strength range; a low alloy structural steel, a super duplex stainless steel, and a super ferritic stainless steel. Significant enhancement of crack growth rates is observed in hydrogen gas at atmospheric pressure in all three materials. Failure occurs via a mechanism of time independent, transgranular, cyclic cleavage over a frequency range of 0.1-5 Hz. Increasing the temperature in hydrogen up to 80°C markedly reduces the degree of embrittlement in the structural and super ferritic steels. No such effect is observed in the duplex stainless steel until the temperature exceeds 120°C. The temperature response may be understood by considering the interaction between absorbed hydrogen and micro-structural traps, which are generated in the zone of intense plastic deformation ahead of the fatigue crack tip. © 1992.

In-situ XPS study on the reducibility of Pd-promoted Cu/CeO2 catalysts for the oxygen-assisted water-gas-shift reaction

Cu/CeO2, Pd/CeO2, and CuPd/CeO2 catalysts were prepared and their reduction followed by in-situ XPS in order to explore promoter and support interactions in a bimetallic CuPd/CeO2 catalyst effective for the oxygen-assisted water-gas-shift (OWGS) reaction. Mutual interactions between Cu, Pd, and CeO2 components all affect the reduction process. Addition of only 1 wt% Pd to 30 wt% Cu/CeO2 greatly enhances the reducibility of both dispersed CuO and ceria support. In-vacuo reduction (inside XPS chamber) up to 400 °C results in a continuous growth of metallic copper and Ce3+ surface species, although higher temperatures results in support reoxidation. Supported copper in turn destabilizes metallic palladium metal with respect to PdO, this mutual perturbation indicating a strong intimate interaction between the Cu–Pd components. Despite its lower intrinsic reactivity towards OWGS, palladium addition at only 1 wt% loading significantly improved CO conversion in OWGS reaction over a monometallic 30 wt% Cu/CeO2 catalysts, possibly by helping to maintain Cu in a reduced state during reaction.

Plasma assisted femtosecond laser inscription in dielectrics

Principles of the femtosecond fabrication of the optoelectronic components in glass are explained and illustrated by examples of the in-bulk writing. The results of the experimental investigation of the dependence of the induced index change on the pulse energy and the numerical modelling of the corresponding laser-glass interaction are presented. The distribution of the plasma density is simulated that may bridge the gap between the models of the pulse propagation and the induced permanent refractive index change. © 2006 American Institute of Physics.