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171.
A novel architecture for an optical time-division switch is investigated and demonstrated at a bit rate of 625 Mbit/s. The self-clocked configuration eliminates some of the limitations found in previous optical implementations of time-division switches: the switch operation is asynchronous, no read gate is required, and an optimal number of delay lines is used. Furthermore, this paper reports the first experimental demonstration of optical space-sorting demultiplexing. 相似文献
172.
Philippe F. Smet Koen Van den Eeckhout Adrie J.J. Bos Erik van der Kolk Pieter Dorenbos 《Journal of luminescence》2012,132(3):682-689
The evaluation of persistent phosphors is often focused on the processes right after the excitation, namely on the shape of the afterglow decay curve and the duration of the afterglow, in combination with thermoluminescence glow curve analysis. In this paper we study in detail the trap filling process in europium-doped alkaline earth silicon nitrides (Ca2Si5N8:Eu, Sr2Si5N8:Eu and Ba2Si5N8:Eu), i.e., how the persistent luminescence can be induced. Both the temperature at which the phosphors are excited and the spectral distribution of the excitation light on the ability to store energy in the phosphors' lattices are investigated. We show that for these phosphors this storage process is thermally activated upon excitation in the lower 5d excited states of Eu2+, with the lowest thermal barrier for europium doped Ca2Si5N8. Also, the influence of co-doping with thulium on the trap filling and afterglow behavior is studied. Finally there exists a clear relation between the luminescence quenching temperature and the trap filling efficiency. The latter relation can be utilized to select new efficient 5d–4f based afterglow phosphors. 相似文献
173.
François Quitin Claude Oestges Ali Panahandeh François Horlin Philippe De Doncker 《Physical Communication》2012,5(4):308-316
Polarized multi-antenna systems are an effective solution for reducing inter-antenna spacing while still maintaining low inter-antenna correlation. Traditionally, only dual-polarized antenna systems are used for polarized transceivers. In this paper, tri-polarized antenna systems are investigated. Starting from the polarization mechanisms in the wireless propagation channel, it is shown that dual-polarized MIMO systems show high sensitivity to the transmitter and receiver orientation, which may be very critical in practical applications. Tri-polarized MIMO systems are introduced as a solution to obtain a robust MIMO performances, which are independent of the transmitter and receiver orientation. The performances of dual- and tri-polarized MIMO systems are evaluated on real-world measured channels, and the limits of each of these systems is highlighted. 相似文献
174.
Nastasia Grubic Philippe G. LeFloch Cristinel Mardare 《Journal de Mathématiques Pures et Appliquées》2014
To begin with, we identify the equations of elastostatics in a Riemannian manifold, which generalize those of classical elasticity in the three-dimensional Euclidean space. Our approach relies on the principle of least energy, which asserts that the deformation of the elastic body arising in response to given loads minimizes over a specific set of admissible deformations the total energy of the elastic body, defined as the difference between the strain energy and the potential of the loads. Assuming that the strain energy is a function of the metric tensor field induced by the deformation, we first derive the principle of virtual work and the associated nonlinear boundary value problem of nonlinear elasticity from the expression of the total energy of the elastic body. We then show that this boundary value problem possesses a solution if the loads are sufficiently small (in a sense we specify). 相似文献
175.
Roux P Sabra KG Kuperman WA Roux A 《The Journal of the Acoustical Society of America》2005,117(1):79-84
It has been experimentally demonstrated that the Green's function between two points could be recovered using the cross-correlation function of the ambient noise measured at these two points. This paper investigates the theory behind this result in the simple case of a homogeneous medium with attenuation. 相似文献
176.
Carmona Philippe; Petit Frederique; Yor Marc 《Journal London Mathematical Society》1998,58(1):239-256
The paper shows that the times spent in [0, +) by certain processesY which are defined by perturbations of Brownian motion involvingreflection at maxima and minima are beta distributed. This resultrelies heavily on RayKnight theorems for such perturbedBrownian motions. 相似文献
177.
Monte Carlo methods have extensively been used and studied in the area of stochastic programming. Their convergence properties
typically consider global minimizers or first-order critical points of the sample average approximation (SAA) problems and
minimizers of the true problem, and show that the former converge to the latter for increasing sample size. However, the assumption
of global minimization essentially restricts the scope of these results to convex problems. We review and extend these results
in two directions: we allow for local SAA minimizers of possibly nonconvex problems and prove, under suitable conditions,
almost sure convergence of local second-order solutions of the SAA problem to second-order critical points of the true problem.
We also apply this new theory to the estimation of mixed logit models for discrete choice analysis. New useful convergence
properties are derived in this context, both for the constrained and unconstrained cases, and associated estimates of the
simulation bias and variance are proposed.
Research Fellow of the Belgian National Fund for Scientific Research 相似文献
178.
Philippe Gravejat Mathieu Lewin éric Séré 《Communications in Mathematical Physics》2009,286(1):179-215
We study the reduced Bogoliubov-Dirac-Fock (BDF) energy which allows to describe relativistic electrons interacting with the
Dirac sea, in an external electrostatic potential. The model can be seen as a mean-field approximation of Quantum Electrodynamics
(QED) where photons and the so-called exchange term are neglected. A state of the system is described by its one-body density
matrix, an infinite rank self-adjoint operator which is a compact perturbation of the negative spectral projector of the free
Dirac operator (the Dirac sea). We study the minimization of the reduced BDF energy under a charge constraint. We prove the
existence of minimizers for a large range of values of the charge, and any positive value of the coupling constant α. Our
result covers neutral and positively charged molecules, provided that the positive charge is not large enough to create electron-positron
pairs. We also prove that the density of any minimizer is an L
1 function and compute the effective charge of the system, recovering the usual renormalization of charge: the physical coupling
constant is related to α by the formula αphys ≃ α(1 + 2α/(3π) log Λ)−1, where Λ is the ultraviolet cut-off. We eventually prove an estimate on the highest number of electrons which can be bound
by a nucleus of charge Z. In the nonrelativistic limit, we obtain that this number is ≤ 2Z, recovering a result of Lieb. This work is based on a series of papers by Hainzl, Lewin, Séré and Solovej on the mean-field
approximation of no-photon QED. 相似文献
179.
180.