288 research outputs found
Negative reflection of elastic guided waves in chaotic and random scattering media
The propagation of waves in complex media can be harnessed either by taming
the incident wave-field impinging on the medium or by forcing waves along
desired paths through its careful design. These two alternative strategies have
given rise to fascinating concepts such as time reversal or negative
refraction. Here, we show how these two processes are intimately linked through
the negative reflection phenomenon. A negative reflecting mirror converts a
wave of positive phase velocity into its negative counterpart and vice versa.
In this article, we experimentally demonstrate this phenomenon with elastic
waves in a 2D billiard and in a disordered plate by means of laser
interferometry. Despite the complexity of such configurations, the negatively
reflected wave field focuses back towards the initial source location, thereby
mimicking a phase conjugation operation while being a fully passive process.
The super-focusing capability of negative reflection is also highlighted in a
monochromatic regime. The negative reflection phenomenon is not restricted to
guided elastic waves since it can occur in zero-gap systems such as photonic
crystals, chiral metamaterials or graphene. Negative reflection can thus become
a tool of choice for the control of waves in all fields of wave physics.Comment: 9 pages, 6 figure
Experimental phase-space-based optical amplification of scar modes
Waves billiard which are chaotic in the geometrical limit are known to
support non-generic spatially localized modes called scar modes. The
interaction of the scar modes with gain has been recently investigated in
optics in micro-cavity lasers and vertically-cavity surface-emitting lasers.
Exploiting the localization properties of scar modes in their wave analogous
phase space representation, we report experimental results of scar modes
selection by gain in a doped D-shaped optical fiber
Optical Scar in a chaotic fibre
We propose to use a multimode optical fiber with a D-shaped cross section as
a privileged system to image wavefunctions of a chaotic system. Scar modes are
in particular the sub ject of our investigations. We study their imprints on
the statistics of intensity and we show how the introduction of a localized
gain region in the fiber is used to perform a selective excitation of scar
modes.Comment: 13 page
Adaptive FPGA NoC-based Architecture for Multispectral Image Correlation
An adaptive FPGA architecture based on the NoC (Network-on-Chip) approach is
used for the multispectral image correlation. This architecture must contain
several distance algorithms depending on the characteristics of spectral images
and the precision of the authentication. The analysis of distance algorithms is
required which bases on the algorithmic complexity, result precision, execution
time and the adaptability of the implementation. This paper presents the
comparison of these distance computation algorithms on one spectral database.
The result of a RGB algorithm implementation was discussed
Gain-controlled wave chaos in a chaotic optical fibre
International audienceIn this paper, we present a non-standard fibre amplifier specially designed to amplify scar modes of a multimode chaotic optical fibre. More precisely, we introduce Ytterbium in the optical fibre as a gain medium localised on the maximum of intensity of the scar modes. After briefly recalling the relevance of a chaotic optical fibre as a device to visualise quantum chaos, we describe the amplification process of scars. We present some numerical results that demonstrate the selective amplification of scar modes, with an amplification rate proportional to the overlap between these modes and the gain area
Negative refraction of Lamb modes: A theoretical study
This paper provides a theoretical investigation of negative refraction and
focusing of elastic guided waves in a free-standing plate with a step-like
thickness change. Under certain conditions, a positive phase velocity (forward)
Lamb mode can be converted into a negative phase velocity (backward) mode at
such interface, giving rise to negative refraction. A semi-analytical model is
developed in order to study the influence of various parameters such as the
material Poisson's coefficient, the step-like thickness, the frequency and the
incidence angle. To this end, all the Lamb and shear horizontal propagating
modes, but also a large number of their inhomogeneous and evanescent
counterpart,s are taken into account. The boundary conditions applied to the
stress-displacement fields at the thickness step yields an equation system. Its
inversion provides the transmission and reflection coefficients between each
mode at the interface. The step-like thickness and Poisson's ratio are shown to
be key parameters to optimize the negative refraction process. In terms of
material, Duralumin is found to be optimal as it leads to a nearly perfect
conversion between forward and backward modes over broad frequency and angular
ranges. An excellent focusing ability is thus predicted for a flat lens made of
two symmetric thickness steps. Theoretical results are confirmed by a numerical
FDTD simulation and experimental measurements made on an optimized Duralumin
flat lens by means of laser interferometry. This theoretical study paves the
way towards the optimization of elastic devices based on negative refraction,
in particular for cloaking or super-focusing purposes.Comment: 8 pages, 6 figure
Evaluation and Design Space Exploration of a Time-Division Multiplexed NoC on FPGA for Image Analysis Applications
The aim of this paper is to present an adaptable Fat Tree NoC architecture
for Field Programmable Gate Array (FPGA) designed for image analysis
applications. Traditional NoCs (Network on Chip) are not optimal for dataflow
applications with large amount of data. On the opposite, point to point
communications are designed from the algorithm requirements but they are
expensives in terms of resource and wire. We propose a dedicated communication
architecture for image analysis algorithms. This communication mechanism is a
generic NoC infrastructure dedicated to dataflow image processing applications,
mixing circuit-switching and packet-switching communications. The complete
architecture integrates two dedicated communication architectures and reusable
IP blocks. Communications are based on the NoC concept to support the high
bandwidth required for a large number and type of data
Selective amplification of scars in a chaotic optical fiber
In this letter we propose an original mechanism to select scar modes through
coherent gain amplification in a multimode D-shaped fiber. More precisely, we
numerically demonstrate how scar modes can be amplified by positioning a gain
region in the vicinity of specific points of a short periodic orbit known to
give rise to scar modes
Mean Oriented Riesz Features for Micro Expression Classification
Micro-expressions are brief and subtle facial expressions that go on and off
the face in a fraction of a second. This kind of facial expressions usually
occurs in high stake situations and is considered to reflect a human's real
intent. There has been some interest in micro-expression analysis, however, a
great majority of the methods are based on classically established computer
vision methods such as local binary patterns, histogram of gradients and
optical flow. A novel methodology for micro-expression recognition using the
Riesz pyramid, a multi-scale steerable Hilbert transform is presented. In fact,
an image sequence is transformed with this tool, then the image phase
variations are extracted and filtered as proxies for motion. Furthermore, the
dominant orientation constancy from the Riesz transform is exploited to average
the micro-expression sequence into an image pair. Based on that, the Mean
Oriented Riesz Feature description is introduced. Finally the performance of
our methods are tested in two spontaneous micro-expressions databases and
compared to state-of-the-art methods
Système de contrôle et de décision par thermographie active infrarouge : application à la détection d'occlusion de colle à l'intérieur de bouchons plastiques
- La thermographie infrarouge est une méthode efficace de contrôle et d'évaluation non destructive. La technique de thermographie active permet de mettre en évidence les différences de structures internes grâce à l'étude de l'évolution et de la propagation d'une impulsion thermique appliquée sur la surface du matériau à inspecter. Un système autonome de contrôle par thermographie active infrarouge qui détecte et caractérise des occlusions de colle à l'intérieur de bouchons plastiques est présenté
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