Coefficient de Fresnel | EPFL Graph Search

Les coefficients de Fresnel, introduits par Augustin Jean Fresnel (1788-1827), interviennent dans la description du phénomène de réflexion-réfraction des ondes électromagnétiques à l'interface entre deux milieux, dont l'indice de réfraction est différent. Ils expriment les liens entre les amplitudes des ondes réfléchies et transmises par rapport à l'amplitude de l'onde incidente. Généralités On définit le coefficient de réflexion en amplitude r et le coefficient de transmission en amplitude t du champ électrique par : : r = \frac{E_{r}}{E_{i}} : t = \frac{E_{t}}{E_{i}} où Ei, Er et Et sont les amplitudes associées respectivement au champ électrique incident, réfléchi et transmis (réfracté). En général, ces coefficients dépendent :

des indices optiques des milieux d'entrée et de sortie, respectivement n1 et n2
de la fréquence f de l'onde incidence
des angles d'incidence θi=θ1 et de réfraction-transmission θt=θ2,
de la polarisation de

Sound pressure estimation at an electroacoustic transducer's voicing face by way of all-electrical sensing

Hervé Lissek, Florian Sandoz

When it comes to designing active control disposals, sound pressure sensing is usually required. In the framework of active control of acoustic impedance, this work aims at conceiving a digital estimator that processes voltage and current measurement to compute an estimation of the pressure in the vicinity of the diaphragm of an electro-dynamic speaker. After dealing with modeling concerns of the actuator, the bilinear transform is used to obtain equations of two IIR filters. An adaptive identification using the LMS algorithm and a SVD-based identification are tested to compute filters coefficient, both leading to poor experimental results. Accordingly, the actual transfer functions of the speaker are measured and optimal IIR filters are designed using the simulated annealing algorithm. It is then shown that good results can be achieved experimentally.

2008

Compact fast-steering tip/tilt laser scanner for high power material processing applications

Moustapha Hafez

The common configuration for deflecting laser beams in two dimensions is based on two single-axis galvanometric scanners placed perpendicular to each other. Over recent years, these fast scanners have proved to be both very reliable and very accurate. They are widely available in the marketplace and have wide-ranging technical specifications. However, such a configuration is rather bulky and can not satisfy the needs of industry which seeks compact solutions for an easy integration of these scanning heads into their production lines. In addition, two separate mirrors leads to optical aberrations when these scanning heads are used in conjunction with scanning lenses, which are specially designed for a single entrance pupil. Therefore, a compact fast-steering tip/tilt laser scanner with a single mirror would act as a key component in diverse applications, e.g. in material processing, astronomy, intersatellite laser communications, imaging systems, bio-medical and ophthalmologic applications. The compact two-axis tilt laser scanner presented in this work can perform a variety of functions such as image tracking, beam stabilization and alignment, line-of-sight pointing, and scene scanning. Designing such a scanner required a multi-disciplinary approach involving optics, mechanics, electromagnetics, sensors and control theories. The scanner is composed of one single mirror with a large active area and has a single point of rotation for the two axes of tilt. Moreover, the center of gravity coincides with the center of rotation to reach optimum dynamic behavior. The scanner's compact dimensions allow an easy integration in the various optical systems. Furthermore, the mirror is easily interchangeable which gives greater flexibility in the choice of the laser source. Another major advantage of the proposed design is its ability to be scaled down to miniaturized versions. The scaling depends mainly on the active surface of the mirror, which is generally specified by the application. The larger the mirror, the higher its inertia and the larger are the actuators in order to guarantee high performance. Two pairs of push-pull linear electromagnetic actuators are used to drive the mirror of the scanner. The mirror suspension is based on a sliding bearing composed of a cone-ball contact with optimized friction and wear behavior. A magnetic pre-load force is used to hold the mirror and to give sufficient rotational stiffness. Furthermore, a position transducer based on a photodetector and a miniature laser pen is integrated in the module and is used for closed loop feedback positioning. The mirror can be tilted around both axes by ± 61mrad (± 3.5°) with an accuracy better than 50μrad. Moreover, a differential resolution in the order of 2.5μrad, and settling times for maximum tip and tilt deflections of 10ms and 14ms respectively are achieved. The overall volume of the scanner is 30 x 40 x 50 mm3, and its total weight does not exceed 90g. Design, simulations and experimental investigations were carried out in this work in order to optimize the different components of the tip/tilt scanner. These components are: the motors, the bearing system, the position transducer and the feedback control loop. An analytical model that allows an optimization of the geometrical parameters for an electromagnetic actuator based on rectangular coils and magnets is proposed. This model proved to be very reliable and can be extended to the design of a variety of electromagnetic driving systems. In addition, an extensive analysis of the beam path distortions generated when a single mirror is pivoted around two axes is covered. Furthermore, correcting equations used to compensate for the systematic errors resulting from this configuration are proposed. The tip/tilt scanner is suited to a variety of applications in industrial material laser processing. These applications are not just restricted to conventional processes like welding, cutting, drilling, and marking, but also to new technologies such as micro-manipulation of parts by laser. In this new approach, bending and shrinkage of the material result due to laser irradiation, allowing fast adjustment and positioning of micro-mechanical and opto-mechanical components with sub-micron accuracy. Furthermore, local annealing of shape memory alloys, leading to stiffness variation in monolithic parts such as micro-grippers, represents a very innovative application in smart materials. On the other hand, the scanner is also a key component in bio-medical and ophthalmologic applications. The first measurements carried out with the scanner for optical tomography of the retina appeared very promising. Furthermore, an optical tweezers set-up has been realized using the scanner, allowing easy and flexible manipulation of laser beam trapped cells and molecules.

EPFL2000

Acoustic metamaterial properties of a 2D closed cellular solid with entrained fluid

Vladimir Dorodnitsyn

Metamaterials are often defined as artificial compositions designed to exhibit desired physical properties. These materials attract a lot of research attention due to unusual behavior that may not yet have been seen in nature. Although there is no commonly accepted definition for metamaterials, they are typically associated with peculiar macroscale properties resulting from their substructure. The electromagnetic metamaterial concept was first developed in 1968. As the wave theory is similar in every field, the achievements in optics were reflected in acoustics several decades later. This allowed developing acoustic metamaterials with such extraordinary properties as negative refractive index, negative bulk modulus and mass density, acoustic lensing, sound wave spectral decomposition, and acoustic bandgaps. All of those features are not only attractive scientifically, but are of interest for plenty of potential applications, including sound and vibration insulation, waveguiding, audible and high-frequency filtering, and even seismic absorption. On the other hand, cellular solids and saturated porous media have been studied for a long time. These media are abundant in nature as granular soils, wood, rocks, bones, and foams. Wave analysis in such environments typically requires some crucial assumptions which do not allow extending a theory to other configurations. An example of such a constraint is the openness of the cells in a medium. Many porous media applications are found in geophysics - particularly gas and oil extractions - the permeability of the cells plays an important role. The ad-hoc dynamic models for such media operate only with open-cell configuration. Moreover, the study is limited to the low-frequency analysis, omitting the influence of wave scattering. The latter, however, is the key source of dispersion in acoustic metamaterials. Scattering may have two origins, geometrical or resonant. Bragg's scattering is determined by the geometrical configuration, such that constructive interference occurs only when the incident wave matches the characteristic size of a unit cell. This makes such systems practically inconvenient. The concept of resonant scattering, introduced about a decade ago, has much fewer limitations and is mostly determined by the dynamics of matrix inclusions. In this thesis, a closed-cell cellular solid with thin vibrating walls and fluid-filled cells is proposed as a new class of acoustic metamaterials. First, the dynamics of a prototypical square cell is investigated numerically considering periodic boundary conditions and taking into account fluid-structure interaction. The results are compared to Biot's theory of saturated porous media in the limit of a closed-cells system. The proposed configuration is studied with respect to dispersion sources, showing the presence of local resonant behavior for different combinations of relative density and entrained fluid. Surprisingly, semi-analytical models can be used to provide a bottom-up explanation of the structure's dynamic behavior. The presence of two pressure waves, slow and fast, is confirmed numerically and analytically. Finally, an experimental proof-of-concept was carried out. Periodic cellular solids represent a versatile acoustic metamaterial platform characterized by low cost, simple, scalable design, which makes possible achieving the desired macroscopic behavior using different types of fluids and bulk materials.