38 research outputs found
Phase noise of a microwave photonic channel: direct-current versus external electro-optic modulation
We characterize the phase noise of a microwave photonic channel, where a 10
GHz signal is carried by an intensity-modulated light beam over a short optical
fiber, and detected. Two options are compared, (i) an electro-optic modulator
(EOM), and (ii) the direct modulation of the laser current. The 1.55~m
laser and the detector are the same. The effect of experimental parameters is
investigated, the main of which are the microwave power and the laser bias
current. The main result is that the upper bound of the phase flicker is
~dBrad in the case of the EOM, limited by the background noise of the
setup. In contrast, with direct modulation of the laser, the flicker is of
to ~dBrad, depending on the laser bias current (50--90~mA),
and the highest noise occurs at the lowest bias. Our results are of interest in
communications, radar systems, instrumentation and metrology.Comment: 5 pages, 5 figures, 1 table, 43 bibliographic reference
Quantitative thermal microscopy using thermoelectric probe in passive mode
International audienceA scanning thermal microscope working in passive mode using a micronic thermocouple probe is presented as a quantitative technique.We show that actual surface temperature distributions of microsystems are measurable under conditions for which most of usual techniques cannot operate. The quantitative aspect relies on the necessity of an appropriate calibration procedure which takes into account of the probe-to-sample thermal interaction prior to any measurement. Besides this consideration that should be treated for any thermal contact probing system, the main advantages of our thermal microscope deal with the temperature available range, the insensitivity to the surface optical parameters, the possibility to image DC, and AC temperature components up to 1 kHz typically and a resolution limit related to near-field behavior
WOod OPtical Scope (WOOPS) : Détermination des propriétés du bois par interaction lumineuse interprétation, mesure et identification des mécanismes et des paramètres d'influence
Le bois sera l'un des matériaux de construction de base du 21ème siècle. Ses propriétés de légèreté, de rigidité, d'isolation et sa capacité à capturer le carbone font de lui un matériau incontournable dans le contexte actuel, où les préoccupations écologiques sont omniprésentes.
Une de ses particularités réside dans sa structure multi-échelle complexe et hétérogène qui résulte de la croissance adaptative et singulière d’un arbre dans son environnement (Figure 1). Comme toute ressource issue du vivant, sa variabilité naturelle est très forte et représente un enjeu essentiel dans sa valorisation comme matériau de construction, d’emballage ou d’aménagement intérieur ou extérieur. Les pays nordiques, aidés par l'homogénéité de leur ressource forestière, ont su industrialiser sa transformation et inonder le marché français si bien que près d’une planche sur deux mise en œuvre sur notre territoire est importée.
Le principal frein à l’utilisation de bois local dans la construction réside dans la difficulté à prédire ses propriétés mécaniques du fait de son hétérogénéité intrinsèque plus forte que celle des bois du nord. En particulier, la construction à partir de bois feuillus, qui représentent les 2/3 de notre ressource, est aujourd'hui freinée par la complexité à modéliser leur comportement mécanique.
Le premier paramètre d'hétérogénéité du bois réside dans l'orientation de ses fibres, qui gouverne ses propriétés mécaniques et physiques. La connaissance de cette orientation pour un placage ou un sciage peut améliorer son usage. Aujourd'hui, cette mesure peut être réalisée par des techniques laser et l'effet trachéide : lorsqu'un point laser rouge est appliqué sur une surface de bois, la lumière se distord et une ellipse est observée chez les résineux dont le grand axe suit les fibres. Ce phénomène, peu coûteux à instrumenter, peut être utilisé pour développer des modèles de comportement mécanique et de séchage. La plupart des auteurs se contente de l'utiliser et personne n'a véritablement publié sur le potentiel réel de l'exploitation de cet effet qui est considérable pour valoriser les bois locaux riches en nœuds
Réduction du bruit de phase à 10 GHz d’une diode laser par asservissement du courant de polarisation
International audienceLe transfert de signaux micro-ondes bas bruit par fibre optique a connu un essor considérable [1]. Dans ce domaine, un enjeu majeur est de pouvoir disséminer des signaux micro-ondes de haute pureté et haute stabilité sur de longues distances en bénéficiant des avantages majeurs des liens fibrés (faibles pertes, infrastructures déjà présentes [2], techniques de compensation plus robustes qu’en présence de schémas de distribution à base de câbles coaxiaux micro-ondes standard, etc.)
Evaluation de la température de fonctionnement de composants microélectroniques par caméra à comptage de photons et détection synchrone vidéo
National audienceLa diminution de la taille des circuits intégrés à semi-conducteurs a permis de multiplier les applications de l'électronique et de rendre en particulier les circuits numériques CMOS très performants. Cependant, avec la miniaturisation, le problème de l'échauffement du circuit est devenu prépondérant. Cette évolution implique la présence d'un dispositif de refroidissement augmentant le coût et le volume du bloc processeur pour assurer sa fiabilité (radiateur, ventilateur). Pour évaluer les niveaux de température atteints dans les circuits et permettre l'optimisation de leur architecture, il est donc nécessaire d'accéder, de manière non destructive, à sa température en cours de fonctionnement. Dans ce cas, la thermographie infrarouge classique ne peut pas être employée, car elle est limitée à des objets dont la taille est compatible avec le critère de Rayleigh (environ 3 µm actuellement). Compte tenu de l'intérêt suscité par l'étude thermique aux micro-échelles d'espace, nous proposons une méthode dont le principe est fondé sur l'utilisation d'une caméra à comptage de photons pour mesurer le rayonnement thermique émis par la surface du composant à caractériser dans le très proche infrarouge. Etant donné le très faible niveau de signal émis à basse température et à courtes longueurs d'onde (T≡573 K, Λ<1µm), nous avons associé à la caméra intensifiée (ICCD) une détection synchrone vidéo. Nous montrons la validité de cette technique en réalisant des micro-thermographies de transistors NPN
Towards full-field photothermoelastic microscopy using a CCD camera
International audienceWe report on a new method which allows extracting thermoelastic images by full-field method using in-plane measurement. The aim of this work is to show that it is possible to extract the in-plane thermoelastic field. Recently, a vibrometric measurement method has been developed at FEMTO-ST Institute for measuring in-plane displacement with nanometer and sub nanometer resolutions. The use of this method with thermography imaging allowed us to extract thermal expansion of micro samples heated by photothermal or thermoelectrical excitation. In this paper, we present an innovating technique to image thermoelastic field of these investigated samples. This technique is based on a CCD (charge-coupled device) camera which has been optimized for the previously presented applications
MEMS in-plane motion/vibration measurement system based CCD camera
International audienceWe report on the development of a vibrometer for in-plane motion which is particularly suitable for Micro Electro Mechanical System (MEMS) samples. The system combines a conventional microscope, coherent electronics and a Full Frame CCD camera. Stroboscope lighting allows the system to freeze the motion. The obtained images correspond to different phases of the sample motion. Different subpixel motion measurement algorithms are compared in terms of precision and computation time. An algorithm that we specially design for this application proved to be the best. It is based on the shift Fourier theorem and uses first harmonics, where most energy is present. Thus the system allows computing the phase and magnitude of the sample displacement. With the system we can obtain a measurement resolution of 100 pm demonstrated on the AFM cantilever vibrations measurement. The effect of the edge roughness was studied. It decreases the performance of the algorithm but roughness is very low on most MEMS applications. This method based on a CCD camera, is very well suited for measuring in-plane MEMS vibration since it does not require surface roughness for scattering the light as with the speckle methods (for example). It can quickly obtain full field motion, with a high precision. Furthermore, the used algorithm is simple, fast and very noise insensitive
Development and characterization of a differential interferometer setup using ultra-wideband SAW devices
International audienceOptical interferometry is a well-known technique for studying wave propagation and is often preferred over other methods for its highprecision in studying wave profiles, amplitude and phase variations, standing wave patterns, group delay measurements, etc., of waves like surface acoustic waves, Scholte- Stoneley waves, dilatational waves, etc... In the past, different interferometer setups have been employed to detect and measure surface acoustic waves. Differential interferometers have proved to be useful in various fields, like piezoelectric thin film characterization, angular measurements of the order of nano-radians and displacement measurements in the picometre range, of acoustic signals, etc. High sensitivity to the phase and amplitude of the acoustic signals and easy calibration of the setup are the key advantages of a differential interferometer. They are a solution to a common disadvantage of interferometry, i.e., path length sensitivity. Our objective is to construct an interferometer that allows to study transient surface acoustic waves in the time domain.The differential interferometer used in this work is an extended version of a Michelson’s interferometer, where the laser beamreflected from the SAW device, carrying phase and amplitude information of the surface acoustic waves, is divided into two orthogonally polarized beams that traverse different arm lengths, and are recombined using a polarizing beam splitter. This causesshearing in the time domain, with the change in phase along the two arms of the interferometer being a function of the path lengths ofthe two orthogonal beams causing interference. The signal received on the photodiode as a function of time is proportional to thenormal velocity at the surface of the sample. The experimental setup is designed to measure displacements of the order of a fewnanometers, of a SAW device producing transient short pulses with a chirped input IDT. An output IDT with only one finger-pair isused for comparison purposes.The setup being extremely sensitive, requires noise calibration. In order to improve the alignment and test the accuracy of theresponse, different parameters of the experimental setup are varied. An input chirp is used to excite SAW pulses within a frequencyrange of 200 – 400 MHz. The Fourier transform limited pulse duration is about 5 ns. The response measured at the output IDT,directly and by using the interferometer setup, are compared, and the change in amplitude and phase for the two measurements arestudied for two cases: varying position of focus between the two IDTs on the SAW device, and varying path length
Thermal detectivity enhancement of visible and near infrared thermography by using super-resolution algorithm: possibility to generalize the method to other domains
5 pagesInternational audienceThis paper reports on a method which allows a decrease in the minimal detectable temperature in visible and near infrared thermography. This original method permits an increase in the thermal sensitivity without loss of good spatial resolution. It is based on a binning operation and a super-resolution algorithm. The radiometric model and super-resolution method are presented. Measurements on two different samples show the enhancement of the thermal sensitivity and the capability of the method. Finally, the authors propose different ways in which the method can be applied. © 2009 American Institute of Physics
A differential optical interferometer for measuring short pulses of surface acoustic waves
International audienceThe measurement of the displacements caused by the propagation of a short pulse of surface acoustic waves on a solid substrate is investigated. A stabilized time-domain differential interferometer is proposed, with the surface acoustic wave (SAW) sample placed outside the interferometer. Experiments are conducted with surface acoustic waves excited by a chirped interdigital transducer on a piezoelectric lithium niobate substrate having an operational bandwidth covering the 200–400 MHz frequency range and producing 10-ns pulses with 36 nm maximum out-of-plane displacement. The interferometric response is compared with a direct electrical measurement obtained with a receiving wide bandwidth interdigital transducer and good correspondence is observed. The effects of varying the path difference of the interferometer and the measurement position on the surface are discussed. Pulse compression along the chirped interdigital transducer is observed experimentally