323 research outputs found

    Bandpass filters for unconstrained target recognition and their implementation in coherent optical correlators

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    An up-dateable correlator is simulated which is based on the non-degenerate four wave mixing (NDFWM) interaction in the photorefractive material bismuth silicon oxide (Bi12SiO20). Specifically, it is shown that variable bandpass filters can be implemented directly in the correlator by adjusting the relative strengths of the signal and reference beams used to write the Fourier transform hologram into the photorefractive. The synthetic discriminant function (SDF) method of grey-level multiplexing is reviewed. A bandpass modification of this technique is used in the design of a multiplexed filter for the recognition of an industrial test component from a limited number of known stable state orientations when viewed from an overhead camera position. Its performance in this task when implemented in the up-dateable correlator is assessed through simulation. The conclusion of this work is that filter multiplexing must be used judiciously for orientation invariant recognition. Only a limited number of images, typically under ten, may be multiplexed into each filter since correlation peak heights and peak-to-sidelobe ratios inevitably progressively deteriorate as images are added to the filter. The effect of severe amplitude disruptions in the frequency plane on correlation peak localisation is examined. In two or higher dimensions simulations show the localisation is very robust to this disruption; an analysis is developed to indicate the reason for this. The effect is exploited by the implementation of an algorithm that locally removes the spatial frequencies that exhibit close phase matching between intra- and inter-class images. The inter-class response can be forced to zero while simultaneously improving the intra-class tolerance to orientation changes. The technique is assessed through simulation with images of two types of motor vehicle, in a variety of orientations, and shown to be effective in improving discrimination and intra-class tolerance for examples in which these were initially very poor. Bandpass filters are experimentally implemented in a joint transform correlator (JTC) based on a NDFWM interaction in Bi12SiO20. The JTC is described and its full bandwidth performance initially assessed. As anticipated from the previous considerations, inter-class discrimination was high but the intra-class tolerance very poor due to the high sensitivity of the filter. The difference of Gaussian approximation to a Laplacian of a Gaussian filter is described and its experimental implementation in the JTC detailed. Experimental results are presented for the orientation independent recognition of a car while maintaining discrimination against another car. An intra-class to inter-class correlation ratio of 7.5 dB was obtained as a best case and 3.6 dB as a worst case, the intra-class variation being at 11 ° increments in orientation at zero elevation angle. The results are extrapolated to estimate that approximately 80 filters would be required for a full 2 steradian orientation coverage. The implementation of the frequency removal technique and the Wiener filter in the JTC is briefly considered in conclusion to this work

    Engineering evaluations and studies. Volume 3: Exhibit C

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    High rate multiplexes asymmetry and jitter, data-dependent amplitude variations, and transition density are discussed

    Field-resolved studies of ultrafast light-matter interaction

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    Field-resolved studies of ultrafast light-matter interaction

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    The fastest light-matter interactions between electrons and optical laser pulses occur on attosecond timescales below the half-cycle oscillation period of the electric field. The investigation of such ultrafast processes and ultimately their control, therefore, requires field-resolved measurements. In this work, the understanding of well-established and newly emerging sub-cycle-resolved techniques for the characterization of optical pulses and ultrafast light-induced processes is expanded and the application of the methods includes gases, bulk solids, and nanostructures. In the first part, the mechanism behind the macroscopic current generation in optical field-induced photocurrent measurements in gases is studied theoretically and experimentally. A rigorous model is developed that connects the measured current to the microscopic movement of charge carriers and includes scattering with atoms and the interaction of charges via the Coulomb force. The model is validated against an extensive set of experiments which measure the carrier-envelope-phase dependent strong-field photoemitted current induced on a pair of electrodes surrounding the focus of a few-cycle laser pulse. The role of the mean-free path as well as the Coulomb interaction is identified. The model provides a fundamental understanding of the signal generation mechanism in photoconductive sampling of laser pulses which had been missing before and which will allow to identify fundamental limitations and strategies for further optimization of the detection. The second set of experiments aims at the transient change of the reflectivity after excitation with a near-infrared pump pulse. Electro-optic sampling is used for the field-resolved characterization of the mid-infrared probe pulses which covers a wavelength range from below 3 micron to above 6 micron. Measurements on semiconductors are performed and dynamics occuring on the femtosecond to the picosecond timescale after photoexcitation are studied. The demonstrated experiments represent an important milestone in pushing field-sampling methods from the THz into the PHz domain. The investigation of the generation of isolated attosecond pulses in the extreme ultra-violet photon energy range using high-harmonic generation (HHG) in noble gases is the topic of the third section. The focus is put on the overdriven regime, where the driving laser undergoes severe reshaping due to plasma effects. Experimentally, attosecond streaking is used to demonstrate isolated attosecond pulses for the first time in this regime. Theoretically, the phasematching mechanism in this regime is studied using extensive numerical simulations. An extension of conventional phasematching expressions is introduced which describes the contribution of the HHG dipole phase due to the blue-shift of the driving laser. The results are important for a complete understanding of HHG phasematching and might help to find routes towards more efficient HHG in the water window. Finally, attosecond measurements on metal nanotips are presented. The attosecond field-resolved characterization of the nanoscale near-fields on a nanotip and the response function using attosecond streaking is demonstrated. Moreover, another field-reconstruction method based on the modulation of the strong-field photocurrent is used for the measurement of the enhanced near-fields at the nanotip apex and different aspects of the methods are studied. Combining the latter approach with the concept of the nanotip as nanoscale localized field sensor, the attosecond characterization of an orbital angular momentum beam in free-space below the diffraction limit is demonstrated. These results pave the way towards nanoscale attosecond field-resolved measurements on generic nanostructures.Die schnellsten Prozesse der fundamentalen Licht-Materie-Wechselwirkung zwischen Elektronen und sichtbaren Laserpulsen finden auf der Attosekunden-Zeitskala statt, unterhalb der halben Schwingungsperiode des elektrischen Feldes. Feldaufgelöste Messungen der beteiligten Laserpulse sind daher für die Untersuchung solch schneller Prozesse und ihrer Kontrolle unabdingbar. In der vorliegenden Arbeit wird das fundamentale Verständnis von etablierten als auch neu aufkommenden subzyklen-aufgelösten Messtechniken erweitert mit Anwendungen in Gasen, Festkörpern und Nanostrukturen. Im ersten Teil wird der Mechanismus hinter der Erzeugung makroskopischer Ströme in den Messungen feld-induzierter Photoströme experimentell und theoretisch untersucht. Die Entwicklung eines rigorosen Models wird präsentiert, das die gemessenen Ströme mit der mikroskopischen Bewegung der Ladungsträger verknüpft. Es beinhaltet außerdem die Streuung an Atomen und die Coulomb-Wechselwirkung. Das Model wird in einer Reihe von umfassenden Experimenten bestätigt, in denen die Abhängigkeit der Ströme von der Phase der Trägerwelle zur Einhüllenden des Laserpulses gemessen werden. Zur Messung der Ströme wird ein Elektrodenpaar, das den Fokus eines intensiven Wenigzyklen-Pulses in verschiedenen Gasen umgibt, verwendet. Der Einfluss der mittleren freien Weglänge und der Ladungs-Wechselwirkung wird aufgeklärt. Das Model liefert ein fundamentales Verständnis der Signalerzeugung in der auf Photoleitung beruhenden Messung von Laserpulsen. Dies erlaubt die Identifizierung der fundamentalen Grenzen und eröffnet Wege zur Optimierung der Messmethode. Die zweite Reihe von Experimenten hat die Messung der transienten Änderung der Reflektivität nach der Anregung durch einen Pumppuls in nahen Infrarotbereich zum Gegenstand. Elektro-optisches Sampling wird verwendet für die feldaufgelöste Charakterisierung der Probepulse im mittleren Infrarotbereich, von unter drei bis über sechs Mikrometern Wellenlänge. Messungen an Halbleitern werden durchgeführt und die Dynamik, die auf der Zeitskala von Femtosekunden bis Pikosekunden nach der Photoanregung stattfindet, wird untersucht. Die gezeigten Experimente sind ein wichtiger Meilenstein für die Erweiterung feldaufgelöster Messtechniken vom THz- in den PHz-Frequenzbereich. Die Erzeugung von isolierten Attosekundenpulsen im extrem ultravioletten Wellenlängenbereich durch hohe Harmonische (HHG) in Edelgasen ist das Thema des dritten Abschnitts. Der Schwerpunkt liegt auf der Untersuchung des übersteuerten Regimes, in dem der Anregungslaser im Erzeugungsprozess eine starke Umformung durch Plasmaeffekte erfährt. Auf experimenteller Ebene wird die Attosekunden Streaking Technik angewendet, um zum ersten Mal die Erzeugung isolierter Attosekundenpulse unter diesen Umständen nachzuweisen. Anhand umfangreicher numerischer Simulationen wird der Mechanismus der Phasenanpassung in diesem Regime theoretisch untersucht. Eine Erweiterung der konventionellen analytischen Beschreibung der Phasenanpassung wird eingeführt, die den Beitrag der Dipolphase der hohen Harmonischen aufgrund der Blauverschiebung des Anregungslasers berücksichtigt. Die Ergebnisse sind von grundlegender Bedeutung für ein vollständiges Verständnis der HHG-Phasenanpassung und helfen möglicherweise dabei, Wege zu effizienter HHG-Erzeugung im sogenannten Wasserfenster zu finden. Zuletzt werden Attosekundenmessungen an metallischen Nanospitzen vorgestellt. Die feldaufgelöste Charakterisierung von Nahfeldern an einer Nanospitze auf der Attosekunden- und Nanometerskala und der entsprechenden Antwortfunktion durch die Attosekunden Streaking Methode werden demonstriert. Darüber hinaus wird eine andere Technik zur Feldrekonstruktion auf die Messung der verstärkten Nahfelder an dem Ende der Nanospitze angewandt. Verschiedene Aspekte der Messmethode, die auf der Modulation des durch Starkfeldemission erzeugten Photostroms beruht, werden untersucht. Schließlich wird die Kombination der demonstrierten Methodik mit dem Konzept der Nanospitze als nanolokalisiertem Feldsensor demonstriert. Damit wird die zeitlich und räumlich aufgelöste Charakterisierung eines frei propagierenden Laserstrahls mit Drehimpuls auf der Attoekunden-Skala und unterhalb des Beugungslimits gezeigt. Die Resultate ebnen den Weg hin zu feldaufgelösten Messungen im Nanometer-Attosekunden Bereich an beliebigen Nanostrukturen

    Texture Segregation By Visual Cortex: Perceptual Grouping, Attention, and Learning

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    A neural model is proposed of how laminar interactions in the visual cortex may learn and recognize object texture and form boundaries. The model brings together five interacting processes: region-based texture classification, contour-based boundary grouping, surface filling-in, spatial attention, and object attention. The model shows how form boundaries can determine regions in which surface filling-in occurs; how surface filling-in interacts with spatial attention to generate a form-fitting distribution of spatial attention, or attentional shroud; how the strongest shroud can inhibit weaker shrouds; and how the winning shroud regulates learning of texture categories, and thus the allocation of object attention. The model can discriminate abutted textures with blurred boundaries and is sensitive to texture boundary attributes like discontinuities in orientation and texture flow curvature as well as to relative orientations of texture elements. The model quantitatively fits a large set of human psychophysical data on orientation-based textures. Object boundar output of the model is compared to computer vision algorithms using a set of human segmented photographic images. The model classifies textures and suppresses noise using a multiple scale oriented filterbank and a distributed Adaptive Resonance Theory (dART) classifier. The matched signal between the bottom-up texture inputs and top-down learned texture categories is utilized by oriented competitive and cooperative grouping processes to generate texture boundaries that control surface filling-in and spatial attention. Topdown modulatory attentional feedback from boundary and surface representations to early filtering stages results in enhanced texture boundaries and more efficient learning of texture within attended surface regions. Surface-based attention also provides a self-supervising training signal for learning new textures. Importance of the surface-based attentional feedback in texture learning and classification is tested using a set of textured images from the Brodatz micro-texture album. Benchmark studies vary from 95.1% to 98.6% with attention, and from 90.6% to 93.2% without attention.Air Force Office of Scientific Research (F49620-01-1-0397, F49620-01-1-0423); National Science Foundation (SBE-0354378); Office of Naval Research (N00014-01-1-0624

    Requirements Study for System Implementation of an Atmospheric Laser Propagation Experiment Program, Volume II

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    Program planning, ground support and airborne equipment for laser space communication syste

    Determination of optical technology experiments for a satellite

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    Optical technology experiments for satellite - communications, acquisition, tracking, lasers, photometry, and atmospheric

    A multi-channel system for use in cardiac electrophysiologic studies

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    The location of accessory pathways in Wolff-Parkinson-White syndrome patients is performed manually during open heart surgery at Groote Schuur Hospital, using a hand-held roving electrode. This manual procedure is slow and tedious, prolonging the operation and the time for which the patient remains on cardiac bypass. A multichannel electrogram acquisition and display system with a storage facility would significantly reduce the time taken and improve the reliability of locating the accessory pathways. Having considered a number of currently available cardiac mapping systems it was decided that a new system be developed for specific application within Groote Schuur Hospital. The main design goals of this system are to improve accuracy, increase reliability and enhance the speed of the entire mapping procedure with direct benefit to staff and patients. The system is based on an IBM compatible computer and allows for the acquisition of a maximum of thirty-two electrogram inputs. A typical configuration would acquire twenty epicardial, two references (one each from atrium and ventricle), one roving electrode and two surface lead signals. The epicardial signals are obtained from a custom-built electrode belt which is placed around the heart over the atrioventricular groove. The project includes the development of front-end hardware and software for processing, display and storage of electrogram signals. The relative activation times of the signals are displayed under software control in order to facilitate the location of any accessory pathway(s)
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