Vortex laser development employing an interferometric output coupler

This thesis focuses on the design and implementation of new laser cavity designs for vortex mode generation. The vortex generation method is integrated into both solid-state laser cavities and systems using doped fibre gain mediums. Optical vortex laser beams have attracted a lot of attention due to their proposed applications in a wide range of industries from communication to particle manipulation to microscopy to material processing. More recently, vortex generation directly from the laser has attracted research due to the potential for higher purity, higher power and a compact system. In this work a modified Sagnac interferometer, dubbed the vortex output coupler (VOC), is integrated as the output coupler of a laser cavity and used to convert the fundamental Gaussian intracavity mode into a first order Laguerre Gaussian output. The VOC was first implemented into various solid-state cavities, with a Vanadate gain medium operating at 1064 nm, where vortex generation was successfully demonstrated. A record, from-the-source generated, vortex power of 31.3 W was achieved, with a laser slope efficiency of 62.5%. The mode purity was 95.2% and the M2 = 2.25. The handedness of the generated vortex was pure and switchable during operation. First order Hermite Gaussian modes with the same power were also demonstrated. The VOC was also shown to function in a pulsed cavity without any detrimental effects. It was found that the VOC has mode filtering properties, which helped maintain the a fundamental Gaussian in the cavity, despite mode mismatch between the pump beam and the fundamental Gaussian mode in the cavity. Fibre laser systems have the advantage of being compact, alignment insensitive and maintaining a close resemblance to the fundamental Gaussian mode through the use of a single mode fibre. To demonstrate the implementation versatility of the VOC and capitalise on its power scaling potential, the VOC was integrated into a non-polarisation maintaining fibre laser system as a bulk output coupler. An Ytterbium-doped gain fibre was used, operating at 1064 nm, which allowed for the same optics as in previous work to be used for the VOC. 5.08 W of vortex output power was achieved from the fibre laser system, with a mode purity and quality of 96.1% and M2 (X/Y) = 2.03/2.22, respectively. This system was also used as a first order vortex source for higher order vortex generation, using a spiral phase plate. Vortices with orbital angular momentum values of l = +2 and +3 were generated from the first order vortex (l = +1) input using spiral phase plates, which imparted +1 and +2 orbital angular momentum (helical phase ramps of 2π and 4π respectively). The VOC is made up of a beamsplitter and three turning mirrors, which are all high power damage threshold components. By choosing appropriate optical coatings for these components, considering wavelength and polarisation, the VOC can be implemented across the output spectrum making it incredibly versatile. The VOC is shown to function in a pulsed laser system, with a vortex pulse with duration 20 ns and energy 303 μJ shown in this work. The output mode can be switched between left and right vortex handedness and also between the first order Hermite Gaussian modes, all during operation. This pulsed operation and output mode versatility make it very interesting for material surface processing, particle levitation and manipulation, free-space communication and broadband, or ultrashort pulse, vortex generation. A VOC enhanced vortex laser can also be used as a high power and purity first order Hermite-Gaussian or Laguerre-Gaussain source for further conversion to higher order modes using other methods.Open Acces

On the complexity of curve fitting algorithms

We study a popular algorithm for fitting polynomial curves to scattered data based on the least squares with gradient weights. We show that sometimes this algorithm admits a substantial reduction of complexity, and, furthermore, find precise conditions under which this is possible. It turns out that this is, indeed, possible when one fits circles but not ellipses or hyperbolas.Comment: 8 pages, no figure

tinygarden - A java package for testing properties of spanning trees

Spanning trees are fundamental objects in graph theory. The spanning tree set size of an arbitrary graph can be very large. This limitation discourages its analysis. However interesting patterns can emerge in small cases. In this article we introduce tinygarden, a java package for validating hypothesis, testing properties and discovering patterns from the spanning tree set of an arbitrary graph.Fil: Dubinsky, Manuel. Universidad Nacional de Avellaneda; ArgentinaFil: Massri, Cesar Dario. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Investigaciones Matemáticas "Luis A. Santaló". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Matemáticas "Luis A. Santaló"; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Matemática; ArgentinaFil: Taubin, Gabriel. University Brown; Estados Unido

Elastic circuits

Elasticity in circuits and systems provides tolerance to variations in computation and communication delays. This paper presents a comprehensive overview of elastic circuits for those designers who are mainly familiar with synchronous design. Elasticity can be implemented both synchronously and asynchronously, although it was traditionally more often associated with asynchronous circuits. This paper shows that synchronous and asynchronous elastic circuits can be designed, analyzed, and optimized using similar techniques. Thus, choices between synchronous and asynchronous implementations are localized and deferred until late in the design process.Peer ReviewedPostprint (published version

Анализ эффективности каскадного кодирования для повышения выносливости многоуровневой NAND флеш-памяти

Повышение плотности записи в современных чипах NAND флеш-памяти, достигаемое как за счет уменьшающегося физического размера ячейки, так и благодаря возрастающему количеству используемых состояний ячейки, сопровождается снижением надежности хранения данных – вероятности ошибки, выносливости (числа циклов перезаписи) и времени хранения. Стандартным решением, позволяющим повысить надежность хранения данных в многоуровневой флеш-памяти, является введение помехоустойчивого кодирования. Эффективность введения помехоустойчивого кодирования в существенной степени определяется адекватностью модели, формализующей основные процессы, связанные с записью и чтением данных. В работе приводится описание основных искажений, сопровождающих процесс записи/считывания в NAND флеш-памяти, и явный вид плотностей распределения результирующего шума. В качестве аппроксимации полученных плотностей распределения результирующего шума рассматривается модель на основе композиции гауссова распределения и распределения Лапласа, достаточно адекватно отражающая плотности распределения результирующего шума при большом числе циклов перезаписи. Для этой модели проводится анализ помехоустойчивости каскадных кодовых конструкций с внешним кодом Рида-Соломона и внутренним многоуровневым кодом, состоящим из двоичных компонентных кодов. Выполненный анализ позволяет получить обменные соотношения между вероятностью ошибки, плотностью записи и числом циклов перезаписи. Полученные обменные соотношения показывают, что предложенные конструкции позволяют за счет очень незначительного снижения плотности записи обеспечить увеличение граничного значения числа циклов перезаписи (определяемого производителем) в 2–2.5 раза при сохранении требуемого значения вероятности ошибки на бит

CrowdCam: Instantaneous Navigation of Crowd Images Using Angled Graph

We present a near real-time algorithm for interactively exploring a collectively captured moment without explicit 3D reconstruction. Our system favors immediacy and local coherency to global consistency. It is common to represent photos as vertices of a weighted graph, where edge weights measure similarity or distance between pairs of photos. We introduce Angled Graphs as a new data structure to organize collections of photos in a way that enables the construction of visually smooth paths. Weighted angled graphs extend weighted graphs with angles and angle weights which penalize turning along paths. As a result, locally straight paths can be computed by specifying a photo and a direction. The weighted angled graphs of photos used in this paper can be regarded as the result of discretizing the Riemannian geometry of the high dimensional manifold of all possible photos. Ultimately, our system enables everyday people to take advantage of each others' perspectives in order to create on-the-spot spatiotemporal visual experiences similar to the popular bullet-time sequence. We believe that this type of application will greatly enhance shared human experiences spanning from events as personal as parents watching their children's football game to highly publicized red carpet galas.Swiss National Science FoundationEuropean Commission (ERC grant #210806 4DVideo, 7th Framework Programme (FP7/2007- 2013)

Feature based volumes for implicit intersections.

The automatic generation of volumes bounding the intersection of two implicit surfaces (isosurfaces of real functions of 3D point coordinates) or feature based volumes (FBV) is presented. Such FBVs are defined by constructive operations, function normalization and offsetting. By applying various offset operations to the intersection of two surfaces, we can obtain variations in the shape of an FBV. The resulting volume can be used as a boundary for blending operations applied to two corresponding volumes, and also for visualization of feature curves and modeling of surface based structures including microstructures

Novel 3D compression methods for geometry, connectivity and texture

A large number of applications in medical visualization, games, engineering design, entertainment, heritage, e-commerce and so on require the transmission of 3D models over the Internet or over local networks. 3D data compression is an important requirement for fast data storage, access and transmission within bandwidth limitations. The Wavefront OBJ (object) file format is commonly used to share models due to its clear simple design. Normally each OBJ file contains a large amount of data (e.g. vertices and triangulated faces, normals, texture coordinates and other parameters) describing the mesh surface. In this paper we introduce a new method to compress geometry, connectivity and texture coordinates by a novel Geometry Minimization Algorithm (GM-Algorithm) in connection with arithmetic coding. First, each vertex (x, y, z) coordinates are encoded to a single value by the GM-Algorithm. Second, triangle faces are encoded by computing the differences between two adjacent vertex locations, which are compressed by arithmetic coding together with texture coordinates. We demonstrate the method on large data sets achieving compression ratios between 87%—99% without reduction in the number of reconstructed vertices and triangle faces. The decompression step is based on a Parallel Fast Matching Search Algorithm (Parallel-FMS) to recover the structure of the 3D mesh. A comparative analysis of compression ratios is provided with a number of commonly used 3D file formats such as VRML, OpenCTM and STL highlighting the performance and effectiveness of the proposed method

Lazy transition systems: application to timing optimization of asynchronous circuits

The paper introduces Lazy Transitions Systems (LzTSs). The notion of laziness explicitly distinguishes between the enabling and the firing of an event in a transition system. LzTSs can be effectively used to model the behavior of asynchronous circuits in which relative timing assumptions can be made on the occurrence of events. These assumptions can be derived from the information known a priori about the delay of the environment and the timing characteristics of the gates that will implement the circuit. The paper presents necessary conditions to synthesize circuits with a correct behavior under the given timing assumptions. Preliminary results show that significant area and performance improvements can be obtained by exploiting the extra "don't care" space implicitly provided by the laziness of the events.Peer ReviewedPostprint (author's final draft

Handover in Mobile WiMAX Networks: The State of Art and Research Issues

The next-generation Wireless Metropolitan Area Networks, using the Worldwide Interoperability for Microwave Access (WiMAX) as the core technology based on the IEEE 802.16 family of standards, is evolving as a Fourth-Generation (4G) technology. With the recent introduction of mobility management frameworks in the IEEE 802.16e standard, WiMAX is now placed in competition to the existing and forthcoming generations of wireless technologies for providing ubiquitous computing solutions. However, the success of a good mobility framework largely depends on the capability of performing fast and seamless handovers irrespective of the deployed architectural scenario. Now that the IEEE has defined the Mobile WiMAX (IEEE 802.16e) MAC-layer handover management framework, the Network Working Group (NWG) of the WiMAX Forum is working on the development of the upper layers. However, the path to commercialization of a full-fledged WiMAX mobility framework is full of research challenges. This article focuses on potential handover-related research issues in the existing and future WiMAX mobility framework. A survey of these issues in the MAC, Network and Cross-Layer scenarios is presented along with discussion of the different solutions to those challenges. A comparative study of the proposed solutions, coupled with some insights to the relevant issues, is also included