M.Phil. / Linearly chirped fibre Bragg gratings can be used to compensate dispersions in a fibre transmission system. Various methods have been developed to chirp fibre gratings. It is proposed that a uniform period grating can be chirped by applying an axially linear strain gradient. In this project, we shall demonstate a novel method for making chirped fibre Bragg gratings, which involves bonding an unchirped fibre Bragg grating of 5cm length to the surface of a tapered stainless steel plate which is strained by bending or dilating. This allows a strain gradient to be formed along the grating length through the transferring of strains from the plate to the fibre. The profiles of tapered stainless steel plates have been determined by means of the finite element method and computer simulations to establish the strain gradient during loading. Ten conventional resistive strain gauges are also bonded on the other surface for strain measurements. Because of the strain gradient, the local Bragg wavelength is a function of the position along the length of the fibre grating so that the grating chirp is an automatic consequence of the strain gradient. This method provides the dynamic control of Bragg wavelength shift, peak reflectivity and spectral bandwidth, and its tunability is also suitable for optimising pulse compression and optical fibre dispersion compensation. Using couple-mode equations, we have also calculated the reflection response of a chirped fibre grating. It is shown that the close agreement between the theoretical and the experimental results suggests that the strain gradient technique provides good control of the Bragg grating chirp and the center wavelength of a chirped fibre Bragg grating.

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Light Detection and Ranging (LiDAR) systems are expected to become the de facto sensors of choice for autonomous vehicles and robotics systems due to their long range and high resolution, allowing them to map the environment accurately. Current available LiDAR systems are based on mechanical apparatus and discrete components that result in large, bulky, and expensive systems with yet-to-be-proven reliability. The advent of Silicon Photonics technology, advanced CMOS foundries allow us to fabricate miniaturized optical components such as phased arrays that combined enable reliable, solid-state, and cost-effective chip-scale LiDAR systems. Furthermore, Silicon Photonics based platform has the advantage of integrating many complex optical components in to a single chip. It is possible to realize an optical phased array based on waveguides with gratings for emitters. These emitters allow to steer the beam by tuning the source's wavelength exploiting the grating's sensitivity to wavelength in one axis and standard phase tuning on the other axis. Such a steering scheme requires only N phase shifters for an N-channel system thus leading to high power efficiency. Another example that could leverage the Silicon Photonics platform is a full coherent LiDAR system utilizing Frequency-Modulated Continuous-Wave (FMCW) detection scheme that was recently reported. However, miniaturizing a LiDAR system to chip-scale has many challenges. The work in this dissertation presents solutions to some of the key challenges we face in order to demonstrate high performance LiDAR based on phased array. One key challenge is the trade-off between beam divergence and field of view. Here, we show a platform based on silicon-nitride/silicon that achieves simultaneously minimal beam divergence and maximum field of view while maintaining performance that is robust to fabrication variations. In addition, in order to maximize the emission from the entire length of the grating, we design the grating’s strength by varying its duty cycle (apodization) to emit uniformly. We fabricate a millimeter long grating emitter with diffraction-limited beam divergence of 0.089°. Another challenge that is intertwined with the aperture length mention before is how maximizing the steering range in an optical phased array. The array's field of view that is perpendicular to the light propagation is governed by the spacing between emitters. In contrast to Radio Frequency based devices, achieving maximum field of view by placing the emitters at half wavelength pitch to avoid side lobes, is challenging for optical phased arrays as the size of the mode is comparable to the wavelength that give rise to cross-talk issues. Emitter pitch that is larger than half the wavelength induce grating lobes in the steered range, effectively limiting the field of view. The closer together the waveguides, the shorter emitters must be to avoid cross-talk, fundamentally limiting the spot size at the farfield. Cross-talk between waveguides induces wavefront aberrations in the beam, thereby increasing beam divergence and limiting the system resolution and range. Here, we improve the mode confinement in the waveguide by increasing the index along the waveguide axis. We use thin Silicon rods, known as metamaterials, between the emitters to tightly confine the mode in the waveguide. Concentrating the mode in the waveguide reduces cross-talk between emitters and maximizes the optical phased array field of view. By embedding an array in a Mach–Zehnder interferometer we demonstrate a sensitive method of measuring cross-talk between the waveguide. We also measure in the nearfield the width of an array of waveguides over a millimeter long emitters. We show that by using the metamaterials we can realize a dense array with a pitch of 1.2 µm over a millimeter long waveguides with gratings at negligible cross-talk. This short pitch allows for 83° steering angle range (Field of View). Combining this the work of Silicon Nitride based long gratings, will allow for a LiDAR system with minimal beam divergence while achieving record large Field of View. Finally, the last chapter discusses Subwavelength Grating structures that due to their sub-wavelength dimensions guide light without diffraction. These structures allow us to tailor the required effective index by varying their duty cycle. We evaluate their robustness to fabrication variations by embedding them inside a sensitive race track. Using this resonator we measured the sensitivity of Subwavelength Grating structures to an off-set in the element's location, elements' width, duty cycle variation, and width change of a single element. Lastly, we show that due to their periodic structure, they are also robust to as many as three consecutive missing elements. This protection property opens the possibility of realizing a plethora of new devices not possible using wire waveguides. One such example is a T-splitter in which an incoming Transverse Magnetic polarized mode could be split to two separate branches at a 90° angle. The demonstrated platform we show here paves the way for on-chip LiDAR systems for autonomous automotive, robotics, wireless communications, and particle trapping.

Three different optical fiber phase modulators utilizing the magnetostrictive properties of the metallic glass alloy Fe74Co10B16 were constructed. By binding the optical fiber to the magnetostrictive metallic glass, the strain imparted to the metallic glass from the magnetic field is transferred to the optical fiber. The strain on the optical fiber shifts the phase of the light, which can be controlled indirectly by varying the current producing the magnetic field permeating the metallic glass. The performance of the modulators on the basis of optical phase shift as a function of bias magnetic field and optical phase shift as a function of excitation frequency was measured. Speculations were made on the loss mechanism inherent in the various modulator designs in order to explain the deviation in performance of the three modulator designs.

Orientador: José Brás Barreto de Oliveira / Banca: José Humberto Dias da Silva / Banca: Euzi Conceição Fernandes da Silva / O Programa de Pós Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividadesde pesquisa em materiais de diversos campi / Resumo: Estruturas dot-in-a-well (DWELL) têm sido extensivamente estudadas nos últimos anos devido a sua relevância para a fabricação de dispositivos ópticos e eletrônicos, em especial os fotodetectores e lasers de alto desempenho. inserir pontos quânticos (QDs) em poços quânticos (QWs) aumenta a densidade dos pontos quânticos que, somado ao confinamento adicional fornecido pelo poço quântico, resulta em eficiência óptica elevada. Muitos trabalhos se dedicam ao estudo da dinâmica dos portadores, a fim de compreender melhor o funcionamento dos dispositivos, principalmente em altas temperaturas. Em nosso estudo, as amostras DWELL, em que os pontos de InAs são confinados em poços de InGaAs, foram crescidas sobre um substrato de GaAs por epitaxia de feixe molecular (MBE) e analisadas a partir de medidas de fotoluminiscência (PL). Os pontos quânticos foram obtidos pelo método de Stranski-Krastanov. Para realizar as medidas de PL foi utilizado um monocromador Jobin Yvon T64000. As amostras foram inseridas em um criostato de Janis, com circuito fechado de He, e excitadas por um laser de Ar. O sinal da PL, depois de espalhado, foi detectado por um detector de Ge e registrado usando um amplificador lock-in. Nos espectros de emissão foram identificados três picos em medidas com alta potência de excitação e baixa temperatura. Os resultados das medidas de PL em função da potência de excitação indicaram que se tratava de transições entre níveis confinados nos QDs. Não foi identificado comportamento bi-modal no crescimento dos pontos quânticos nas amostras analisadas. Os resultados das medidas de PL, para diferentes regimes de excitação, mostraram que em todas as transições ópticas investigadas há éxcitons envolvidos e as recombinações não radiativas são insignificantes. Os gráficos de intensidade da PL integrada em função da... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The dot-in-well (DWELL) structure has been exensively studied in recent years because of its relevance to the fabrication of optical and electronic devides, in particular, high-performance photodetectors. The insertion of quantum dots (QD) in the well increases the density of dots which, added to the additional confinement provided by the quantum well, results in a higher optical efficiency. Many works has been devoted to the study of the dynamics carriers in order to properly understand the functioning of devices mostly at high temperatures. In our study, a DWELL sample, in which InAs QDs are confined in InGaAs wells, was grown on a GaAs substrate by molecular beam epitaxy (MBE) and was analyzed by photoluminescence (PL) measurements. The quantum dots were grown by typical Stranski-Krastanov method. The PL spectra were obtained in a T64000 Jobin Yvon Monochromator. The sample was inserted in a Janis He-closed-circuit crystat and excited using an Ar-ion laser. The PL signal, after dispersed, was detected by a Ge-detector and recorded using a loock-in amplifier. In the emission spectra is possible to identify 3 peaks in high power and low temperature. The results of PL measurements depending on the power excitation showed that the peaks are related to transitions between the confined levels in the QDs. Not been identified bi-modal behavior in the growth of quantum dots in the samples analyzed. The results of the PL measurments for different excitation regimes showed that in all the investigated optical transitions there excitons involved and non radiative recombinations are insignificant. The graphics of integrated PL intensity as a function or inverse temperature allowed finding different ranges of temperature and the activation energies associated with the processes of thermal escape of... (Complete abstract click electronic access below) / Mestre

Active vision systems are about mobile platform equipped with one or more than one cameras. They perceive what happens in their surroundings from the image streams the cameras grab. Such systems have a few fundamental tasks to tackle---they need to determine from time to time what their motion in space is, and should they have multiple cameras, they need to know how the cameras are relatively positioned so that visual information collected by the respective cameras can be related. In the simplest form, the tasks are about finding the motion of a camera, and finding the relative geometry of every two cameras, from the image streams the cameras collect. / On determining the ego-motion of a camera, there have been many previous works as well. However, again, most of the works require to track distinct features in the image stream or to infer the full optical flow field from the normal flow field. Different from the traditional works, utilizing no motion correspondence nor the epipolar geometry, a new method is developed that operates again on the normal flow data directly. The method has a number of features. It can employ the use of every normal flow data, thus requiring less texture from the image scene. A novel formulation of what the normal flow direction at an image position has to offer on the camera motion is given, and this formulation allows a locus of the possible camera motion be outlined from every data point. With enough data points or normal flows over the image domain, a simple voting scheme would allow the various loci intersect and pinpoint the camera motion. / On determining the relative geometry of two cameras, there already exist a number of calibration techniques in the literature. They are based on the presence of either some specific calibration objects in the imaged scene, or a portion of the scene that is observable by both cameras. However, in active vision, because of the "active" nature of the cameras, it could happen that a camera pair do not share much or anything in common in their visual fields. In the first part of this thesis, we propose a new solution method to the problem. The method demands image data under a rigid motion of the camera pair, but unlike the existing motion correspondence-based calibration methods it does not estimate the optical flows or motion correspondences explicitly. Instead it estimates the inter-camera geometry from the monocular normal flows. Moreover, we propose a strategy on selecting optimal groups of normal flow vectors to improve the accuracy and efficiency of the estimation. / The relative motion between a camera and the imaged environment generally induces a flow field in the image stream captured by the camera. The flow field, which is about motion correspondences of the various image positions over the image frames, is referred to as the optical flows in the literature. If the optical flow field of every camera can be made available, the motion of a camera can be readily determined, and so can the relative geometry of two cameras. However, due to the well-known aperture problem, directly observable at any image position is generally not the full optical flow, but only the component of it that is normal to the iso-brightness contour of the intensity profile at the position. The component is widely referred to as the normal flow. It is not impossible to infer the full flow field from the normal flow field, but then it requires some specific assumptions about the imaged scene, like it is smooth almost everywhere etc. / This thesis aims at exploring how the above two fundamental tasks can be tackled by operating on the normal flow field directly. The objective is, without the full flow inferred explicitly in the process, and in turn no specific assumption made about the imaged scene, the developed methods can be applicable to a wider set of scenes. The thesis consists of two parts. The first part is about how the inter-camera geometry of two cameras can be determined from the two monocular normal flow fields. The second part is about how a camera's ego-motion can be determined by examining only the normal flows the camera observes. / We have tested the methods on both synthetic image data and real image sequences. Experimental results show that the developed methods are effective in determining inter-camera geometry and camera motion from normal flow fields. / Yuan, Ding. / Adviser: Ronald Chung. / Source: Dissertation Abstracts International, Volume: 70-09, Section: B, page: . / Thesis submitted in: October 2008. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 121-131). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

Thesis (Elec.E)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Roy Sidney Bondurant. / Elec.E

Thesis (Ph.D.)--University of Delaware, 2007. / Principal faculty advisor: Dennis W. Prather, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Fouad Kiamilev, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

CMOS imagers are replacing CCD imagers in many applications and will continue to make new applications possible. CMOS imaging offers lower cost implementations on standard CMOS processes which allow for mixed signal processing on-chip. A system-on-a-chip approach offers the ability to perform complex algorithms faster, in less space, and with lower power and noise. Our transform imager is an implementation of a mixed focal plane and peripheral computation imager which allows high fill factor with high computational rates at low power. However, in order to use the technology effectively a need to verify and further understand the behavior and of the pixel elements in this transform imager was needed. This thesis presents a study of the pixel elements and mismatches and errors in the pixel array of this imager. From there, a discussion about removing offsets and an implementation of a circuit to remove the largest offsets is shown. To further enhance performance, initial work to develop light adaptive readout circuits is presented. Finally, an overview is given of a newly designed one-megapixel transform imager with many design improvements.