[pt] Com o objetivo de se aumentar a confiabilidade de um sistema de micro-ondas tem-se utilizado, em alguns casos, a polarização circular. De modo a se evitar o emprego de vários dispositivos para a obtenção de fontes de polarização circular (os quais concorrem para um aumento do coeficiente de onda estacionária global do sistema) utiliza-se um único componente capaz de efetuar a transformação da polarização circular desejada. Tal componente é conhecido como polarizador. Este trabalho de tese visa apresentar o desenvolvimento e o projeto de um polarizador construído em guia circular, para a faixa de SHF. O efeito da transformação de polarização é conseguido através de diafragmas indutivos e capacitivos dispostos ao longo do guia circular. O coeficiente de onda estacionária máximo encontrado em um modelo construído para a faixa de 6,8 a 7,8 GHz foi de 1,1 e a relação axial de 1,2 dB. Devido a não utilização de dielétricos com perdas na estrutura, pode-se esperar baixos valores de perda por inserção. Nestas medidas incluem-se a transição guia retangular-guia circular. Na Introdução do presente trabalho é apresentada a obtenção de polarização circular em guia circular através de dois modos TE 11, ortogonais, defasados de 90 0. no Capítulo 1 são analisados os fundamentos teóricos em que se apóia o polarizador e no Capítulo 2 é analisada a sistemática de projeto da estrutura. Os Capítulos 3 e 4 descrevem, respectivamente, o projeto de um modelo experimental e os ensaios efetuados. / [en] Circular polarization hás been used in some cases to increase the capability and reliability of microwave systems. The desing employed here avoids introduccing several components to generate circular polarization (as such elements tend to increase the total voltage standing wave ratio of the system) by utilizing a single component capable of making the transformation from the original linear polarization in the retangular guide to the desired circular polarization. This component is known as a polarizer. This paper presents the development and results of a thesis project involving the construction of a polarizer in circular waveguide for the SHF band. The transformation from linear to circular polarization is obtained by using inductive and capacitive diaphrams dispposed longitudinaly in circular guide. The maximum voltage standing wave ratio measured in the model constructed was 1,1 and the maximum axial ratio was 1.2 dB for the band 6,8 to 7.8 GHz. Because lossy dielectric materials were not used in this polarizer low values of insertion loss are expected. The effects of the transition from retangular to circular guide are included in the measurements. In the introduction section of this paper the author describes the method for realizing circular polarization in circular waveguides by the use of two orthogonal TE 11 modes phased 90 0 apart with respect to each other. In Chapter 1 the theoretical fundamentals upon which the polarizer is based are analysed and in Chapter 2 a systematic analysis of the structure is reported, Chapter 3 and 4 describe, respectively, the investigations of the experimental model and the test results achieved.

This thesis focuses on the development of high harmonic generation (HHG) by using polarisation controlled quasi-phase matching QPM as well as related topics. A new class of QPM techniques called polarisation-controlled QPM is introduced where linear or circlar birefringence enables the modulation of the driving field's polarisation state called polarisation-beating QPM (PBQPM) for linear birefringence and optical rotation QPM (ORQPM) for circular birefringence respectively. PBQPM uses a linear birefringence to modulate periodically the driving pulse between linear and circular/elliptical polarisation states. Because elliptical or circular polarisation of the driving pulse suppresses harmonic generation, by appropriately matching the beat length of the driving field's polarisation state to the coherence length of the harmonic generation, QPM can be achieved. In the second technique, ORQPM, propagation of the driving radiation in a system exhibiting circular birefringence causes its plane of polarisation to rotate; by appropriately matching the period of rotation to the coherence length, it is possible to avoid destructive interference of the generated radiation. Not only does ORQPM have similar enhancements as true-phase matching, it is also the first proposed QPM source for circularly polarised high harmonics. The importance of phase modulation in QPM, especially relating to modebeating in hollow-core waveguides where harmonics is being generated are also explored theoretically. Based on this, a novel technique for analyzing random phase matching using a continuous phase-diffusion treatment has been developed; theoretical analytical models are shown to produce excellent agreement with simulations. It is further shown that random phase matching may be responsible for additional broadening of the high harmonic spectrum, especially at higher harmonic orders. Because mode and polarisation control is central to polarisation-controlled QPM, four waveguide mode decomposition techniques from single shot CCD data have been developed. The extraction of phase and coupling coefficients are demonstrated experimentally. A novel analytical general solution for the phase introduced by a phase-only spatial light modulator to generate a given far-field phase and amplitude was developed. The solution was demonstrated experimentally and shown to enable excellent control of the far-field amplitude and phase. Finally, circular and linear birefringent waveguides were explored. Analytic solutions to rectangular birefringent hollow-core waveguides were developed and some initial demonstration experiments were performed.

[pt] A aplicação de um guia circular corrugado como alimentador é analisada a partir do conhecimento das distribuições de campo elétrico e magnético no plano focal de refletores parabólicos. O desenvolvimento matemático dos campos no interior da estrutura conduz a expressões para os diagramas de radiação e demais parâmetros de interesse. Diversas peculiaridades inerentes à propagação em estruturas corrugadas ficam caracterizadas ao longo da análise, alcançando-se a notável situação de simetria dos diagramas e polarização cruzada nula. Inclui-se um projeto para utilização de refletores de pequena e grande distância focal. / [en] The use of a corrugated circular waveguide as a feeder is analysed by considering the distributions of eletric and magnetic fields at the focal plane of parabolic reflectors. The mathematical field development inside the structure leads to expressions of radiation patterns and the necessary parameters. Several peculiarities in connection with the propagation in corrugated structures are characterized through the analysis, obtaining the remarkable situation of patterns simmetry and zero cross- polarization. A feeder design for using small and large focal lenght reflectorsis included.

The realization of wavelength scale and sub-wavelength scale fabrication of integrated optical devices has led to a concurrent need for computational design tools that can accurately model electromagnetic phenomena on these length scales. This dissertation describes the physical, analytical, numerical, and software developments utilized for practical implementation of two particular frequency domain design tools: the modal method for multilayer waveguides and one-dimensional lamellar gratings and the Rigorous Coupled Wave Analysis (RCWA) for 1D, 2D, and 3D periodic optical structures and integrated optical devices. These design tools, including some novel numerical and programming extensions developed during the course of this work, were then applied to investigate the design of a few unique integrated waveguide and grating structures and the associated physical phenomena exploited by those structures. The properties and design of a multilayer, multimode waveguide-grating, guided mode resonance (GMR) filter are investigated. The multilayer, multimode GMR filters studied consist of alternating high and low refractive index layers of various thicknesses with a binary grating etched into the top layer. The separation of spectral wavelength resonances supported by a multimode GMR structure with fixed grating parameters is shown to be controllable from coarse to fine through the use of tightly controlled, but realizable, choices for multiple layer thicknesses in a two material waveguide; effectively performing the simultaneous engineering of the wavelength dispersion for multiple waveguide grating modes. This idea of simultaneous dispersion band tailoring is then used to design a multilayer, multimode GMR filter that possesses broadened angular acceptance for multiple wavelengths incident at a single angle of incidence. The effect of a steady-state linear loss or gain on the wavelength response of a GMR filter is studied. A linear loss added to the primary guiding layer of a GMR filter is shown to produce enhanced resonant absorption of light by the GMR structure. Similarly, linear gain added to the guiding layer is shown to produce enhanced resonant reflection and transmission from a GMR structure with decreased spectral line width. A combination of 2D and 3D modeling is utilized to investigate the properties of an embedded waveguide grating structure used in filtering/reflecting an incident guided mode. For the embedded waveguide grating, 2D modeling suggests the possibility of using low index periodic inclusions to create an embedded grating resonant filter, but the results of 3D RCWA modeling suggest that transverse low index periodic inclusions produce a resonant lossy cavity as opposed to a resonant reflecting mirror. A novel concept for an all-dielectric unidirectional dual grating output coupler is proposed and rigorously analyzed. A multilayer, single-mode, high and graded-index, slab waveguide is placed atop a slightly lower index substrate. The properties of the individual gratings etched into the waveguide's cover/air and substrate/air interfaces are then chosen such that no propagating diffracted orders are present in the device superstrate and only a single order is present outside the structure in the substrate. The concept produces a robust output coupler that requires neither phase-matching of the two gratings nor any resonances in the structure, and is very tolerant to potential errors in fabrication. Up to 96% coupling efficiency from the substrate-side grating is obtained over a wide range of grating properties.

Waveguide architectures were inscribed within two different stimuli-responsive hydrogels capable of actuation. An electroactive hydrogel, which deforms when placed within an electric field, is demonstrated as a method for remote actuation and steering of light outputs. Lattices of waveguide with diameters on the microscale were embedded within hydrogel prisms, achieved through a nonlinear light propagation process known as self-trapping. This process is a result of balance between the natural divergence of light and self-focusing effects caused by an irreversible positive refractive index change during photopolymerization. Waveguiding structures are inscribed in the material because of this process. Square (2D) and near-cubic (3D) lattices were inscribed in hydrogel prisms, demonstrating the ability to remotely steer one or two light outputs simultaneously using an electric field. The overall optical effect is reminiscent of camouflaging techniques observed in marine creatures (ie. cephalopods). Additionally, a novel volumetric 3D printing technique previously demonstrated by the Saravanamuttu group was implemented to fabricate hydrogel cylinders capable of photothermal actuation. Coupling a thermoresponsive hydrogel material with a photoabsorber, areas irradiated by a light source are observed to contract. These contractions lead to the deflection of waveguiding cylinders towards the light source, reminiscent of the phototropic behaviours observed in particular plants (ie. sunflowers). The results of these studies provide insight for the fabrication of functional materials through nonlinear light propagation. Understanding these systems could provide knowledge for the fabrication of other stimuli-responsive materials with light-guiding properties. / Thesis / Master of Science (MSc)

No description available.

<p> An optical temperature sensor and Fourier spectrometer, working in the 1550nm telecommunications wavelength range, were fabricated in silicon-on-insulator. Both devices were based on asymmetric Mach-Zehnder Interferometer waveguide geometries. The temperature sensor underwent a two phase design. The various asymmetry factors, due to different path length differences, of the Mach-Zehnder arms resulted in different levels of temperature sensitivity, which in turn was the driving mechanism behind the Fourier spectrometer. Due to the asymmetry of the Mach-Zehnder arms, there exists an inherent optical path length difference which is further changed with temperature variation due to the thermo-optic effect. The phase I temperature sensor showed an accuracy of 1-2°C and a sensitivity of 0.5°C for ΔL of 37.23μm and 23.46μm, respectively. The phase II temperature sensor design, which allowed for self normalization, resulted in a 1°C temperature accuracy and a 0.5°C sensitivity for a ΔL of 27.85μm. Both the phase I and II temperature sensors showed repeatable and stable results for the temperature range of 20-100°C, and agreed well with the theoretical design performance. Upon analysis of the highly asymmetric Mach-Zehnder designs it was found that both the 1.05cm and 3.05cm path length differences resulted in a temperature accuracy of 0.1°C, with a 0.05°C sensitivity over a small temperature range.</p> <p> The Fourier spectrometer exhibited decent agreeability with theoretical design performance and demonstrated proof of concept. A 1.05cm path length difference was insufficient to resolve two wavelengths at 1546.12nm and 1564.68nm, which agreed with the theoretical model. However, the 3.05cm ΔL was sufficient to resolve the two wavelengths in a repeatable manner.</p> / Thesis / Master of Applied Science (MASc)

<p> Ion implanted Bragg gratings integrated in rib waveguide structures were simulated, fabricated and characterized for the silicon-on-insulator (SOI) photonics platform. After selective silicon self-implantation, to an amorphizing dose of 2x10^15 ions/cm^2, the approximately 0.3 damage-induced increase in the refractive index provided the modulation mechanism necessary for the formation of a Bragg grating. The benefits of implanted Bragg gratings compared to the more widely utilized surface relief type gratings include planar surface retention, desirable for subsequent processing and wafer bonding, and a smaller depth of the index modulation, important for minimizing filtering bandwidths. To our knowledge, this is the first time ion implantation has been utilized to produce Bragg gratings integrated in an SOI rib waveguide. The benefits of using SOI for an optoelectronics platform include: cost minimization, reduced device size, and compatibility with silicon based microelectronics.</p> <p> Device performance was simulated using coupled mode theory (CMT) in conjunction with beam propagation methods (BPM), to determine transverse modal profiles for computing coupling coefficients and to determine geometric dimensions suitable to achieve adequate grating strength and single-mode operation. The Monte Carlo ion implantation simulator SUSPREM4, implementing the binary collision approximation (BCA), was used to determine the amorphous silicon grating profiles. Implanted grating devices were then fabricated into SOI having a 2.5 μm device layer and were optically characterized. For a grating length of 2100 μm and an implant energy of 60 keV, the extinction ratio of the resonant wavelength was found to be -18.11 dB and -0.87 dB for TE and TM polarizations respectively. The excess loss per unit length was measured to be 1.2 dB/mm for TE polarization and 0.6 dB/mm for TM polarization. After annealing the gratings at temperatures of up to 300 °C, used to annihilate low energy point defects responsible for absorption, it was found that the excess loss per unit length was reduced to 0.3 dB/mm for TE polarization. Compared to etched gratings with similar dimensions, it was determined that the strength of the implanted gratings was approximately 2.5 times stronger for grating lengths one third the length as result of mode-shifting due to the higher index of refraction. This is of great consequence to the miniaturization and densification of Bragg grating based devices in silicon photonics.</p> / Thesis / Master of Applied Science (MASc)

The nonlinear propagation of a visible, continuous wave laser beam was studied in three types of polymer systems that harness photochemical reactions: (i) a photopolymerization to create permanent self-written structures, (ii) a photo-oxidation hosted within a polymer matrix and (iii) a reversible photoisomerization that triggers the contraction of a photoresponsive hydrogel. The process of self-trapping was characterized by monitoring the spatial intensity profiles over time. The mechanism of each material was determined with a series of control experiments in order to confirm the nature of the nonlinear response, including their reversibility and intensity-dependence. These observations led to the study of interactions between self-trapped beams. Two beams under linear conditions will pass through one another, but two beams travelling in a nonlinear medium will interact and influence one another. The interactions of two beams introduced into the aforementioned photochemical systems were investigated and revealed a rich diversity of phenomena including: (i) the attraction between beams, (ii) merging of beams into a single waveguide, (iii) nonlocal attraction between beams, (iv) orbiting of beams, (v) switching of beam positions, and (vi) inhibition of the self-trapping of a neighbouring beam. Each observation is dependent on a detailed understanding of the underlying mechanism of refractive index change. Numerical simulations supplement some of these experiments and provide further evidence for the nonlinear mechanisms. The formation of permanent self-written structures with these nonlinear waves offers the opportunity to create seamless 3D printed materials with prismatic geometries. Several macroscopic objects were constructed using nonlinear waves from incoherent LEDs and amplitude masks. Decomposition of 3D objects into prismatic elements was carried out using an algorithm that breaks an object into individual pieces. Using a multi-step printing process, several prismatic elements can be combined to form a target object. The results of these experimental and theoretical studies improve upon the current understanding of the dynamics of nonlinear light propagation in photochemical systems. These insights may allow us to harness other nonlinear effects and develop new materials for applications such as optical communication, computing and 3D printing. / Thesis / Doctor of Science (PhD) / The nonlinear propagation of a visible, continuous wave laser beam was studied in three types of polymer systems that harness photochemical reactions: (i) a photopolymerization to create permanent self-written structures, (ii) a photo-oxidation hosted within a polymer matrix and (iii) a reversible photoisomerization that triggers the contraction of a photoresponsive hydrogel. Photochemical changes to the material lead to self-induced light-guiding structures that influence the behaviour of light. These self-trapped beams can interact with one another inside of a nonlinear medium, giving rise to a rich diversity of phenomena including: (i) the attraction between beams, (ii) merging of beams into a single waveguide, (iii) nonlocal attraction between beams, (iv) orbiting of beams, (v) switching of beam positions, and (vi) inhibition of the self-trapping of a neighbouring beam. Each observation is dependent on a detailed understanding of the underlying mechanism of refractive index change. Numerical simulations supplement some of these experiments and provide further evidence for the nonlinear mechanisms. The formation of permanent self-written structures with these nonlinear waves offers the opportunity to create seamless 3D printed materials with prismatic geometries. Several macroscopic objects were constructed using nonlinear waves from incoherent LEDs and amplitude masks. Decomposition of 3D objects into prismatic elements was carried out using an algorithm that breaks an object into individual pieces. Using a multi-step printing process, several prismatic elements can be combined to form a target object. The results of these experimental and theoretical studies improve upon the current understanding of the dynamics of nonlinear light propagation in photochemical systems. These insights may allow us to harness other nonlinear effects and develop new materials for applications such as optical communication, computing and 3D printing.

Microstructure and photonic crystal fibers with periodic as well as random refractive-index distributions are investigated. Two cases corresponding to fibers with one-dimensional (1D) radial index distributions and two-dimensional (2D) transverse index distributions are considered. For 1D geometries with an arbitrary number of cladding layers, exact analytical solutions of guided modes are obtained using a matrix approach. In this part, for random index distributions, the average transmission properties are calculated and the influence of glass/air ratio on these properties is assessed. Important transmission properties of the fundamental mode, including normalized propagation constant, chromatic dispersion, field distributions, and effective area, are evaluated. For 2D geometries, the numerical techniques, FDTD (Finite-Difference Time-Domain) method and FDM (Finite Difference Method), are utilized. First, structures with periodic index distributions are examined. The investigation is then extended to microstructure optical fibers with random index distributions. Design of 2D microstructure fibers with random air-hole distributions is undertaken with the aim of achieving single-mode guiding property and small effective area. The former is a unique feature of the holey fiber with periodic air-hole arrangement and the latter is a suitable property for nonlinear fiber devices. Measurements of holey fibers with random air-hole distributions constitute an important experimental task of this research. Using a section of a holey fiber fabricated in the draw tower facility at Virginia Tech, measurements of transmission spectra and fiber attenuation are performed. Also, test results for far-field pattern measurements are presented. Another objective of this dissertation is to explore new applications for holey fibers with random or periodic hole distributions. In the course of measuring the holey fibers, it was noticed that robust temperature-insensitive pressure sensors can be made with these fibers. This offers an opportunity for new low-cost and reliable pressure fiber-optic sensors. Incorporating gratings into holey fibers in conjunction with the possibility of dynamic tuning offers desirable characteristics with potential applications in communications and sensing. Injecting gases or liquids in holey fibers with gratings changes their transmission characteristics. These changes may be exploited in designing tunable optical filters for communication applications or making gas/liquid sensor devices. / Ph. D.