M.Ing. (Mechanical Engineering) / The dissertation explores the applications and limitations of optic Fibre Bragg Grating (FBG) sensors for the purpose of structural health monitoring of high performance composite aerospace structures. The absence of a set of stringent performance criteria governing the form and function of a sensory system for embedded high performance applications highlights the major hurdle to be overcome before widespread acceptance of these technologies becomes apparent. The dissertation therefore develops through an extensive literature study a basic framework of performance criteria to be met by the sensory system upon which a prototype Structural Health Monitoring (SHM) system can be further developed. The resolution of the performance criteria into categories of mechanical and non-mechanical performance allows independent evaluation of factors that directly affect the performance of the sensor (in terms of strength, embeddability and load carrying ability) as well as its functional performance (in terms of orientation, spatial resolution and measurement philosophy). The literature study uses the non-mechanical performance limitations as a guideline for the selection of Fibre Bragg Grating (FBG) sensors as the sensory mechanism. The mechanical performance limitations of these specific sensors are then called into question and evaluated. Independent experiment campaigns are therefore developed to evaluate the mechanical and non-mechanical performance limitations such that a set of performance criteria can be developed governing the use of embedded sensory systems. Non mechanical performance with particular emphasis on sensor placement and orientation is investigated by simulating a fixed-free Euler Bernoulli cantilever using the Finite Element Method (FEM). The ability of the sensor to identify structural changes by measuring changes in modal response shows good results. Furthermore the inability of modal based monitoring to identify structural changes in the vicinity of modal inflection points is identified as an opportunity to locate structural deficiencies by monitoring multiple modes with known inflection point positions. The method also provides recommendations of sensor placement and orientation (close to the beam fixture and parallel with the neutral axis) such that the effectiveness of strain component measurements from all measurable modes is maximised. Mechanical performance of embedded FBG sensors is evaluated through an extensive fracture testing program which measures the fracture strains of fibre samples subjected to two-point bending. The fracture test program allows the quantification of the effects of the presence of the fibre’s protective polymer coating on fibre embeddability in composites, the consequent effects that the removal of this coating has on the mechanical performance and fracture behaviour of FBG sensors. These effects are qualified and mitigatory measures developed to improve the mechanical performance. A system of crack masking, hydrofluoric acid etching and fibre treatment is developed and statistical data analysis methods are employed and refined such that improvements in the mechanical properties of the FBG sensors can be quantified. An evaluation of the effectiveness of the proposed mechanical performance improvements yields good results culminating in the development of a comprehensive set of mechanical performance criteria to facilitate further development of a reliable SHM system.

M.Ing. / Bragg gratings are important components for sensing and for wavelength-division multiplexed optical communication systems. These gratings are manufactured by either side-writing of the fiber with a high intensity UV light through a phase mask, or by exposing the fiber to interference fringes through an interferometer arrangement. With one phase mask, only a small range of grating wavelengths is possible. This is achieved by pre-straining the fiber during the writing process. The limitation arises from the break strength of the fi ber, allowing a maximum range of Bragg wavelengths of only approximately 10 nm. The interferometric technique uses a beam splitter to divide a single input UV beam into two and intersecting them at the fiber. The angle at which the beams intersect will determine the period of the interference fringes and thus the Bragg grating written in the optical fiber. The Argon-ion laser is used with a 1060 nm phase mask (used to split beam) to write Bragg gratings with reflections from 1012 to 1600 nm. Three accurate- translation and rotation stages are used to keep the fiber at the beam intersection. Alignment, mechanical stability and coherence of laser are critical.

M.Ing. / Fibre Bragg gratings (FBGs) have been manufactured for the first time in South Africa by means of the phase mask method. It is possible to manufacture not only uniform FBGs, but also chirped FBGs. The optical fibre that is used for imprinting the FBGs can also be hydrogen loaded locally. FBGs with a reflectivity of 99.7% and higher can be written by making use of the experimental setup presented in this thesis. It is possible to manufacture a FBG with a centre wavelength that has any value between the Bragg wavelength and approximately 6 nm lower than the Bragg wavelength. This is done by stretching the optical fibre prior to the writing process. FBGs have been simulated in MATLAB to get an idea of what one may expect during the manufacturing process. The program makes it possible to simulate the effects of changes in grating length, index modulation, pressure, temperature and strain on the centre wavelength of an FBG. Dispersion is explained in detail. Chromatic dispersion, which is part of dispersion as a whole, can be cancelled by making use of an FBG. The different techniques for the measurement of chromatic dispersion is explained. Some insight is given on dispersion (the pulse broadening in the time domain due to the different velocities of different wavelengths from the source's finite optical bandwidth) compensation. An FBG that was manufactured locally has been tested as a dispersion compensator. It was found that an FBG is effective in performing this function.

Much of the analytical and computational work necessary for the development of distributed fiber-optic strain sensors using photo induced Bragg gratings is presented. The one dimensional wave equation is solved for a slowly varying sinusoidal modulation of the index of refraction. The solution is found to take the form of a fourier transform for low reflectivity «15%) gratings. As a result, the process can be inverted, and if the phase and amplitude of the reflected light can be measured over frequency, the phase and amplitude of the bragg grating as a function of length can be computed using the inverse fourier transform. These results are computationally verified, and then further analysis of critical engineering parameters is carried out. A measurement system and procedure are described. A method of writing long, low-reflectivity bragg grating is proposed. / Master of Science

No description available.

M.Ing. / Optical fibre Bragg gratings are a periodic variation of the refractive index in the core of an optical fibre andmay be formed by exposure to intense UV laser light under specific conditions. Light at a certain wavelength, called the Bragg wavelength, is reflected back when illuminating the grating with a light source. Bragg gratings can relatively easily be employed as strain and temperature sensors, but have small sensitivity for pressure. Special transducers are required to increase the sensitivity. A pressure sensor was manufactured by coating a fibre Bragg grating with a polymer. The polymer coating converts transverse pressure into longitudinal strain through the Poisson effect inside the polymer coating. This thesis investigates the sensitivity of themanufactured Bragg grating pressure sensor, by using the method of finite element analysis. An account of the experimental setup, whereby the Bragg grating is written with a frequency tripled Nd:YAG laser, is given. The process whereby the fibre is coated with the polymer is described. The sensor is characterized through experimental results and a comparison is made between theoretical and experimental results. Uses for this sensor and ways with which the sensitivity may be increased are suggested as future work.

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (p. 140-144).

The main topic of this thesis is realization and characterization of fibre Bragg gratings. A novel versatile grating fabrication technique is developed and a number of gratings are realized, showing the potential of the system. Arbitrarily-shaped gratings are sequentially imprinted in the fibre by a moving interference pattern created with a continuous-wave ultraviolet (UV) source. This scheme allows for a very good control and stability of the grating shape, which is also shown experimentally. As opposed to most other present fabrication techniques, the proposed method offers a total control over the grating parameters by software, enabling simple implementation of new designs. Different kinds of error sources when stitching long gratings are identified and investigated regarding impact on the final grating result. Another important question within this field is how to characterize gratings. We propose a new characterization method based on optical low-coherence reflectometry (OLCR). A new interferometer design allows for simple simultaneous detection of the reflection response from two different points in the interrogated grating, so that differential measurements can be performed. The advantage of this is that the sensitivity to noise caused by e.g. thermal fluctuations in the system is substantially reduced. Several test gratings have been investigated and a very good agreement to the expected results is noted. A second characterization technique using interferometric detection of the side diffraction from the grating under test is investigated both theoretically and experimentally. With aid of two-dimensional theory for wave propagation, it is shown that there is a linear relation between the detected phase and modulation depth and the corresponding grating properties. The technique is evaluated with a novel scheme of implementation where the UV source provided in a fabrication setup is used assource for the side probe. This approach results in a very simple implementation and opens for an integration of the characterization and fabrication systems. Finally, a tuning method for transmission filters based on local heating of linearly chirped fibre Bragg gratings is analysed and further developed to allow for fully software-controlled operation. The potential of this technique is illustrated by some promising initial experimental results. / QC 20100812

[pt] O objetivo principal deste trabalho foi o de estudar as redes de Bragg em fibras ópticas e suas principais aplicações, com especial interesse na compensação de dispersão de pulsos ópticos em sistemas de telecomunicações, onde uma implementação original foi construída. Faz-se uma revisão geral das técnicas existentes para fabricá-las e caracterizá-las, além de suas principais aplicações, não somente na área de telecomunicações mas também na área sensoriamento óptico. São mostrados os resultados experimentais obtidos nas montagem realizadas para escrever redes de Bragg em fibras ópticas e para caracterizá-las. Algumas experiências usando redes de Bragg para o controle do comprimento de onda de emissão de um laser de semicondutor e para medição de força aplicada numa estrutura são mostradas. São mostrados também os resultados experimentais inéditos obtidos com a compensação de dispersão de pulsos ópticos em enlaces de telecomunicações, onde várias montagens foram realizadas para comprovar a superioridade da utilização da redes de Bragg para esse fim. Demonstra-se também um sistema para gerar pulsos solitônicos sintonizável em comprimento de onde numa faixa de MHZ. Finalmente são mostrados os resultados obtidos com a utilização de fibras ópticas para chaveamento interferométrico de luz. Neste sistema são utilização como elemento óptico não linear fibras tipo D, com e sem redes de Bragg, revestidas com filme de materiais semicondutores amorfos. / [en] Bragg grating in optical fibers are used to compensate the dispersion of optical pulses in telecommunication systems. Initially, an overview is presented of the existing fabrication techniques of such gratings, as well as the techniques used for characterization. Some useful applecations in telecommunications and sensing are also shown. The results obtained in the set-up to write and to characterize Bragg gratings are presented. Some measurements were also done with typical applications such as controlling the emitting wavelength in semiconductors lasers, and monitoring the applied pressure in an optical fiber. Some new results are described on dispersion compensation in fibers. Using specially chirped Bragg gratings to pre- compensatedispersion of an optical pulse in a 80 km fiber link, we demonstrated that the poor quality of the input laser pulses has little impact on the duration of the pulses recovered after transmission. Finally, an interferometric switching arrangement using a semiconductor coated D fiber as nonlinear control element, is presented. The configuration uses infrared optical pulses to control a continuous wave light signal. It is also shown that the original reflected wavelength of a semiconductor coated D-fiber with Bragg grating can be shited with an external optical control signal.

Neste trabalho é apresentado um sistema de leitura de redes de Bragg derivado da técnica por filtro Fabry-Perot sintonizável. A técnica apresentada é implementada através de um sistema atuador piezelétrico que modula mecanicamente uma rede de Bragg de referência, com seu espectro de reflexão convoluido com o espectro de reflexão de uma rede sensora. A técnica desenvolvida apresenta vantagens em relação ao processo original por ter implementação mais simples, menor custo e melhor discriminação de posição espectral. / This work presents a process to detect Bragg grating spectra based on the tunable Fabry Perot filter technique. The apparatus is assembled with a piezoelectric actuator that mechanically modulates a reference Bragg grating. The spectrum of that grating is convoluted with the spectrum of the sensor during a sweep. The device presents some advantages as, eg., simplicity, lower cost and better discrimination for the spectral position of the reflected band.