An investigation into performance criteria for Fibre Bragg Grating sensors embedded in composite structures
- Authors: Roberson, Craig Valentine
- Date: 2014-09-17
- Subjects: Bragg gratings , Fiber optics , Diffraction gratings
- Type: Thesis
- Identifier: uj:12338 , http://hdl.handle.net/10210/12124
- Description: 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.
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Manufacturing of fibre bragg gratings for dispersion compensation
- Authors: De Bruyn, Louis
- Date: 2011-11-30
- Subjects: Optical fibers , Diffraction gratings , Fibre bragg gratings
- Type: Thesis
- Identifier: uj:1751 , http://hdl.handle.net/10210/4106
- Description: 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.
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Add-drop multiplexers using fibre bragg gratings and optical couplers
- Authors: Naude, Riaan
- Date: 2009-02-26T12:21:04Z
- Subjects: Diffraction gratings , Optical fibers
- Type: Thesis
- Identifier: uj:8168 , http://hdl.handle.net/10210/2173
- Description: M.Ing. , This thesis, devoted to fibre optics, is primarily concerned with the utilization of fibre Bragg gratings and optical couplers to realize optical add-drop multiplexers (OADMs). A comparative study regarding various OADM configurations is undertaken on the basis of manufacturing and the performance in terms of insertion loss, channel isolation, tuning ranges, stability and cost. The heart of most of the OADMs is fibre Bragg gratings. The Runge-Kutta numerical integration method is used to solve the coupled-mode equations in order to simulate the spectral dependence of Bragg gratings numerically. Properties such as the grating strength, the grating length and the grating index profile governing the spectral dependence of Bragg gratings are investigated. In recent years, there has been an increasing interest in the dispersive properties of Bragg gratings. We investigate methods to limit the amount of dispersion induced by fibre Bragg gratings. The tuning of Bragg gratings for dynamic OADMs is also reviewed. High channel isolation Bragg gratings are theoretically and experimentally investigated. DC-apodized gratings were designed and manufactured by using the phase mask method through the use of a preconditioning technique. Bragg gratings with channel isolations of up to 24.61 dB have been realized by using this technique. The spectral dependence of DC-apodized gratings on the amount of preconditioning and the smoothness of the index envelope is simulated and in agreement with the experimental results. An athermal Bragg grating was designed and manufactured, exhibiting an average wavelength-temperature sensitivity of 2.76 pm/oC. An OADM comprising a DC-apodized Kaiser grating and an optical circulator was realized. The device showed an insertion loss of 1.84 dB and a channel isolation of 22.84 dB. The coupling mechanisms for different types of optical couplers are investigated. The distribution of power was established to be either by evanescent field coupling (etched, polished and weakly fused couplers) or due to the beating phenomenon (strongly fused couplers). The beating phenomenon of the HE11 and HE12 modes in the waist of the tapered-fused coupler is modelled and used to simulate different characteristics, such as wavelength, polarization and external refractive index dependence of tapered-fused couplers, in order to realize OADMs.
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Realization of chirped fibre Bragg gratings by strain gradients and their applications for fibre dispersing compensation
- Authors: Zhu, Yinian
- Date: 2012-09-05
- Subjects: Optical fibers , Optical detectors , Diffraction gratings
- Type: Thesis
- Identifier: uj:9621 , http://hdl.handle.net/10210/7040
- Description: 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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