Optical fiber sensors are widely applied in modern sensing systems. Taking advantage of the high sensitivity of optical interference, optical coherence fiber sensors, such as fiber Bragg gratings (FBGs) and Fabry-Perot interferometers (FPIs), have been investigated intensively and utilized broadly in optical sensing systems. Multimode fibers (MMFs) offer low coupling loss, high compatibility with various light sources, and insensitivity to ambient fluctuations, which are preferred for reliable and low-cost sensing systems. Therefore, the combination of optical coherence sensors and MMFs have facilitated and will continue to contribute to various optical sensor designs with desirable performances. This dissertation addresses the design and construction of optical coherence sensors in multimode fibers and presents several fully multimode fiber sensing systems with low coherence light source. Moreover, a theoretical analysis of fiber mode excitations, model coupling, and multimodal interference is conducted, and a numerical model is constructed to study the behaviors of optical coherence sensors in MMFs. With the femtosecond laser point-by-point inscription method, parallel fiber Bragg gratings (pFBGs), scattering array interferometers (SAIs), and densely multiplexable scattering array interferometers (DMSAIs) in sapphire fibers are proposed, fabricated and characterized, achieving excellent performance in multiplexed high temperature sensing. The study on SAI signals also revealed the unique coherence properties of MMFs. This work points out that the coherence properties in MMFs play a significant role in affecting the performances of optical coherence sensors, and such properties are closely related to both geometrical and optical properties of the fibers. This work also presents both theoretical and experimental tools to explore such properties and predict and test the performance of optical coherence sensors in MMFs, which is of great significance in the applications of such sensors in the real world. / Doctor of Philosophy / Optical fibers have been considered a powerful media that opened a new era in the field of telecommunication and optical sensing. Owing to their excellent resistance to chemical corrosion, immunity to electromagnetic interference, extremely low loss transmission at long distance, small size, and large aspect ratio, optical fibers are considered an ideal media to construct optical sensors. Optical coherence sensors are a very important type of optical fiber sensors that utilize the optical coherence property, such as interference, for sensing purposes. A lot of such sensors are generally constructed with single mode fibers (SMFs) owing to the high-quality coherence interaction supported by the fundamental-mode-only operation. Multimode fibers (MMFs), however, process high compatibility with various light sources owing to the high power-coupling efficiency because of large core size, which is desirable for industrial applications that requires low-cost and robust sensing systems. Meanwhile, the high modal volume of MMFs causes severe challenges on the design and fabrication of optical coherence sensors in MMFs. In this work, theoretical analysis of the mode excitation and coupling in MMFs is discussed, and a numerical model to simulate the behaviors of optical coherence sensors in MMFs is built. Then, using femtosecond laser point-by-point fabrication technique, parallel fiber Bragg gratings (pFBGs), scattering array interferometers (SAIs), and densely multiplexable scattering array interferometers (DMSAIs) are demonstrated and proven to be effective in multiplexable temperature sensing. Furthermore, using the numerical model and the SAI structures, coherence properties in MMFs are studied. This research may facilitate a deeper understanding of coherence properties in optical waveguides and support the design of novel fiber sensors that can be utilized in the real world.

Renewable, recyclable, and high performing barrier materials were made from cellulose nano fibers. Various strategies to enhance performance in dry, wet and humid conditions were proposed. These methods included thermal treatment to induce hornification, PAE resin based cross linking, and inclusion of high aspect ratio filler materials to form composites. Results indicated that hornification alone, even though effective in enhancing the barrier properties comes at the cost of severe degradation of mechanical properties. In the second case, where a cross linker was used, lower heating temperature limited the degradation of mechanical properties. Moreover, the new bonds included due to cross linking also modified the mechanical properties of the material and cause significant improvement. In the case of inclusion of filler materials, improvement of mechanical properties due to reinforcing effect was observed, and additionally the improvement in barrier properties was observed due to increased tortuosity of the materials. Furthermore, when the composites were made with cross linker, there was a significant improvement in barrier and mechanical properties as compared to the barrier material made from the pure cellulose nano fibers. In all cases the barrier materials were found to be resistant to degradation by water, as measured by water retention value, and surface contact angle. The resistance to water in the first case was as a result of severe hornification of the material. Whereas in the second and third case the cross linking and concomitant limited hornification played a significant role in water resistance. In addition to the three methods to improve barrier properties, the use of nano fibers made from cellulose II was also studied. Different stages of fibrillation of the starting cellulose pulps were studied and the fibers and films made from them were characterized in detail. Results from this study indicated that fibers made from cellulose II pulp are much harder to fibrillate as compared to cellulose I fibers. Moreover, due to fibril aggregation it is harder to form nano fibers from cellulose II. Even though from the perspective of better inter and intra fibril bonding cellulose II might be favorable over cellulose I, significant work in the formation of nano fibers from cellulose II is required before they can be used for making barrier materials.

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LD2668 .T4 1966 W137 / Master of Science

M.Ing. / A study was done on optical fibre hydrophones. Integrated optic hydrophone components as well as optical fibre components were investigated. An optical fibre coupler was successfully manufactured with an etch process. The coupling ratio can be varied mechanically or by means of refraction index variation. Experimental procedures and results will be shown. Bending losses excited in an optical fibre coil were investigated as a function of different coil diameters. The results were extrapolated to determine the amount of losses arising in a hydrophone coil.

The replacement of asbestos fibres with cellulose fibres in producing corrugated fibre reinforced cement sheets by the Hatschek process resulted in edge cracking for stacked sheets. This was due to the hydrophilic nature of cellulose, which increases its tendency for exchanging water with the surroundings. The drying process of corrugated sheets, in a stack, resulted in shrinkage hence edge cracking along the sheet. To reduce the magnitude of drying shrinkage and edge cracking potential, several mitigation strategies were proposed including the surface treatment of cellulose fibres, incorporation of wollastonite microfibres, addition of admixtures and superplasticizers, kaolin inclusion as partial replacement of cement and different exposure conditions. A fundamental understanding in mechanisms behind volume changes and how cracks form was crucial for optimization of the mitigation strategies. This thesis initially used a review approach to understand the mechanisms involved in different types of shrinkage and the role of different mitigation techniques. The ultimate goal was to achieve lower drying shrinkage and cracking risks in corrugated sheets along with reducing its economic impact. As a result, surface treatment of cellulose fibres, based on transforming the hydrophilic nature of cellulose to hydrophobic state, was investigated. Furthermore, inclusion of wollastonite/ kaolin as partial replacement of cement, were evaluated. Also, the potential of adding admixtures/ superplasticizers was explored. Finally, investigation on development of edge cracks in stacked corrugated fibrecement sheets was conducted under different exposure conditions. The results and findings of this research showed no significant improvement in permeability with cellulose surface treatment. Wollastonite microfibres promoted pore discontinuity hence significant reduction in permeability thus lower drying shrinkage. However, the resultant sheets were brittle. By reducing water content with addition of superplasticizers, density was enhanced thus reducing volume change from drying and wetting. Kaolin acted as internal restraint for shrinkage, refining the microstructure at the interfacial transition zone thus increasing density and its pozzolanic reaction enhanced mechanical properties. The inclusion of kaolin in the fibrecement mix in conjunction with controlling exposure conditions managed to eliminate edge cracking.

A Research Report submitted to the Faculty of Architecture, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Building. / Environmental and health considerations have led to the substitution of cellulosic fibres for asbestos fibres in many cement-based products. Inevitably, the substitution has required modifications to the manufacturing process. Certain production techniques associated with the switch to cellulosic fibres and other additives needed elucidation. It is possible that to ensure the required concentration of certalr additives in the final product, an excess is being used in the process because of uncertainties about the quantitative balance of the process. My intention was to examine the potential tole for Fourier Transform Infrared (FTtR) Spectroscopy in the investigation of the materials constituting fibre-reinforced cement boards. I was able to show that qualitative and quantitative determination of certain of the components in the final product are possible with adequate precision and reproducibility to be of value to the manufacturer, I also showed that care taken in the preparation of standard and analytical samples was essential for the success of such analyses. / Andrew Chakane 2018

A Research Report submitted to the Faculty of Architecture, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Building. / Environmental and health considerations have led to the substitution of cellulosic fibres for asbestos fibres in many cement-based products. Inevitably, the substitution has required modifications to the manufacturing process. Certain production techniques associated with the switch to cellulosic fibres and other additives needed elucidation. It is possible that to ensure the required concentration of certaln additives in the final product, an excess is being used in the process because of uncertainties about the quantitative balance of the process. / Andrew Chakane 2019

by Hung Wing-yiu. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1988. / Bibliography: leaves 98-103.

This work presents mechanical nano-grinding as an alternative technique for the fabrication of optical fibers endface microlenses. It also presents a novel surface-roughness improvement technique called Loose Abrasive Blasting (LAB). Traditionally, the majority of such microlenses are made using either chemical etching or heating and pulling methods. Despite the success of these methods, they suffer some common drawbacks such as the lack of controllability on the produced lens profile. Consequently the possible variations of the lens profiles that can be made by these methods are also limited. The difficulty to center the lens on the fiber core is another problem associated with heating and pulling method. The exposure to hazardous chemical such as hydrofluoric acid is yet another problem associated with chemical etching. Nano-grinding technique described in this thesis should provide a much better alternative to the traditional optical fabrication techniques. Nano-grinding experiments were conducted on a nano-grinding machine (NGM) specially built for this purpose. The machine incorporates state-of-the-art air-bearing spindles, piezo electric actuators, and capacitive displacement sensors with accuracy down to 2 nm. Such precise motion provided by this system is the key for the success of this technique. With such system, it was possible to produce a multitude variety of lens profiles with high profile accuracy and with surfaces of optical quality without the need for exposure to any kind of hazardous chemicals. In achieving this objective, the research was conducted on many frontiers. First, the possibility of grinding optical fibers without inducing surface and subsurface damages was investigated. Micro-indentation, nano-indentation, and nano-scratch tests were conducted to determine the critical depth of cut that can be achieved before the occurrence of surface and subsurface cracks. Nano-scratch test in particular provided a clear insight to the cracking and the chipping mechanisms that might unfold if the critical depth of cut was exceeded in an actual grinding situation. The knowledge gained from this exercise laid the ground base for the design of the NGM. Using the NGM, further experiments were carried out to determine the optimal grinding parameters for an efficient and successful grinding process. Parameters investigated include the grit size, the cutting speed, and the in-feed rates. The optimum parameters have to ensure the best endface surface quality and the same time maintain a high throughput. This study shows that based on these optimal parameters, it should be possible to produce endface microlenses of optical surface quality free surface and/or subsurface damages in less than 30 seconds with surface roughness (Ra) less than 3 nm. A novel post-grinding surface improvement techniques was also developed. The technique called loose abrasive blasting (LAB) can be used for polishing at and non-flat surfaces. Experiments were conducted on a loose abrasive blasting machine built specially for this purpose. The performance of this technique was compared with other techniques such as slurry polishing and chemical etching used for polishing of brittle materials. The results showed that while chemical etching was found unsuitable for polishing of at optical fiber endfaces, LAB outperformed slurry polishing by significant margin. After the optimal grinding conditions were established, the NGM was used for grinding of different kinds of optical fiber microlens profiles. Among the endface profiles produced were conical lenses, tapered lenses, D-shaped lenses and others. It has also been shown, in case of conical lenses for instance, that there is almost unlimited number of profiles that can be produced by simply changing the contact angle between the fiber endface and the grinding film. The effect of surface roughness on light coupling efficiency between a fiber endface and a laser diode was also investigated. Cleaved fiber endfaces as well as ground endfaces with variant degrees of surface roughness were used in this experiment. The results showed that surface roughness has significant effect on light coupling efficiency. The effect of lens eccentricity on light coupling was also investigated.