In this thesis, the work focuses on developing distributed optical fiber vibration sensors based on phase-sensitive optical time domain reflectometry (Φ-OTDR). Three works have been accomplished to improve the performances of Φ-OTDR for distributed vibration sensing. Firstly, Φ-OTDR based on a polarization diversity scheme is demonstrated to mitigate the polarization mismatch effect occurring in traditional systems. A theoretical analysis is performed in different polarization cases corresponding to coherent and polarization diversity detection. Φ-OTDR based polarization diversity shows a great potential in the multi-events sensing application. Two vibration events are simultaneously detected and their signal to noise ratios are improved by 10.9 dB and 8.65 dB, respectively, compared to the results obtained by a conventional coherent scheme. Intensity fluctuation in a phase-sensitive optical-time domain reflectometry (Φ-OTDR) system caused by stochastic characteristics of Rayleigh backscattering has limited relative vibration strength measurement, which is proportional to dynamic strain. A trace-to-trace correlation coefficient is thus proposed to quantify the Φ-OTDR system stability and a novel approach of measuring the dynamic strain induced by various driving voltages of lead zicronate titanate (PZT) is demonstrated. Piezoelectric vibration signals are evaluated through analyzing peak values of the fast Fourier transform spectra at fundamental frequency and high-order harmonics based on Bessel functions. Experimental results show high correlation coefficients and good stability of our Φ-OTDR system, as well as the small measurement uncertainty of measured peak values. To reduce the intra-band noise caused by the finite extinction ratio of optical pulses, Φ-OTDR based on high extinction ratio generation is studied. Two methods are developed for achieving high extinction ratio of optical pulse generation. One of the approaches is to synchronize two cascaded electro-optic modulators to achieve high extinction ratio operation. The other one is to use the nonlinear optical fiber loop mirror as an optical switch to suppress the continuous wave portion of optical pulse. The sensing range of 1.8 km and 8.4 km with corresponding spatial resolution of 0.5 m and 2 m have been demonstrated based on cascaded two electro-optic modulators and nonlinear optical fiber loop mirror setup, respectively.

Oceans cover 71% Oceans cover 71% surface of the earth and are rich in oil and gas resources, marine living resources, renewable energy, mineral resources. The depths of the oceans are often thought of as a silent world, but that was never the case, and oceans have become noisier as human technologies have advanced. Humans have not only added noise to the ocean; they have also eliminated natural sounds. One of the primary noise sources is sonar. Sonar technology is widely used in fish detection, ocean floor mapping, and vehicle navigation. The noise in the ocean dramatically affects the animal’s survival and breaks marine ecological balance. Herein, underwater wireless optical communication (UWOC) and fiber communication and sensing (FC&S) technologies are proposed to minimize the acoustic noise in the ocean. Compared to noisy, powerful acoustic communication technology, UWOC has the merit of silence and takes advantage of high bandwidth, high transmission speed, and power efficiency. Multi-functions FC&S system turns the submarine telecommunication cable network into sensor network. UWOC and underwater FC&S technology can boost the development of Underwater Internet of things (UIoT) by establish large-scale underwater sensor networks. This dissertation aims to investigate and address noisy ocean issues and build large-scale underwater sensor networks by optical communication and sensing technology. The dissertation proposes using UWOC and FC&S technology to replace the conventional acoustic communication technology and reduce the noise in the ocean. UWOC helps achieve high-speed wireless communications between sensors, vehicles, and even humans for UIoT. The significant challenges of developing UWOC systems are the complex underwater environment's attenuation, scattering, and turbulence effects. This dissertation studied the turbulence effects on the UWOC system’s performance and addressed the pointing-acquisition-and-tracking issues. The diffuse-line-of-sight configuration and scintillating-fiber-based detector help the mobile UWOC systems relieve the strict requirements on PAT. FC&S technology is proposed to build underwater communication and sensor networks. Studies pave the way for UIoT and keep ocean silent. Such modality is much sought-after for implementing robust UWOC links in a complex oceanic environment, building large-scale sensor networks across the oceans, and minimizing noise pollution in the ocean.

The power industry relies on high voltage transformers as the backbone of power distribution networks. High voltage transformers are designed to handle immense electrical loads in hostile environments. Long term placement is desired, however by being under constant heavy load transformers face mechanical, thermal, and electrical stresses which lead to failures of the protection systems in place. The service life of a transformer is often limited by the life time of its insulation system. Insulation failures most often develop from thermal faults, or hotspots, and electrical faults, or partial discharges. Detecting hotspots and partial discharges to predict transformer life times is imperative and much research is focused towards these topics. As these protection systems fail they often generate gas or acoustic signals signifying a problem. Research has already been performed discovering new ways integrate optical fiber sensors into high voltage transformers. This thesis is a continuation of that research by attempting to improve sensor sensitivity for hydrogen and acetylene gasses. Of note is the fabrication of new hydrogen sensing fiber for operation around a larger absorption peak and also the improvement of the acetylene sensor's light source stability. Also detailed is the manufacturing of a field testable prototype and the non-sensitivity testing of several other gasses. The developed sensors are capable but still could be improved with the use of more powerful and stable light sources. / Master of Science

The objective of this thesis was to investigate if an optical fiber sensing method with the use of two overlapping fiber Bragg gratings to measure Lamb wave $S_0$ modes in a steel plate, and how it would compare to traditional PZT transducers. A solution was proposed where the use of an optical fiber sensing system was built and took advantage of the strain dependence of a fiber Bragg grating mounted to a steel plate. Together with an overlapping reference fiber Bragg grating, the system can translate strain to light intensity. A method of controlling the Bragg wavelength of the reference fiber Bragg grating to optimize the overlap between the two fiber Bragg gratings, enabling the system to compensate for drift in the sensing fiber Bragg grating. Testing of the system was performed and yielded promising results, being able to measure the Lamb wave signal from the steel plate. The system showed some sensitivity limitations and signal to noise ratio, as well as the software created to compensate for the drift. With the improvement proposed for further work with the system in terms of improving the system's sensitivity, signal to noise ratio and drift control should make the system able to perform at levels as traditional PZT transducers.

Made available in DSpace on 2016-03-15T19:38:17Z (GMT). No. of bitstreams: 1 Rafael Euzebio Pereira de Oliveira.pdf: 3969043 bytes, checksum: cfa66f3e9f3a18e9e86b364cd01a2c0a (MD5) Previous issue date: 2010-05-13 / In this dissertation the transmission properties of photonic crystal fibers are assessed under the influence of pressure and temperature for sensing applications. A review is given of the light propagation in conventional optical fibers and the effects of pressure and temperature on the refractive index of silica are studied. Photonic crystals in 1 and 2 dimensions are also studied. Then, microstructured fibers and photonic crystal fibers are addressed and their guidance properties and general applications are shown. A review is made of temperature and pressure sensors based on conventional fibers, Bragg gratings and microstructured and photonic crystal fibers described in the literature. Finally, results are shown and discussed of the experiments with the application of pressure on a hollow-core photonic crystal fiber, for which high-order bandgaps, in the visible spectral region, were identified and characterized for the first time. These bandgaps are shown to be pressure sensitive. Applying external hydrostatic pressure over the fiber, optical transmission variations of a few dB were observed for pressures of hundreds of kgf/cm², while when the fiber holes were internally pressurized a similar variation was achieved for units of kgf/cm². The response of light polarization, and hence of the fiber birefringence, to internal pressure was also analyzed.. In the other part of the experimental work, the temperature-induced bandgap spectral shift of photonic bandgap fibers was characterized. The response of a hollow-core fiber, a hybrid fiber and an all-solid fiber was characterized with bandgap shifts of up to 36 pm/°C in straight fibers. For the all-solid fiber the influence of bending over the bandgaps was characterized, as well as temperature-induced bandgaps shifts under these conditions. For a straight fiber, a simple model for the bandgap shift calculation is presented and showed good agreement with the experimental results. Closing the work, the conclusions are presented. / Nessa dissertação são avaliadas as propriedades de transmissão em fibras de cristal fotônico sob influência da pressão e da temperatura visando aplicações em sensoriamento. Antes é feita uma revisão da propagação da luz em fibras ópticas convencionais e os efeitos da pressão e temperatura sobre o índice de refração da sílica. Também são estudados os cristais fotônicos em 1 e 2 dimensões. Em seguida são estudadas as fibras microestruturadas e de cristal fotônico e são mostradas suas propriedades de guiamento e aplicações gerais. Em seguida é mostrada uma revisão sobre sensores de temperatura e pressão utilizando fibras convencionais, redes de Bragg e fibras de cristal fotônico. Finalmente são mostrados e discutidos os resultados dos experimentos com pressão em uma fibra de cristal fotônico de núcleo oco, para a qual foram identificados, caracterizados e reportados pela primeira vez bandgaps de ordem superior localizados na região visível do espectro, que são sensíveis à pressão. Aplicando-se pressão hidrostática na parte externa da fibra obteve-se variação de alguns dB na transmissão óptica para pressões de centenas de kgf/cm², enquanto que quando a pressão era aplicada no interior dos buracos da fibra obteve-se variação semelhante para unidades de kgf/cm². Também foi analisada a resposta da polarização da luz guiada, e portanto da birrefringência da fibra, à pressão interna. Em outra parte do trabalho experimental foi caracterizada a resposta à temperatura do deslocamento espectral dos bandgaps em fibras de bandgap fotônico. Foram caracterizadas as respostas de uma fibra de núcleo oco, uma fibra híbrida e uma fibra totalmente sólida com deslocamentos de até 36 pm/°C para fibras esticadas. Para a fibra totalmente sólida foi também caracterizada a influência da curvatura da fibra sobre os bandgaps e como o deslocamento devido à temperatura é afetado nessas condições. Para uma fibra esticada é apresentado um modelo para o cálculo aproximado do deslocamento dos bandgaps em função da temperatura obtendo-se resultados satisfatórios com o experimento. Na ultima parte do trabalho são apresentadas as conclusões finais.

This thesis explores and evaluates the temperature measuring capabilities of a phase-sensitive optical time-domain reflectometer (φ-OTDR), which exploits Rayleigh backscattering in normal single mode optical fibers. The device is constructed and its setup explained, and a protocol for making temperature measurements with it is developed. Performance tests are made and the device is shown to achieve fully distributed temperature measurements on fibers hundreds of meters in length with a spatial resolution of 1 m and a temperature resolution of 0.1 K. In addition, the capabilities of the device to measure normal strain in the measurement fiber are tested using the same approach, albeit with less success. The device is capable of very precise measurements, making it very sensitive to the environmental conditions around the measuring fiber but also susceptible to disturbances. Some discussion is had on how to avoid or deal with these disturbances. Furthermore, the technique is shown to be able to run in conjunction with other φ-OTDR measurement techniques from the same device simultaneously. / Det här examensarbetet utforskar och utvärderar förmågorna att mäta temperatur hos en fas-känslig optisk tidsdomän-reflektometer (φ-OTDR), som utnyttjar bakåtriktad Rayleigh-spridning i vanliga optiska singelmodfibrer. Anordningen konstrueras och dess komponentstruktur förklaras, och ett protokoll tas fram för att utföra mätningar med den. Prestandatester utförs och anordningen visas kapabel att göra fullt distribuerade temperaturmätningar längs hundratals meter långa fibrer, med en rymdsupplösning på 1 m och en temperaturupplösning på 0.1 K. Dessutom testas förmågan att mäta normaltöjning hos testfibern med samma metod, dock med mindre framgång. Anordningen är väldigt känslig för förhållandena i omgivningen runt mätningsfibern, vilket gör den kapabel till mätningar med mycket hög precision, men också mottaglig för störningar. Lite diskussion hålls kring hur dessa störningar kan undvikas eller hanteras. Vidare visas att mätningstekniken kan köras samtidigt som andra φ-OTDR-baserade tekniker från samma anordning.