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The technique of the distributed fiber optic sensor system, the principle that we use Sagnac interferometer to sense time-varying physical field, can be used to measure the position of the disturbed physical field and have the ability of detecting continuous position. Based on the configuration of the Sagnac interferometer, sensing optic fiber is loop design, which is hard to be set in real surroundings, and a half length of loop fiber have to be the isolated protection of the physical field. Therefore, this essay brings up the In-Line conception to be the design direction. And we make use of the physical field of pipeline leak acoustic to detect disturbance position. The measurable range of systematic structure signal is 3¡Ñ10-4 ~ 3¡Ñ10-2 , and the dynamic range is 40 dB. On the other hand, the structure of polarization insensitive is brought up, the measurable range is 1.5¡Ñ10-3 ~ 3¡Ñ10-2 , and the dynamic range is 26 dB.

In this paper, the fesibility of using a Fiber Bragg Grating (FBG) as a sensing scheme to detect the underwater acoustic signals is analyzed. When a FBG is disturbed by an underwater sound, the wavelength of the FBG is changed. Therefore, the central spectrum of the reflected light is shifted according to the wavelength change of the FBG. This spectrum can be detected by an imbalanced two-arm interferometer. Its transfer function will be studied. Also, the polarization induced signal fading of those two-arm interferometers will be studied.

Hydrogen peroxide is used in various fields, such as food preservative, bleaching, oxidizing, reducing, and chemical reaction reagents. However, inappropriate use may have harmful effects to human health or environment. A number of analytical methods have been developed for the determination of hydrogen peroxide. Herein is described the effort to develop an optical fiber chemical sensor based on the evanescence wave absorbance that can detect the presence of, and measure the concentration of, hydrogen peroxide. For the H2O2 optical fiber sensor, Nafion membrane was coated in the fiber optic. Titanium ions dispersed in a Nafion membrane can form a TiO-H2O2 complex with the H2O2 diffused into the membrane. The complex is shown to absorb light with a maximum absorption near 360 nm. The intensity of the absorbance peak is directly proportional to the concentration of H2O2. At present, this sensor has been tested for detecting H2O2 concentrations ranging from 0.03 ppm to 9 ppm in an aqueous solution at room temperature. Additionally, coating polydimethylsiloxiane (PDMS) outside the fiber optic can detect H2O2 in high concentration 300ppm and high temperature 70oC. Finally, the use of the developed optical fiber chemical sensor allows the direct determination of H2O2 in milk.

Interferometric fiber optic sensors have been extensively used to measure a large variety of physical, chemical and biomedical parameters due to their superior performance. At the Center for Photonics Technology of Virginia Tech, a variety of interferometric fiber optic sensors have been developed in recent years, for efficient oil recovery, partial discharge detection in high voltage transformers, pressure sensing in gas turbine engines, and temperature measurements in gasifiers and boilers. However, interrogating an interferometric sensor involves accurate recovery of a measurand from the phase-modulated lightwaves, and has been a challenge for high performance, high speed, and low-cost, to current white-light interferometry (WLI) techniques, such as the widely used scanning WLI (S-WLI) and spectral-domain WLI (SD-WLI). The performance of a white-light interferometric sensing system depends not only on the design of the probes, but also, to a great extent, on the interrogation strategy to be used. In this Ph.D. research, a tunable optical filter based WLI (TOF-WLI) is proposed and validated as a low cost, yet high performance, solution to the interrogation of various types of interferometric sensors. In addition to the capability of linear/quadrature demodulation, TOF-WLI retains all the features of WLI, is compatible with the SD-WLI, and can be tailored for both static and wideband signals. It also has great potential in surface metrology and biomedical imaging as well as optical spectroscopy. The key, to the success of this new approach in competition with the other available WLI techniques, is that the tunable optical filter (TOF) must be specially designed for sensing and extremely low cost. Therefore, two novel TOFs, a diffraction grating tunable filter (DG-TOF) and an extrinsic Fabry-Perot tunable filter (EFP-TF), are proposed and demonstrated. Laboratory and field test results on using the DG-TOF WLI for partial discharge and thermal fault detection in high voltage power transformers, and the EFP-TF WLI in temperature sensor systems and a turbine engine monitoring system will also be presented to demonstrate the feasibility for efficient sensor interrogation. / Ph. D.

The reason of the essay research find position of leakage point, and design a fiber optical leakage detection system. The research of fiber optical detection system in past year, because property of structure produce SNR smaller, quality of detection system is too bed. The essay brings to a hybrid Mach-Zehnder & Sagnac interferomtric can improve the SNR to 10dB. Therefore, it provides the better SNR. The experiment is added to signal process, such as PTL, PGC structure. To provide the systematic characteristic, such as dynamic range(60dB), percentage error(0.025%). The focal of the essay provide leak detection systematic characteristics how we make use of signal process.

The main frame of the distributed fiber optic leak detection system adopted the hybrid Mach-Zehnder & Sagnac interferomtric. We use the sensing fiber of In-Line frame to detect leak physical field. We can measure the position of the leak physical field through our sensing system and signal process system. In the cause of improving detective ability of leak detection system, we modify three elements of the system, including (1) the choice of the acoustic response of sensing fiber, (2) modification of the PZT phase modulator, and (3) modification of the PGC demodulator. The frame of our experiment is composed of the distributed fiber optic leak detection system and leak system of the fluid pipes. In which leak system of fluid pipes is designed the leaky frame of high-pressure fluid pipes. The main of experiment introduce the leak detection system to measure the leak acoustics of the fluid pipes. Then we can discuss the experimental result. The measurable minimum range of our distributed fiber optic leak detection system is3.3x10^-4(rad/¡ÔHz), and the dynamic range is above 75 dB. The dynamic range of this system can improve the original system to above 15 dB.

[pt] Nesta dissertação são relatadas as etapas que resultam na construção do protótipo de um receptor acústico a Fibra Óptica (RAFO). O trabalho faz parte de um projeto mais amplo visando o desenvolvimento de um medidor de vazão sônico baseado em fibras ópticas. Nesta fase inicial, os esforços foram voltados para a concepção, construção e testes do protótipo do transdutor, responsável pela deteção dos sinais sônicos emitidos por uma fonte, baseado em tecnologia de fibras ópticas. O uso de um sensor extrínseco foi a solução aqui empregada, que adota uma proposta de medição diferente da utilizada em medidores de vazão sônicos convencionais. O sistema de medição de vazão proposto na linha de pesquisa na qual este trabalho está inserido, baseia-se em medidas de tempo de transito para daí inferir a vazão volumétrica do fluido que escoa pelo duto. O sistema foi concebido para operar apenas numa banda estreita de frequências, casada com o sinal senoidal emitido pela fonte sonora. Neste trabalho um receptor acústico a fibra óptica foi projetado, montado e testado, tendo seu funcionamento sido demonstrado para freqüências de operação em torno de 3,2 kHz. A escolha desta faixa de frequências deveu-se basicamente a limitações do sistema de leitura optoeletronico utilizado nos testes de medição. Entretanto, uma vez que o princípio de funcionamento foi demonstrado com sucesso, não existem limitações para o re-dimensionamento do sensor de forma que este possa vir a operar em bandas de freqüência mais elevadas. / [en] This thesis reports the steps that have led to the assemblage and testing of na optical fiber microphone. This is part of a greater effort directed towards the development a sonic flowmeter based on optical fiber technology. In this first phase of the project, focus has been placed on the conception, construction, and testing, of the acoustic receiver first prototype. This transducer will be responsible for capturing the acouustical signails sent by an emitter, also based on optical fiber technology, and which is yet to be developed. In constrast with conventional sonic flowmeters, in which time of flight of acoustical pulses is the measured quantity related to flow rate, we are proposing a system based on measurement of phase difference between emitted and received sinusoidal signals.Hence, the acoustic receiver has been conceived to operate in a narrow frequency band. In particular, the developed prototype has been designed to operate in a frequency band centered at 3.2 kHz. This choice has been dictated by the response of the optoeletronic circuit employed in tests performed with the receiver operating in air and water. Nevertheless, since the principle of operation has been measurement of acoustical signails in higher frequency bands.

Due to their compact size, sensitivity, and ability to be multiplexed, intrinsic Fabry-Perot interferometers (IFPIs) are excellent candidates for almost any multi-point temperature or strain application, and it is well-known that using a single-mode lead-in fiber, a multi-mode fiber section as the Fabry-Perot cavity, and an additional single-mode fiber as the tail results in a structure that generates strong interference fringes while remaining robust. Though the basic principles behind these sensors are understood, to the best of the author's knowledge there hasn't been a thorough investigation into the design and fabrication of a chain of multiplexed IFPI sensors for industrial use in an environment where serious issues associated with the size of the test coupon, sensor placement, and mechanical reinforcement of the fiber could arise. This thesis details the preparation and results of this investigation. It turns out that fabricating a sensor chain with appropriate sensor spacing and excellent temperature response characteristics proved a significant challenge, and issues addressed include inter-sensor interference, high-temperature mechanical reinforcement for bare fiber sections, and high bending losses. After overcoming these problems, a final sensor chain was fabricated and characterized. This chain was then subjected to a battery of tests at the National Energy Technology Laboratory (NETL), where four multiplexed sensors were installed on a 2â x2â coupon in a simulated gas turbine environment. Final results are presented and analyzed. The work that went into developing this chain lays the foundation for future efforts in developing quasi-distributed temperature sensors by identifying potential obstacles and fundamental limitations for certain approaches. / Master of Science

Several techniques for measuring humidity are presented. The goal of the study is to use the electrostatic self-assembled monolayer synthesis process to fabricate a Fabry-Parot Cavity based optical fiber humidity sensor. The sensing scheme bases on the refractive index change with relative humidity of the film applied to the end of optical fiber. That is, the change in reflected optical power indicates certain humidity. To achieve this, some chemicals induce on specific coating materials were applied at the end of optical fiber. In this thesis, experimental results are given to prove that the humidity sensor has high sensitive and fast response time. Furthermore, we investigate the potential for the use of human breathing monitoring and air flow rate detection. Results from preliminary tests of each are given. / Master of Science