Accurate measurement of temperature is essential for the safe and efficient operation and control of a vast range of industrial processes. Many of these processes involve harsh environments, such as high temperature, high pressure, chemical corrosion, toxicity, strong electromagnetic interference, and high-energy radiation exposure. These extreme physical conditions often prevent conventional temperature sensors from being used or make them difficult to use. Novel sensor systems should not only provide accurate and reliable temperature measurements, but also survive the harsh environments through proper fabrication material selections and mechanical structure designs. This dissertation presents detailed research work on the design, modeling, implementation, analysis, and performance evaluation of novel optical high temperature sensors suitable for harsh environment applications. For the first time to our knowledge, an optical temperature sensor based on the broadband polarimetric differential interferometric (BPDI) technology is proposed and tested using single crystal sapphire material. With a simple mechanically structured sensing probe, in conjunction with an optical spectrum-coded interferometric signal processing technique, the proposed single crystal sapphire optical sensor can measure high temperature up to 1600 oC in the harsh environments with high accuracy, corrosion resistance, and long-term measurement stability. Based on the successfully demonstrated sensor prototype in the laboratory, we are confident of the next research step on sensor optimization and scale-up for full field implementations. The goal for this research has been to bring this temperature sensor to a level where it will become commercially viable for harsh environment applications associated with industries. / Ph. D.

This dissertation addresses several theoretical issues in low-finesse fiber optic Fabry-Perot Interferometric (FPI) sensors. The work is divided into two levels: modeling of the sensors, and signal processing based on White-Light-Interferometry (WLI). In the first chapter, the technical background of the low-finesse FPI sensor is briefly reviewed and the problems to be solved are highlighted. A model for low finesse Extrinsic FPI (EFPI) is developed in Chapter 2. The theory is experimentally proven using both single-mode and multimode fiber based EFPIs. The fringe visibility and the additional phase in the spectrum are found to be strongly influenced by the optical path difference (OPD), the output spatial power distribution and the working wavelength; however they are not directly related to the light coherence. In Chapter 3, the Single-Multi-Single-mode Intrinsic FPI (SMS-IFPI) is theoretically and experimentally studied. Reflectivity, cavity refocusing, and the additional phase in the sensor spectrum are modeled. The multiplexing capacity of the sensor is dramatically increased by promoting light refocusing. Similar to EFPIs, wave-front distortion generates an additional phase in the interference spectrogram. The resultant non-constant phase plays an important role in causing abrupt jumps in the demodulated OPD. WLI-based signal processing of the low-finesse FP sensor is studied in Chapter 4. The lower bounds of the OPD estimation are calculated, the bounds are applied to evaluate OPD demodulation algorithms. Two types of algorithms (TYPE I & II) are studied and compared. The TYPE I estimations suffice if the requirement for resolution is relatively low. TYPE II estimation has dramatically reduced error, however, at the expense of potential demodulation jumps. If the additional phase is reliably dependent on OPD, it can be calibrated to minimize the occurrence of such jumps. In Chapter 5, the work is summarized and suggestions for future studies are given. / Ph. D.

The purpose of this study is to characterize the plasma spray deposits used for attaching intrinsic Fabry-Perot interferometric fiber optic strain sensors. The deposits must maintain adhesion at elevated temperatures without distorting the sensors' signals. Two different material systems were tested and modeled, a nickel based alloy and yttria-stabilized zirconia. The material properties of the deposits and the thermal stresses in the system were evaluated to determine attachment lifetime of the sensors. The encapsulated sensors' signals were collected before and after plasma spraying and at elevated temperatures. The material properties of the deposits were evaluated by electron microscopy, energy dispersive x-ray spectroscopy, scratch testing, thermal fatigue testing, and nanoindentation. The thermal stresses were evaluated by Raman spectroscopy and from finite element analysis in COMSOL® Multiphysics®. Several of the sensors broke during encapsulation due to the plasma spray processing conditions and the signals experienced distortion at elevated temperatures. The sensors can be treated to remove this interference to allow for this deposit attachment. The nickel based alloy's ductility and lamellar microstructure allowed for non catastrophic relaxation mechanisms to relieve induced thermal stresses. The yttria stabilized zirconia failed catastrophically at elevated temperatures due its lack of compliance to mismatches in thermal expansion. A high melting point metallic deposit, similar to the nickel based alloy, is desirable for fiber optic sensor attachment due to its ductility, thermal expansion, and dominant relaxation mechanisms. The processing conditions may need to be optimized to allow for the sensors' protection during encapsulation. / Master of Science

This dissertation reports the development of two new categories of optical fibers. These are the Random Hole Optical Fiber (RHOF) and the Crucible Technique Hybrid Fiber (CTF). The RHOF is a new class of microstructure fiber which possesses air holes which vary in diameter and location along the length of the fiber. Unlike all prior microstructure fibers, these RHOF do not have continuous air holes which extend throughout the fiber. The CTF is a method for incorporating glasses with vastly differing thermal properties into a single optical fiber. Each of these two classes of fiber brings a new set of optical characteristics into being. The RHOF exhibit many of the same guidance properties as the previously researched microstructure fibers, such as reduced mode counts in a large area core. CTF fibers show great promise for integrating core materials with extremely high levels of nonlinearity or gain. The initial goal of this work was to combine the two techniques to form a fiber with exceedingly high efficiency of nonlinear interactions. Numerous methods have been endeavored in the attempt to achieve the fabrication of the RHOF. Some of the methods include the use of sol-gel glass, microbubbles, various silica powders, and silica powders with the incorporation of gas producing agents. Through careful balancing of the competing forces of surface tension and internal pressure it has been possible to produce an optical fiber which guides light successfully. The optical loss of these fibers depends strongly on the geometrical arrangement of the air holes. Fibers with a higher number of smaller holes possess a markedly lower attenuation. RHOF also possess, to at least some degree the reduced mode number which has been extensively reported in the past for ordered hole fibers. Remarkably, the RHOF are also inherently pressure sensitive. When force is applied to an RHOF either isotropically, or on an axis perpendicular to the length of the fiber, a wavelength dependent loss is observed. This loss does not come with a corresponding response to temperature, rendering the RHOF highly anomalous in the area of fiber optic sensing techniques. Furthermore an ordered hole fiber was also tested to determine that this was not merely a hitherto undisclosed property of all microstructure fibers. Crucible technique fibers have also been fabricated by constructing an extremely thick walled silica tube, which is sealed at the bottom. A piece of the glass that is desired for the core (such as Lead Indium Phosphate) is inserted into the hole which is in the center of the tube. The preform is then drawn on an fiber draw tower, resulting in a fiber with a core consisting of a material which has a coefficient of thermal expansion (CTE) or a melting temperature (Tm) which is not commonly compatible with those of silica. / Ph. D.

The extrinsic Fabry-Perot interferometer (EFPI) has been extensively used as a strain sensor in various applications. However, like other interferometric sensors, the EFPI suffers from ambiguity in detecting directional changes of the applied perturbation, when the operating point is at a maxima or a minima on the transfer function curve. Different methods, or sensor configurations have been proposed to solve this problem. This thesis investigates the use of dual wavelength interferometry to overcome this limitation. Possible systems configurations based on dual wavelength interferometry were considered, and the comprehensive design and implementation of a dual laser time division multiplexed (TOM) system based is presented. The system operates by alternately pulse modulating two laser diodes, which are closely spaced in center wavelength. Although the strain rate measurement capability of the system is dependent primarily on the speed of its hardware and the accuracy of its software, it is shown that it can be considerably enhanced by employing digital signal processing techniques. / Master of Science

ITC/USA 2006 Conference Proceedings / The Forty-Second Annual International Telemetering Conference and Technical Exhibition / October 23-26, 2006 / Town and Country Resort & Convention Center, San Diego, California / In this paper we will present a new fiber sensor integrated monitor (FSIM) to be used in an embedded instrumentation system (EIS). The proposed system consists of a super luminescent diode (SLD) as a broadband source, a novel high speed tunable MEMS filter with built in photodetector, and an integrated microprocessor for data aggregation, processing, and transmission. As an example, the system has been calibrated with an array of surface relief fiber Bragg gratings (SR-FBG) for high speed, high temperature monitoring. The entire system was built on a single breadboard less than 50 cm² in area.

In this dissertation, the rate of the BFD during body armor impact is characterized with fiber Bragg gratings for the first time ever. The depth rate is characterized using a single fiber optic sensor, while the entire shape rate can be characterized using multiple fiber optic sensors. This is done with a final depth accuracy of less than 10% and a timing accuracy of 15% for BFDs as deep as 50 mm and impact event of less than 1 millisecond. The shape sensing method introduced in this dissertation is different from traditional fiber optic sensor shape reconstruction methods in the fact that strain from the kinetic friction regime is used rather than the static friction regime. In other words, information from the fiber optic sensors slipping is used to reconstruct the shape in this work, whereas strain from the fiber optic sensor remaining fixed to a reference is used for typical fiber optic shape sensing purposes.

In this thesis, a method to improve survivability is developed for fiber Bragg gratings under high velocity impact in dynamic body armor shape sensing applications by encasing the fiber in silicone. Utilizing the slipping of the fiber within the silicone channel, a proportionality relationship between the strain of the fiber to the acceleration of the impacting projectile is found and is used to obtain the rate of the back-face deformation. A hybrid model is developed to handle errors caused by the stick-slip of the fiber by fitting an inverse exponential to stuck sections found in a captured strain profile and double integrated to transform the stuck section to its equivalent slipping. Displacement errors below 10% was achieved using the hybrid model. A graphical user interface with a step-by-step walkthrough and a fiber Bragg grating interrogation system was designed for test engineers to utilize this technology. Test engineers from the Army Test Center in Aberdeen, MD were trained on this technology and successfully captured and processed shots using this technology. A method for cutting Silicon through wire-EDM machining is developed by utilizing the photoconductive properties of Silicon. Cut rates for unilluminated and illuminated Silicon was compared and a 3x faster cut was achieved on the illuminated cuts.

Gas sensing is essential for safety and maintenance operations in many industries, including power generation, petrochemical, capture and storage technologies, and the food-processing sector. The properties of fiber-optic sensors make them a superior choice for environmental monitoring applications, especially in extreme conditions, and particularly when compared against conventional electro-optical sensors. Their advantageous properties include immunity to electromagnetic radiation, high temperature durability, high sensitivity and the ability for high resolution detection, as well as multifunctional sensing capabilities such as temperature, humidity, pressure, strain, and corrosion. Among different types of interferometers, Mach-Zehnder Interferometers (MZI) have received significant attention because they are robust, compact, and have high levels of precision. In this dissertation, we present an in-line and compact MZI point sensor designed for sensing refractive index. In comparison with various types of interferometers, fiber MZI based RI sensing was selected based on its enhanced sensitivity and fabrication simplicity. The MZI sensor is developed using photonic crystal fiber and demonstrated for high sensitivity detection and measurement of pure gases. The transmission spectrum of MZI sensors is formed by interference between the cladding and core modes. To construct the device, the sensing element fiber was placed and aligned between two single-mode fibers with air gaps at each side. Two linear-translation micro stages were used to accurately differ and adjust gap lengths from 0 to 5mm. Great measurement repeatability was shown in the cyclic test for the detection of various gases such as methane and helium. A high RI measurement resolution of 2.1 E-7 and a sensitivity of 4629 nm/RIU was achieved, which is among the highest reported. Results show that the sensitivity of the fabricated MZI increases from 3000 nm/RIU to 4600 nm/RIU when the length of the sensing element fiber decreases from 5 mm to 3.3 mm. Furthermore, the device was packaged to demonstrate the laboratory-scale monitoring, as well as leakage detection of different concentrations of CO2 in both subsurface soil and aqueous environments. Two water resistant but gas permeable membranes were used to package the sensor, to achieve a good balance of CO2 permeability and water resistance. The experimental study of this work reveals the great potential of the fiber-optic approach for environmental monitoring of CO2. This study also explores other potential applications. Three types of sensors were fabricated using the proposed configuration employing 4 mm stub of (i) solid core Photonic Crystal Fiber (PCF), (ii) 10 µm Hollow core PCF (HC-PCF), and (iii) 20 µm HC-PCF as the sensing elements. We compared the performance of these sensors for detecting and measuring the quantity of gas present. As the transmission signals correspond to the frequency components in the sensor’s Fast Fourier Transform (FFT) spectrum, the effect of gap distance on the number and amplitude distribution of the modes was examined in an effort to optimize the design elements. The MZI sensors are highly sensitive to low percentages of CH4 and CO2, making them suitable for greenhouse gas measurement. / Graduate

The Multichannel Analysis of Surface Waves (MASW) method traditionally uses an array of collinear vertical geophones to measure seismic wave propagation velocity at discrete points along the ground surface. Distributed fiber optic sensors (FOS) measure the average longitudinal strain over discrete lengths (i.e., zones) of a buried fiber optic cable. Such strain measurements can be used to assess ground motion and thus analyzed with the MASW method. To evaluate the feasibility of using FOS strain measurements in the MASW method, field experiments were conducted with both FOS and surface vertical geophones. Synthetic seismograms were also used to compare FOS to vertical and horizontal geophones and investigate the effect of installation depth and sensor type. Through the MASW method, shear wave (Vs) profiles from the FOS showed comparable results to those obtained with the geophones and achieved the same degree of uncertainty from the non-uniqueness of the MASW inversion process.