Optical fiber sensor multiplexing reduces cost per sensor by designing a system that minimizes the expensive system components (sources, spectrometers, etc.) needed for a set number of sensors. The market for multiplexed optical sensors is growing as fiberoptic sensors are finding application in automated factories, mines, offshore platforms, air, sea, land, and space vehicles, energy distribution systems, medical patient surveillance systems, etc. Optical path length multiplexing (OPLM) is a modification to traditional white-light interferometry techniques to multiplex extrinsic Fabry-Perot interferometers and optical path length two-mode sensors. Additionally, OPLM techniques can be used to design an optical fiber sensor to detect pressure/force/acceleration and temperature simultaneously at a single point. While power losses and operating range restrictions limit the broadscale applicability of OPLM, it provides a way to easily double or quadruple the number of sensors by modifying the demodulation algorithm. The exciting aspect of OPLM is that no additional hardware is needed to multiplex a few sensors. In this way OPLM works with conventional technology and algorithms to drastically increase their efficiency. [1] / Master of Science

Sensing has been essential for the investigation, understanding, exploitation, and utilization of physical phenomena. Traditional single-point sensing methods are being challenged by the multi-point or distributed sensing capabilities afforded by optical fiber sensors. A powerful technique available for distributed sensing involves the use of the Optical Frequency Domain Reflectometry (OFDR). This work focuses on using OFDR as a means of obtaining distributed vibration measurements using the Rayleigh scatter along a single-mode optical fiber. The effort begins by discussing various distributed measurement techniques currently in use before discussing the OFDR technique. Next, a thorough discussion on how high spatially resolved Rayleigh measurements are acquired and how such measurements can be used to make static strain measurements is presented. A new algorithm to resolve strain at regions of high spatial gradient is developed. This results in enhanced measurement performance of systems using the Rayleigh scatter to determine static strain or temperature measurements by improving measurement fidelity at the high gradient locations. Next, discussions on how dynamic strain (vibration) couples to optical fiber in a single point and in a distributed setting are presented. Lessons learned are then used to develop a new and unique distributed vibration measurement algorithm. Various consequential benefits are then reviewed before concluding remarks are stated. A simulation model was developed and used to supplement this investigation in every step of the discussion. The model was used to gain insight on how various physical phenomena interact with the optical fiber. The simulation was also used to develop and optimize the high gradient and vibration algorithms developed herein. Simple experiments were then used to validate the theory and the simulation models. / Ph. D.

The production and fabrication of semi-conductor core optical fibers was shown to be feasible and controllable. This was accomplished through the step sequence of fabrication and characterization of 4 fiber types, an experiment on controlling the grain length in the core and a simple model of the heat transfer during fabrication. Fibers were first made with a silicon core, followed by a phosphorous doped n-type silicon core, then a boron doped p-type silicon core, and a tellurium doped n-type gallium antimonide core. Characterization of the fibers was accomplished with energy dispersive spectroscopy (EDS) for compositional analysis, electron backscatter diffraction (EBSD) for crystal orientation and grain size, optical and electron microscopy for physical fiber quality and optical transmission for core optical quality. A model was developed to relate the heat transfer with the grain structure of the fiber core. All of the fibers fabricated had a polycrystalline core with either no detectable oxygen in the case of the silicon fibers or low amounts of oxygen diffusion into the core as in the case of the GaSb fibers. Fiber lengths ranged from 7 cm for the initial silicon fibers to 60 cm and outside diameters down to 100 µm for n and p type silicon fibers. Core diameters for all fiber types ranged from 10 – 200 µm depending on the fabrication parameters. Lengths of major grains in the core are dependent on the core diameter and the pulling speed. The grain lengths of the major grains in the core generally increase in length with an increase in core diameter. Grain lengths in all fibers are thought to be suitable for use in fabrication of electronic structures in the core region with even the smallest average grain length of around 300 µm. This grain structure satisfies the grain boundary requirements for fabrication of boundary free p-n junctions and other more complicated electronic structures. Small core diameter fibers had better physical quality with fewer cracks and longer continuous length than the larger core fibers. / Ph. D.

A sensing method that can monitor metallic structures continuously would eventually produce safer metallic structures as well as a more efficient and economic way to monitor corrosion. A secondary focus of this research is the implementation of a fiber optic sensor to measure refractive indices of unknown solutions. The surface plasmon sensor, interrogated with white light resulted in attenuations of light at different wavelengths when solutions of different refractive indices were introduced. This sensor has been shown to respond to the three configurations of polished single mode and multimode, as well as the unpolished multimode sensors. The sensitivity calculated was comparable with the sensitivity of the Kretschmann arrangement The transmissive aluminum-clad fiber sensor was shown to be effective in providing a response to the process of corrosion. Varying lengths of aluminum-clad fiber was spliced to acrylate multimode fiber and different wavelengths of sources were used to test the sensor in a bath of NaOH. The results were similar and reproducible. A tapered sensor configuration was attempted and proved to be very useful. / Master of Science

Over the past decade, fiber optic strain sensors have begun to transition from use in laboratory research to commercial sector applications. This transition is somewhat hindered due to the high cost associated with many optical components required for fiber optic-based sensing systems. Multiplexing systems for fiber optic sensors are one approach to reducing the per-channel cost of fiber optic sensor implementation, however, in many applications, on-line monitoring of sensor elements is not required and the periodical addressing of sensor elements is acceptable. Commercially available fiber optic strain sensor systems are now available which support periodical sensor addressing by providing absolute information about the strain state of the sensor. A post-damage inspection fiber optic sensor design which employs a thick metal coating to retain information regarding the strain history of a sensor is demonstrated. Additionally demonstrated is a corrosion sensing technique which exploits the residual strain retention of the post-damage inspection sensor. Finally, the temperature sensing properties of the metal-coated sensor is investigated. / Master of Science

In this conducted research, optical fiber sensors are used to measure low strength alternating magnetic fields. Various fiber sensor configurations are tested and investigated to demonstrate sensing capabilities at different field magnitudes and frequencies. Distributed acoustic sensing fibers (DAS) have been largely studied and documented across a variety of applications and sensing systems. This research uses the DAS technology in tandem with magnetostrictive materials to create a distributed multi-material optical fiber magnetic sensor. Magnetic sensing has high demand across different fields and often runs into challenges of extreme environments including high temperature, corrosion, and areas with poor accessibility. The robust and distributed nature of optical fiber sensors which can be cheaply produced for long lengths is an attractive option over other single point magnetic sensors. In down hole applications specifically, having a distributed sensor able to be deployed easily and over long distances for magnetic sensing would be a large improvement to bulkier traditional magnetometers. In the conducted study, different magnetostrictive materials are implemented in distributed optical fiber sensors to analyze and compare the effective sensitivity and potential commercial viability. Nickel, galfenol alloy, and MetGlas alloy inclusions are drawn into fused silica optical fibers with Bragg gratings inscribed later on for DAS capability. Each was investigated for its response to varying AC magnetic fields to determine relative sensitivity and resolution for distributed magnetic field sensing. / Master of Science / Magnetic sensing has high demand in biomedical applications as well as within the oil and energy industry. This research proposes a series of optical fiber-based sensors to overcome many of the challenges present amidst traditional magnetic sensors. Materials that respond to magnetic fields by either contracting or expanding are coined magnetostrictive. The proposed fiber-based sensors use magnetostrictive materials to create a change in the optical path length of the light being transmitted through the optical fiber. This path difference can be converted to a strain measurement and when compared with a standardized magnetometer, a calibration curve is established for the fiber sensor. Different magnetostrictive materials are studied for measuring various alternating magnetic field amplitude strengths to look at improved sensitivity and/or resolution. This includes nickel, galfenol alloy, which is made up of iron and gallium, and MetGlas, which is composed of primarily of iron. Small wires of the respective materials are drawn out inside the silica fiber while the optical fiber is made so that continuous lengths run the course of the fiber. Different sizes were experimented with. Another simplified tested setup used a ribbon of the MetGlas while a distributed acoustic fiber sensor was laid on top to pick up the strain response while exposed to an alternating magnetic field. All of the mentioned test setups showed success in measuring alternating magnetic field strengths with a clear positive correlation of strain response to magnetic field amplitude. A calibration curve was established for each sensing system and analyzed to show an effective sensitivity range.

Sapphire fiber based temperature sensors are exceptional in their ability to operate at temperatures above 1000C and as high as 1800C. Sapphire fiber technology is emerging and the fiber is available commercially. Sapphire fiber has a high loss, is highly multi-mode and does not have a solid cladding, but it is nonetheless very useful in high temperature applications. Of the available interferometer configurations, Fabry-Perot interferometers are distinguished in their high accuracy and great isolation from sources of error. In this thesis, improvements are reported to an existing design to enhance its reliability and to reduce possible modes of failure. The existing high temperature sensor design has shown a lot of potential in the past by continuously measuring the temperature in a coal gasifier for 7 months, but its true potential has not yet been realized. The goal of this work and the work of many others is to extend the working life and reliability of high-temperature optical sapphire temperature sensors in harsh environments by exploring a solid cladding for sapphire fiber, improved fringe visibility sapphire wafers and a new sensor design. This project is supported by the National Energy and Technology Laboratory of the Department of Energy. / Master of Science

This research describes a method of using distributed acoustic sensing to noninvasively measure volumetric flow rate via multiple unique sensor styles. This work modifies previously used methods of flow detection via fiber optic acoustic sensors affixed onto the exterior body of a flow apparatus. Flow rate measurement methods for two unique sensor styles are described. Weak trends are additionally observed as a function of flow temperature that may represent opportunity for future optimization. A discussion of current noninvasive flow rate measurement methods is given as well as their limitations. A background of distributed acoustic sensing is presented along with a summary of its fundamentals as well as its functionality in noninvasive flow rate measurement. A description of previous techniques that utilized distributed acoustic sensing in conjunction with fiber optic acoustic sensing is shown. The acoustic properties of the fluid-induced vibrations are measured as a function of flow rate and flow temperature utilizing a special type of fiber optic sensor. Numerically smoothed frequency domain acoustic peaks are evaluated by intensity, area, central frequency, and full width at half maximum as flow conditions vary. All tested sensors were found to yield a strong dependence between peak intensity and flow rate. A dependence between central frequency and flow temperature was observed in some cases. The sensor system developed was able to measure fluid-induced vibration intensity and vibrational central frequency and offers potential uses in a myriad of vibrational applications. / Master of Science / This research provides a method of measuring fluid-induced vibrations caused by internal pressure fluctuations stemming from a variety of flow conditions. In this case, a specially fabricated optical fiber is applied to the external surface of the pipe. As water flows at a known volumetric flow rate and temperature, the acoustic signal generated is detected by the optical sensor signal demodulation system. The fiber used is a silicate material designed to transmit optical signals over long distances with minimal loss. Modifications to the fiber can be made to differentiate the measured optical signal loss by frequency band, as well as to designate the spatial position on a fiber sensor to locate where loss is occurring. By measuring optical loss of distinct fiber spatial positions at high sampling frequencies, an abundance of sensing opportunities become available. In knowing optical signal travel time of select wavelengths to corresponding strain characteristics amongst a section of fiber, optoelectronic devices with strong computing power called interrogators can powerfully measure the intensity and rate of fiber strain at a significantly high sampling frequency. Fiber optic sensors have been used in many areas where monitoring of changes in positional microstrain is desired. Such sensors are embedded in-ground for seismic monitoring, as well as on the ocean floor for submarine structural characterization with long singular fibers. Flow rate measurement is performed with fiber coils and various other geometries for active oil wells, fission reactors, and other areas. Improving the performance and applicational flexibility of these sensors allows for greater opportunity for scientific advancement in an array of fields. This research was completed to offer a new method of flow rate measurement while also gauging if flow temperature was able to be measured via a single fiber optic sensor. Fiber strain was observed to be strongly dependent on flow rate, whereas the rate at which strain occurred suggests simultaneous flow and temperature measurement is possible in certain types of fiber arrangements. The work produced in this research is a step towards singular-fiber flow rate and temperature sensing.

Miniature pressure sensors which can endure harsh environments are a highly sought after goal in industrial, medical and research fields. Microelectromechanical systems (MEMS) are the current methods to fabricate such small sensors. However, they suffer from low sensitivity and poor mechanical properties. To fulfill the need for robust and reliable miniature pressure sensors that can operate under high temperatures, a novel type of optical fiber tip sensor only 125μm in diameter is presented in this thesis. The essential element is a piece of hollow fiber which connects the fiber end and a diaphragm to form a Fabry-Pérot cavity. The all-fused-silica structure fabricated directly on a fiber tip has little temperature dependence and can function very well with high resolution and accuracy at temperatures up to 600 °C. In addition to its miniature size, its advantages include superior mechanical properties, biocompatibility, immunity to electromagnetic interference, disposability and cost-effective fabrication. The principle of operation, design analysis, fabrication implementation and performance evaluation of the sensor are discussed in detail in the following chapters. / Master of Science

Spread spectrum techniques offer an alternate solution to the urgency for distributed optical fiber sensors. These techniques are based on the properties of pseudorandom sequences that have triangular autocorrelation functions with peaks only at regions of no delay. This affords an opportunity to give the desired signal a power advantage over many types of interference and noise. A study in employing spread spectrum techniques for multiplexing optical fiber sensors is presented. A mathematical analysis of the system is conducted with due consideration given to performance issues. Simulations in software are conducted to characterize system performance. Hardware developed for this project operates at over 1 Mbps and is capable of simultaneously monitoring four sensors. Real time experiments conducted on these multiplexed sensors affirm the technical feasibility of the system. Configurations for viable applications of the system are also suggested. / Master of Science