Optical fiber sensors have been intensively studied and successfully employed in various human social activities and daily living, such as industrial production, civil engineering, medicine, transportation, national defense and so on. According to different structures, optical sensors could be divided into various categories. This thesis focuses on studying different kinds of in-line fiber Mach-Zehnder interferometers, which have played an important role among the optical interferometric fiber sensors. The structure composition, fabrication process, physical principle and practical applications of two novel in-line fiber Mach-Zehnder interferometers are proposed and discussed in detail in this work. The tapered bend-insensitive fiber Mach-Zehnder interferometer (BIF-MZI) is firstly fabricated and used as a fiber vibrometer. The unique double-cladding structure of bend-insensitive fiber not only provides higher mechanical strength to the sensor, but also guarantees a more uniform transmission spectrum， since only a few inner-cladding modes are left interfering with the core mode. A high sensitivity and fast response intensity demodulation scheme is employed by monitoring the power fluctuation of the BIF-MZI at the operation wavelength. Both damped and continuous vibrations are detected using the proposed sensor. It is demonstrated that this sensor responses to an extremely wide range of frequencies from 1 Hz up to 500 kHz with high signal-to-noise ratios (SNRs). The discrimination of temperature and axial strain is realized based on the dispersion effects of high-order-mode fiber (HOMF) by forming a single mode fiber-high-order-mode fiber-single mode fiber (SMF-HOMF-SMF) structure based in-line Mach-Zehnder interferometer. Unlike some kinds of in-line MZIs such as tapered and core–offset structures whose cladding modes are excited with different types under changing temperature and strain circumstances, the HOMF is capable of supporting three stable core modes, which guarantees a reliable and repeatable measurements within a large temperature or strain range. A new method based on the fast Fourier transform (FFT) is employed to analyze the mode couplings and their chromatic dispersion and intermodal dispersion properties in HOMF. The strong dispersion effects lead to a multi-peak feature in the spatial frequency spectrum. It is found that peaks that denote the waveform periods at positions that are beyond the critical wavelength possess highly sensitive and distinct phase responses to external disturbances, which provides the possibility to realize the discrimination measurements with high sensitivities and smaller errors by selecting appropriate peaks. The phase demodulation scheme is applied to quantify the temperature and strain changes in terms of phase shifts. Appropriate peak selections according to the practical needs would provide an easy access for applications where more than two parameters are required to be discriminated.

Optical fiber sensors have been intensively studied and successfully employed in various human social activities and daily living, such as industrial production, civil engineering, medicine, transportation, national defense and so on. According to different structures, optical sensors could be divided into various categories. This thesis focuses on studying different kinds of in-line fiber Mach-Zehnder interferometers, which have played an important role among the optical interferometric fiber sensors. The structure composition, fabrication process, physical principle and practical applications of two novel in-line fiber Mach-Zehnder interferometers are proposed and discussed in detail in this work. The tapered bend-insensitive fiber Mach-Zehnder interferometer (BIF-MZI) is firstly fabricated and used as a fiber vibrometer. The unique double-cladding structure of bend-insensitive fiber not only provides higher mechanical strength to the sensor, but also guarantees a more uniform transmission spectrum， since only a few inner-cladding modes are left interfering with the core mode. A high sensitivity and fast response intensity demodulation scheme is employed by monitoring the power fluctuation of the BIF-MZI at the operation wavelength. Both damped and continuous vibrations are detected using the proposed sensor. It is demonstrated that this sensor responses to an extremely wide range of frequencies from 1 Hz up to 500 kHz with high signal-to-noise ratios (SNRs). The discrimination of temperature and axial strain is realized based on the dispersion effects of high-order-mode fiber (HOMF) by forming a single mode fiber-high-order-mode fiber-single mode fiber (SMF-HOMF-SMF) structure based in-line Mach-Zehnder interferometer. Unlike some kinds of in-line MZIs such as tapered and core–offset structures whose cladding modes are excited with different types under changing temperature and strain circumstances, the HOMF is capable of supporting three stable core modes, which guarantees a reliable and repeatable measurements within a large temperature or strain range. A new method based on the fast Fourier transform (FFT) is employed to analyze the mode couplings and their chromatic dispersion and intermodal dispersion properties in HOMF. The strong dispersion effects lead to a multi-peak feature in the spatial frequency spectrum. It is found that peaks that denote the waveform periods at positions that are beyond the critical wavelength possess highly sensitive and distinct phase responses to external disturbances, which provides the possibility to realize the discrimination measurements with high sensitivities and smaller errors by selecting appropriate peaks. The phase demodulation scheme is applied to quantify the temperature and strain changes in terms of phase shifts. Appropriate peak selections according to the practical needs would provide an easy access for applications where more than two parameters are required to be discriminated.

Corrosion related deterioration of aging aircraft has proven to cause reduced flight availability, service lifetime, costly repairs, and if undetected, can result in potentially unsafe operating conditions. The purpose of this research is to develop, fabricate and test optical fiber-based chemical sensors for monitoring corrosion from early stages through the entire corrosion event. Although there are several preventative methods under development to address the problem of corrosion degradation, new techniques are still needed that are cost-effective and reliable to ensure an acceptable health status determination of aging aircraft and civil infrastructure. In using optical fiber-based sensors to detect corrosion precursors such as moisture, pH, nitrates, sulfates, chlorates and corrosion related metal-ion by-products the severity of the corrosive environment can be determined allowing predictive health evaluation of the infrastructure. The long period grating (LPG) element is highly sensitive to refractive index changes and with appropriate design geometry a variety of target molecules can be detected. Optical fiber long period gratings are designed to act as spectral loss elements that couple a discrete wavelength out of the optical fiber as a function of the surrounding refractive index. By applying special coating that change refractive index with absorption of target molecules to the LPG surface, it becomes a transducer for chemical measurement. Presented in this research is the incorporation of pH-sensitive hydrogels with long period gratings for the development of a fiber optic-based pH sensor. Optical fiber-based pH sensors offer numerous advantages in wastewater monitoring, blood diagnostics, bioremediation, as well as chemical and food processing. Specifically this research focuses on pH sensors that can be multiplexed with other chemical sensors for a complete chemical analysis of the corrosive environment. / Master of Science

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For the purpose of developing a new highly-sensitive and reliable fiber optical acoustic sensor capable of real-time on-line detection of acoustic emissions in power transformers, this dissertation presents the comprehensive research work on the theory, modeling, design, instrumentation, noise analysis, and performance evaluation of a diaphragm-based optical fiber acoustic (DOFIA) sensor system. The optical interference theory and the diaphragm dynamic vibration analysis form the two foundation stones of the diaphragm-based optical fiber interferomtric acoustic (DOFIA) sensor. Combining these two principles, the pressure sensitivity and frequency response of the acoustic sensor system is analyzed quantitatively, which provides guidance for the practical design for the DOFIA sensor probe and system. To meet all the technical requirements for partial discharge detection, semiconductor process technologies are applied, for the first time to our knowledge, in fabricating the micro-caved diaphragm (MCD) used for the DOFIA sensor probe. The novel controlled thermal bonding method was proposed, designed, and developed to fabricate high performance DOFIA sensor probes with excellent mechanical strength and temperature stability. In addition, the signal processing unit is designed and implemented with high gain, wide band response, and ultra low noise. A systematic noise analysis is also presented to provide a better understanding of the performance limitations of the DOFIA sensor system. Based on the system noise analysis results, optimization measures are proposed to improve the system performance. Extensive experiments, including the field testing in a power transformer, have also been conducted to systematically evaluate the performance of the instrumentation systems and the sensor probes. These results clearly demonstrated the feasibility of the developed DOFIA sensor for the detection of partial discharges inside electrical power transformers, with unique advantages of non-electrically conducting, high sensitivity, high frequency response, and immunity to the electro-magnetic interference (EMI). / Ph. D.

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

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

Polymer fiber sensors have been extensively explored over the past few decades for biomedical, structural health monitoring, and environmental monitoring applications. Their low melting point and well-established processing methods make them easily integrable with other materials, such as metals, semiconductor devices, and composites, to create multimaterial sensors with versatile sensing capabilities. However, the high viscoelasticity of polymer materials and the limitations of existing sensing mechanisms constrain the precision and stability of these sensors. This research focuses on enhancing the sensitivity of multimaterial polymer sensors by improving both the sensing mechanisms (chapter 2 and 3) and sensor structures (chapter 4 and 5). Chapters 2 and 3 discuss the integration of silica optical fiber sensors into magnetostrictive composite materials for distributed magnetic field sensing. A series of Fiber Bragg Gratings (FBGs) were inscribed in the core of a silica fiber, which was then thermally embedded at the center of a magnetostrictive composite made of Terfenol-D and thermoplastic elastomers. The magnetostrictive properties of the composite, using various polymer matrices, were thoroughly investigated. A detailed study of the sensor's response under different boundary conditions and applied tensions demonstrated its tunable frequency response and bandwidth capabilities. Furthermore, the sensor's magnetic field sensing performance was characterized under applied AC magnetic fields, showing a responsivity of up to 4.5 ppm/mT and a resolution of 0.1 mT. Theoretical modeling of the magnetostrictive fiber's behavior was also conducted, with the strain transfer coefficient being calculated and compared to the bulk material's response. This thermally drawn magnetostrictive fiber exhibits significant potential for fully distributed sensing applications. In Chapters 4 and 5, the development of a stretchable fiber strain sensor is presented, with improvements in sensitivity achieved through structural optimizations. Polymer fibers, known for their high stretchability, flexibility, and softness, are promising candidates for sensing applications. However, their high viscoelasticity often leads to significant hysteresis. To address this, a double-coil strain sensor was introduced in this research. A theoretical model of the double-coil capacitance was developed to inform future sensor designs. Based on this model, a stretchable miniature fiber sensor was constructed, featuring a stretchable core tightly coiled with parallel conductive wires. This sensor demonstrated low hysteresis, a theoretical resolution of 0.015%, a response time of less than 30 milliseconds, and outstanding stability after more than 16,000 cycles of testing. Its potential as a wearable device was showcased by embedding it into belts, gloves, and knee protectors, with applications ranging from bladder monitoring to life safety rope systems. The dissertation concludes with a discussion of the research findings and suggestions for future directions in the development of multimaterial fiber sensors. / Doctor of Philosophy / This research focuses on enhancing the sensitivity of polymer fiber sensors, which are widely used in healthcare monitoring, infrastructure safety, and environmental observation. These sensors offer the advantage of integrating with other materials to create versatile, multi-functional devices. However, their soft nature and limited sensing mechanisms pose challenges to measurement accuracy and stability. This dissertation proposes improvements in the sensitivity of multimaterial polymer fiber sensors by enhancing both their sensing mechanisms and structural designs. In the first part, new techniques were developed to improve magnetic field sensing by embedding optical fibers into magnetically responsive materials. A scalable method called thermal drawing was used to fabricate magnetostrictive fibers, enabling the sensors to measure magnetic fields at various locations with a minimum detectable change of 0.1 mT. This approach enhances the accuracy of magnetic field detection, which is valuable for monitoring magnetic field distributions in industrial applications. The second part introduces a stretchable sensor designed for strain detection in wearable, biomedical, and structural health monitoring applications. Featuring a double-coil design, this sensor demonstrated stability, durability, and accuracy in real-time monitoring by detecting changes in relative capacitance. Overall, this research offers significant insights into improving the reliability and effectiveness of polymer fiber sensors, paving the way for future innovations in smart sensing technologies. The dissertation concludes with a discussion of potential improvements and future research directions.

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.

The focus of this thesis is on the development of sensing devices based on optical fiber sensors, specifically optical Fiber Bragg Gratings (FBG), using laser microfabrication methods. FBG is a type of optical fibers whose spectral response is affected by applied strain and temperature. As a result, it can be calibrated for the measurement of physical parameters manifesting themselves in the changes of strain or temperature. The unique features of optical fiber sensors such as FBGs have encouraged the widespread use of the sensor and the development of optical fiber-based sensing devices for structural measurements, failure diagnostics, thermal measurements, pressure monitoring, etc. These features include light weight, small size, long-term durability, robustness to electromagnetic disturbances, and resistance to corrosion. Despite the encouraging features, there are some limitations and challenges associated with FBGs and their applications. One of the challenges associated with FBGs is the coupling of the effects of strain and temperature in the optical response of the sensors which affects the reliability and accuracy of the measurements. Another limitation of FBGs is insensitivity to the index of refraction of their surrounding medium. In liquids, the index of refraction is a function of concentration. Making FBGs sensitive to the index of refraction and keeping their thermal sensitivity intact enable optical sensors with the capability of the simultaneous measurement of concentration and temperature in liquids. Considering the unique features of FBGs, embedding of the sensors in metal parts for in-situ load monitoring is a cutting-edge research topic. Several industries such as machining tools, aerospace, and automotive industries can benefit from this technology. The metal embedding process is a challenging task, as the thermal decay of UV-written gratings can starts at a temperature of ~200 oC and accelerates at higher temperatures. As a result, the embedding process needs to be performed at low temperatures. The objective of the current thesis is to move forward the existing research front in the area of optical fiber sensors by finding effective solutions to the aforementioned limitations. The approaches consist of modeling, design, and fabrication of new FBG-based sensing devices. State-of-the-art laser microfabrication methods are proposed and implemented for the fabrication of the devices. Two approaches are adopted for the development of the FBG-based sensing devices: the additive method and the subtractive method. In both methods, laser direct microfabrication techniques are utilized. The additive method deals with the deposition of on-fiber metal thin films, and the subtractive method is based on the selective removal of materials from the periphery of optical fibers. To design the sensing devices and analyze the performance of the sensors, an opto-mechanical model of FBGs for thermal and structural monitoring is developed. The model is derived from the photo-elastic and thermo-optic properties of optical fibers. The developed model can be applied to predict the optical responses of a FBG exposed to structural loads and temperature variations with uniform and non-uniform distributions. The model is also extended to obtain optical responses of superstructure FBGs in which a secondary periodicity is induced in the index of refraction along the optical fiber. To address the temperature-strain coupling in FBGs, Superstructure FBGs (SFBG) with on-fiber metal thin films are designed and fabricated. It is shown that SFBGs have the capability of measuring strain and temperature simultaneously. The design of the sensor with on-fiber thin films is carried out by using the developed opto-mechanical model of FBGs. The performance of the sensor in concurrent measurement of strain and temperature is investigated by using a customized test rig. A laser-based Direct Write (DW) method, called Laser-Assisted Maskless Microdeposition (LAMM), is implemented to selectively deposit silver thin films on optical fibers and fabricate the superstructure FBGs. To attain thin films with premium quality, a characterization scheme is designed to study the geometrical, mechanical, and microstructural properties of the thin films in terms of the LAMM process parameters. A FBG, capable of measuring concentration and temperature of liquids is developed, and its performance is tested. Femtosecond laser micromachining is successfully implemented as a subtractive method for the sensor fabrication. For this purpose, periodic micro-grooves are inscribed in the cladding of regular FBGs so as to increase their sensitivity to the concentration of their surrounding liquid while keeping their thermal sensitivity intact. This type of sensors has the potential for applications in biomedical research, in which the in-situ measurement of the properties of biological analytes is required. Another accomplishment of this thesis is the development of FBG sensors embedded in metal parts for structural health monitoring using low temperature embedding processes. In this regard, the opto-mechanical model is extended to predict the optical response of the embedded FBGs. The embedding process involves low temperature casting, on-fiber thin film deposition, and electroplating methods. The performance of the embedded sensors is evaluated in structural loading and thermal cycling.