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

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.

Narrowband spectral filters find important applications in optical fiber communication systems, particularly in wavelength demultiplexers and single-frequency semiconductor lasers. Conventional Fabry-Perot resonators provide a narrow spectral width but lack the capability of mode discrimination. A new coupled-waveguide Fabry-Perot resonator made of two parallel waveguides with reflecting mirrors at the ends is proposed for application as narrowband tuned spectral filter in single-mode diode lasers and wavelength demultiplexers. The interference of counter propagating waves from reflection by end mirrors and the coupling of waves between the two parallel guides contribute to the operation of this resonator structure. Thus, the device exhibits the attributes of both Fabry-Perot resonator and directional coupler. The coupled-mode theory of parallel waveguides is employed to analyze the proposed structure. Spectral characteristics are derived from the governing coupled-mode equations and related boundary conditions. Two geometries consisting of identical waveguides, as well as nonidentical waveguides, are examined. The spectral characteristics of the proposed resonator demonstrate that significant improvement in mode discrimination capability and longitudinal mode spacing over the conventional Fabry-Perot resonator is achieved. Numerical results for several example cases are presented and the influence of various parameters on spectral properties are investigated. / Master of Science

A surface acoustic wave sensor based on an extrinsic Fabry-Perot interferometer is described. A single-mode fiber, used as the input/output fiber, and a multimode fiber, used mainly as a reflector, form an air-gap that acts as a low-finesse Fabry-Perot cavity. The Fresnel reference reflection from the glass/air interface at the front of the air-gap interferes with the sensing reflection from the air/glass interface at the far end of the air-gap in the input/output fiber. Strains in the silica tube housing the two fibers change the air-gap length, thereby altering the phase difference between the reference and sensing reflections and modulating the output intensity. A theoretical analysis of the interaction between the strain induced by the acoustic fields and the fiber sensor is presented. Because signal drifting in interferometric sensors is common, a dual optical wavelength stabilization technique is also incorporated into the sensor to minimize the effect. Signal to noise ratios (SNR’s) on the order of 39 dB are obtained with a strain sensitivity of 4°/ μstrain cm⁻¹. / Master of Science

DNA detection technology has developed rapidly due to its extensive application in clinical diagnostics, bioengineering, environmental monitoring, and food science areas. Currently developed methods such as surface Plasmon resonance (SPR) methods, fluorescent dye labeled methods and electrochemical methods, usually have the problems of bulky size, high equipment cost and time-consuming algorithms, so limiting their application for in vivo detection. In this work, an intrinsic Fabry-Perot interferometric (IFPI) based DNA sensor is presented with the intrinsic advantages of small size, low cost and corrosion-tolerance. This sensor has experimentally demonstrated its high sensitivity and selectivity. In theory, DNA detection is realized by interrogating the sensor's optical cavity length variation resulting from hybridization event. First, a microgap structure based IFPI sensor is fabricated with simple etching and splicing technology. Subsequently, considering the sugar phosphate backbone of DNA, layer-by-layer electrostatic self-assembly technique is adopted to attach the single strand capture DNA to the sensor endface. When the target DNA strand binds to the single-stranded DNA successfully, the optical cavity length of sensor will be increased. Finally, by demodulating the sensor spectrum, DNA hybridization event can be judged qualitatively. This sensor can realize DNA detection without attached label, which save the experiment expense and time. Also the hybridization detection is finished within a few minutes. This quick response feature makes it more attractive in diagnose application. Since the sensitivity and specificity are the most widely used statistics to describe a diagnostic test, so these characteristics are used to evaluate this biosensor. Experimental results demonstrate that this sensor has a sensitivity of 6nmol/ml and can identify a 2 bp mismatch. Since this sensor is optical fiber based, it has robust structure and small size ( 125μm ). If extra etching process is applied to the sensor, the size can be further reduced. This promises the sensor potential application of in-cell detection. Further investigation can be focused on the nanofabrication of this DNA sensor, and this is very meaningful topic not only for diagnostic test but also in many other applications such as food industry, environment monitoring. / 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

This dissertation focuses on developing new technologies for ultra-low-cost sapphire fiber-optic high-temperature sensors. The research is divided into three major parts, the souceless sensor, the simple Fabry-Perot (F-P) interrogator, and the sensor system. Chapter 1 briefly reviews the background of thermal radiation, fiber optic F-P sensors, and F-P signal demodulation. The research goal is highlighted. In Chapter 2, a temperature sensing system is introduced. The environmental thermal radiation was used as the broadband light source. A sapphire wafer F-P temperature sensor head was fabricated, with an alumina cap designed to generate a stable thermal radiation field. The radiation-induced optical interference pattern was observed. We demodulated the temperature sensor by white-light-interferometry (WLI). Temperature resolution better than 1°C was achieved. Chapter 3 discusses a novel approach to demodulate an optical F-P cavity at low-cost. A simple interrogator is demonstrated, which is based on the scanning-white-light-interferometry (S-WLI). The interrogator includes a piece of fused silica wafer, and a linear CCD array, to transform the F-P demodulation from the optical frequency domain to the spatial domain. By using the light divergence of an optical fiber, we projected a tunable reference F-P cavity onto an intensity distribution along a CCD array. A model for S-WLI demodulation was established. Performance of the new S-WLI interrogator was investigated. We got a good resolution similar to the well-known traditional WLI. At last, we were able to combine the above two technologies to a sapphire-wafer-based temperature sensor. The simple silica wafer F-P interrogator was optimized by focusing light to the image sensor. This approach improves the signal to noise ratio, hence allows the new integrator to work with the relatively weak thermal radiation field. We, therefore, proved in the experiment, the feasibility of the low-cost sourceless optical Fabry-Perot temperature sensor with a simple demodulation system. / PHD / Temperature measurements for high temperature harsh environments is a challenge industrial task. In this work, a low-cost sapphire fiber high temperature sensor is introduced which uses single crystal sapphire fiber as the light guiding and a sapphire-wafer-based Fabry-Perot (F-P) interferometer as the temperature sensing element. The research goal is to provide an optical sensing system whose price is competitive to the high temperature thermocouples. Two technologies were developed to reduce the cost of the sensing system, the sourceless sensor head design and the low-cost wafer-based F-P interrogator. The sourceless sensor head makes use of the environmental thermal radiation as a broadband light source, together with the white light interferometry signal demodulation method, for temperature measurements. In this case, the system avoids using not only an external light, but also the light driver and the light coupling element. A low-cost F-P cavity interrogation method was introduced to demodulate the sapphire-wafer-based temperature sensing F-P cavity. The signal demodulation is based on the scanning white light interferometry, but a reliable and low-cost reference F-P cavity is introduced. It includes only a piece of transparent wafer and a CCD array to transfer the interference fringe from the spectra domain to the spatial domain and therefore a low cost CCD can be directly applied to identify the optical path distance of the sensing OPD. Eventually, the above two technologies were able to put together and an extremely low-cost F-P temperature sensing system was built. It has a good potential for further applications and commercialization.

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

O objetivo do presente trabalho foi o estudo e análise da resposta óptica de dispositivos de cristal fotônico uni-dimensional (1D) fabricados através do uso da tecnologia de silício poroso. Os resultados obtidos neste trabalho apresentam contribuições significativas no desenvolvimento de uma tecnologia para a fabricação de dispositivos ópticos em silício. As principais contribuições deste trabalho estão direcionadas ao aprimoramento dos processos de fabricação de cristais fotônicos 1D e processos de tratamento térmico. Os resultados da análise estrutural através de microscopia óptica de varredura (MEV) e da resposta óptica (refletância ou absorbância) mostraram que dispositivos de cristal fotônico fabricados em soluções altamente diluídas de HF apresentam melhor desempenho, tendo sido otimizado o processo de fabricação utilizando-se uma célula de duplo compartimento (célula dupla). A otimização da resposta óptica dos dispositivos foi atribuída ao efeito de minimização das rugosidades de interface e minimização de efeitos de anisotropia na taxa de corrosão durante o processo de anodização eletroquímica. O processo eletroquímico utilizado para a fabricação de cristais fotônicos 1D apresentou limitação quanto ao número máximo de camadas, sendo observado que dispositivos com número de camadas acima de 60 apresentavam degradação das suas camadas superficiais, comprometendo a resposta óptica do dispositivo. Este resultado foi atribuído a efeitos de diluição química das camadas expostas à solução por longos períodos de processo. Os dispositivos fotônicos 1D mostraram-se sensíveis a processos de recozimento térmico, deslocando suas bandas fotônicas proibidas para regiões de menor comprimento de onda devido à mudança do índice de refração das camadas e aos efeitos de expansão e compressão das camadas constitutivas do dispositivo. Os dispositivos de micocavidade Fabry-Perot mostraram-se mais sensíveis aos processos de recozimento térmico. Os resultados obtidos no presente trabalho vislumbram grandes possibilidades de aplicação dos cristais fotônicos de PS na fabricação de dispositivos ópticos na tecnologia de silício como filtros, lentes, cavidades ressonantes, guias de ondas, grades de difração e dispositivos sensores. / The aim of the present work was to study and analyze the optical response of one- dimensional (1D) photonic crystal devices obtained by using the porous silicon technology. The experimental results obtained from this work showed the significant contribution to the development of a technological process for optical device fabrication in the silicon substrate. The most important contributions of the work are pointed out to improve the electrochemical process for device fabrication and thermal annealing process in order to improve the optical response of the devices. The results obtained from Scanning electronic microscopy (SEM) and from the optical response of the devices, showed that devices fabricated in the double cell and diluted HF solution improved their optical response due to minimization of the anisotropy of corrosion rate and decreasing of the surface roughness between layers. The electrochemical process used for device fabrication showed the existence of limitation on the numbers of layers because of the existence of chemical dissolution effect that became important for long time process. The 1D photonic crystal devices in PS technology showed high sensibility to thermal annealing process, due to the refraction index change after thermal annealing the photonic band gap position shift down to low wavelength region. The Fabry-Perot devices showed higher sensibility to thermal annealing process improving their optical response after annealing process. The results obtained from the present work showed that the PS 1D photonic device could be applied to optical devices fabrication in silicon technology such as optical filters, lenses, resonant cavities, wave-guide devices, diffraction grade and optical sensor device.

Cette thèse a pour but l’étude théorique de métamatériaux métalliques empilés. Ces structures sont actuellement proposées pour améliorer et élargir les fonctionnalités des métamatériaux métalliques. Nous portons un intérêt particulier aux propriétés de polarisation de ces structures métalliques empilées.En premier lieu, nous précisons le type de métamatériaux que nous étudions et nous présentons la méthode modale qui nous permet de décrire les propriétés électromagnétiques de la structure. A l’aide d’un Formalisme de Jones Etendu (FJE), développé récemment dans notre équipe, nous faisons ressortir les principales propriétés de polarisation linéaire de ces métamatériaux métallique.En alliant le FJE à l’algorithme de propagation de la matrice S, nous étudions un empilement de deux métamatériaux vus comme un montage polariseur-analyseur. Nous établissons ensuite une expression de la transmission de la structure: la loi de Malus étendue. Cela nous permet notamment de démontrer les résonances de type Fabry-Perot qui ont lieu entre les métamatériaux.Pour des structures plus conséquentes, nous montrons qu’il est possible de réaliser une rotation de la polarisation, à très faible perte et spectralement agile, grâce aux résonances de type Fabry-Perot.Fondamentalement, nous révélons une nouvelle façon d’exciter des résonances Fano qui sont induites par les propriétés de polarisation des métamatériaux. Ces résonances peuvent être utilisées pour des applications de capteur ou de filtrage. De plus, ces résonances Fano induites par la polarisation ouvrent de nouvelles possibilités d’applications pour les empilement de métamatériaux métalliques. / This PhD thesis deals with the theoretical study of stacked metallic metamaterials. Such structures are currently investigated to extend the functionalities offered by single metallic metamaterials. We especially focus on the specific polarization properties of the stacked metallic metamaterials.We first present the type of metamaterial that we consider, and we describe the modal method that is used to model its electromagnetic properties. We outline the linear polarization properties characterizing the metamaterial thanks to an Extended Jones Formalism (EJF) recently developed by our team.In combination with the EJF, we apply the S-matrix algorithm to the study of a stack of two metallic metamaterials in a polarizer-analyzer configuration. We derive an analytical expression for the transmission response of the stacked structure: the Extended Malus Law. Mainly, it highlights the Fabry-Perot-like resonances located between the metamaterials.Using larger stacked structures, we demonstrate that spectrally tunable and low loss polarization rotation can be achieved owing to these Fabry-Perot-like resonances.In essence, we reveal a new way of realizing Fano resonances which are induced by the specific polarization properties of the metamaterials. We show that such resonances can be engineered for sensing or filtering applications. Moreover, the polarization-induced Fano resonances expand the possibilities of stacked metallic metamaterials.