Review and Analysis of Peak Tracking Techniques for Fiber Bragg Grating Sensors

Fiber Bragg Grating (FBG) sensors are among the most popular elements for fiber optic sensor networks used for the direct measurement of temperature and strain. Modern FBG interrogation setups measure the FBG spectrum in real-time, and determine the shift of the Bragg wavelength of the FBG in order to estimate the physical parameters. The problem of determining the peak wavelength of the FBG from a spectral measurement limited in resolution and noise, is referred as the peak-tracking problem. In this work, the several peak-tracking approaches are reviewed and classified, outlining their algorithmic implementations: the methods based on direct estimation, interpolation, correlation, resampling, transforms, and optimization are discussed in all their proposed implementations. Then, a simulation based on coupled-mode theory compares the performance of the main peak-tracking methods, in terms of accuracy and signal to noise ratio resilience

Arrayed Waveguide Grating-Based Interrogation System for Safety Applications and High-Speed Measurements

This thesis is focused on the design of two interrogation systems for Fiber Bragg Grating (FBG) sensors based on the Wavelength Domain Multiplexing (WDM) by means of the Arrayed Waveguide Grating (AWG) device. The FBG sensors have been employed in a large number of environments thanks to their intrinsic characteristics. To design a measurement system based on the Fiber Optic Sensor (FOS) technology, it is mandatory to make use of an optoelectronic system with the aim to "read" the wavelength shifting performed by the sensors. This latter is named interrogation system and, actually, sets a limit on the employability of the FBG sensors, due to its cost, design complexity and low reliability in some contests. For this reasons, the researchers are constantly looking on new technologies for the design of innovative interrogation systems. The AWG device seems to provide characteristics which cannot be reached with other devices and, due to its passivity, gives the possibility to increase the system speed to let the FBG sensors to be employed also for the detection of high-speed phenomena. Furthermore, thanks to the robustness and reliability of AWG device, is possible to turn an interrogation system into a full analog monitoring system employable in a safety scenario, such as industrial processes or other kind of environments, in which digital processing does not ensure enough reliability

Design of photonic sensors based on cavities and new interrogation techniques

[ES] Los sensores ópticos son dispositivos fotónicos sensibles a determinadas magnitudes que se usan precisamente para medir, ya sea de forma absoluta o relativa, esas magnitudes. Medir la temperatura, la presión, la tensión, la humedad o la presencia de un determinado gas son algunas de las funcionalidades que llevan a cabo estos sensores. A lo largo de las últimas décadas multitud de sensores y técnicas de interrogación han sido desarrolladas, lo que ha tenido un increíble impacto en multitud de áreas. Uno de los ejemplos más claro es la arquitectura civil, donde los sensores fotónicos juegan un papel fundamental en la monitorización del estado de las estructuras. A pesar de los buenos resultados conseguidos por los sensores ópticos hasta la fecha, las técnicas de interrogación desarrolladas hasta ahora muestran algunas desventajas. Tiempos de medida altos, baja resolución o una gran complejidad son algunas de ellas. En esta tesis doctoral se presenta el diseño y caracterización de diversos sensores fotónicos basados en las ya conocidas redes de difracción de Bragg así como la implementación de nuevas técnicas de interrogación de dichos sensores para intentar eliminar o reducir esas desventajas. Las técnicas de interrogación desarrolladas en este trabajo se basan en la fotónica de microondas, donde la interacción entre las señales ópticas y eléctricas se usa para detectar en este caso los cambios en una determinada magnitud. Las técnicas desarrolladas en este trabajo buscan ser lo más versátiles y escalables posibles para así poder adaptarse a los requerimientos de diferentes escenarios. Encontramos así técnicas que permiten interrogar a miles de sensores con una gran resolución y sensibilidad así como también técnicas de interrogación de sensores puntuales con una enorme sensibilidad y simplicidad tanto en la interrogación como en el sistema usado para ello. También están presentes los resultados obtenidos mediante la colaboración con el instituto de investigación sueco ACREO para el desarrollo de un sensor de campo eléctrico basado en fibras polarizadas donde se han fabricado varias redes de difracción de Bragg.[CA] Els sensors òptics son dispositius fotònics sensibles a determinades magnituds que s'usen precisament per a mesurar, ja siga de forma absoluta o relativa, aquestes magnituds. Mesurar la temperatura, la pressió, la tensió, la humitat o la presència d'un determinat gas són algunes de les funcionalitats que realitzen aquestos sensors. Al llarg de les últimes dècades multitud de sensors i tècniques d'interrogació han sigut desenvolupades, i això ha tingut un impacte increïble a multitud d'àrees. Un dels exemples més clar es l'arquitectura civil, on aquestos sensors juguen un paper fonamental en la monitoratge de l'estat de les estructures. Encara que els resultats aconseguits han sigut bons, les tècniques d'interrogació desenvolupades fins ara mostren alguns desavantatges. Temps de mesurament alt, baixa resolució o una gran complexitat són algun d'ells. A aquesta tesi doctoral es presenta el disseny i caracterització de diversos sensors fotònics basats en les ja conegudes xarxes de difracció de Bragg així com l'implementació de noves tècniques d'interrogació per a intentar eliminar o reduir aquestos inconvenients. Les tècniques d'interrogació desenvolupades en aquest treball es basen en la fotònica de microones, on l'interacció entre les senyals òptiques i elèctriques s'usa per a detectar en aquest cas els canvis en una determinada magnitud. Les tècniques desenvolupades en aquest treball busquen ser el més versàtils i escalables possibles per a poder adaptar-se als requeriments dels diferents escenaris. Trobem així tècniques que permeten interrogar milers de sensors amb una gran resolució i sensitivitat però també tècniques que permeten interrogar sensors puntuals amb una increïble sensitivitat mostrant una gran simplicitat en el seu disseny. També estan presents els resultats obtinguts mitjançant la col¿laboració amb l'institut d'investigació suec ACREO per al desenvolupament d'un sensor de camp eléctric basat en FBGs i en fibres polaritzades.[EN] Optical sensors are photonic devices sensitive to different magnitudes that are used precisely to measure, in an absolute or a relative way, these magnitudes. These optical sensors are nowadays used to measure temperature, pressure, strain, humidity or the presence of a particular gas. In the past few decades a multitude of photonic sensors and different interrogation techniques have been developed, which had a great impact in dozens of different fields. One of the best examples is civil architecture, in which photonic sensors play a fundamental role in order to monitor the condition of the structures. Despite of the good results showed by photonic sensors, the interrogation techniques used show different drawbacks. A large measurement time, low resolution or great complexity are some of them. In this doctoral thesis the design and characterization of a set of different photonic sensors based on the already known fiber Bragg gratings, along with the implementation of new interrogation techniques, are used in order to eliminate or at least reduce these problems. The interrogation techniques developed in this work are based on Microwave Photonics techniques, in which the interaction between optical and electrical signals is used to detect in this case the changes in a particular magnitude. The techniques showed in this work have been designed in order to be as versatile and scalable as possible to have the opportunity to adapt to any requirement in different scenarios. In this work techniques that are able to interrogate hundreds or even thousands of sensors with great sensitivity and resolution can be found in addition to techniques that are developed to interrogate individual sensors with an enormous sensitivity. The work carried out in collaboration with the Swedish research institute ACREO, based on the development of an electric field sensor based on poled fibers together with FBGs is also present.Hervás Peralta, J. (2019). Design of photonic sensors based on cavities and new interrogation techniques [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/119754TESI

Optical Fiber Sensing for SubMillimeter IntrinsicallySafe Liquid Level Monitoring

The popularization and fast growth of the optical fiber sensing technology has stimulated in different fields WHERE measurements of diverse physical and chemical parameters are required. Among these parameters, liquid level sensing plays an essential role in industry applications such as chemical processing, fuel storage, transportation systems, oil tanks/reservoirs, and wastewater treatment plants. In order to measure this parameter different sensing techniques based on acoustical, mechanical, electrical and electromagnetical technologies have been already proposed. Nevertheless, they suffer from intrinsic safety concerns in harsh environments, especially with corrosive, and explosive or flammable atmospheres. Fiber optic based liquid level sensors (FOLLS) can work in harsh environments with inherent advantageous features that only optical fiber offers, such as intrinsic safety, resistance to chemical corrosion, immunity to electromagnetic interference, electric isolation, small size, lightweight sensing heads, high accuracy and resolution, easy multiplexing, and capability for extremely remote monitoring without the need of electrical power at the measuring point. In this context, this doctoral Thesis presents two specific optical fiber sensor technologies to measure liquid level. Both the MachZehnder and FabryPerot interferometers are researched. The Thesis also focus on uniform fiber Bragg grating (FBG). Since these technologies have different operation principle, the liquid level measurement was based on refractive index changes for the MachZehnder sensor and based on hydrostatic pressure in the case of both the FPI and FBG sensors. Furthermore, analysis of temperature crosssensitivity is performed with the aim to improve the pressurebased sensors performance. Despite the FBGs provide high accurate measurements, the interrogation systems are the most important drawback for their large commercial application, due to their high cost. Therefore, a new and lower cost interrogation technique based on FPI microcavities was proposed as a final contribution

Técnicas de metrología óptica basadas en fotónica de microondas

Esta tesis presenta una serie de técnicas de metrología óptica basadas en fotónica de microondas (MWP), cuya incorporación permite la mejora de ciertas figuras de mérito con respecto a sus equivalentes puramente fotónicos en el ámbito de la metrología óptica y de la interrogación de sensores de fibra óptica (OFS). Tras una introducción donde se describen algunos de los tipos de OFS más relevantes y los cuatro principales métodos de reflectometría óptica, se resumen las publicaciones que forman parte del compendio de la presente tesis. Estas se han dividido en aquellas que presentan técnicas basadas en reflectometría óptica incoherente dispersiva en el dominio de la frecuencia (DI-OFDR), o sistemas con mejoras que pueden ser empleadas en DI-OFDR, y aquellas que han buscado y estudiado las aplicaciones metrológicas de los bucles desplazadores de frecuencia (FSL). El primer grupo de técnicas hace referencia a sistemas basados en reflectometría óptica incoherente en el dominio de la frecuencia (I-OFDR) donde la inclusión de un elemento dispersivo en el circuito óptico permite la incorporación de nuevas funcionalidades. En particular, la medición de desplazamiento de la longitud de onda de reflectores de banda estrecha mediante la detección del retraso de grupo diferencial de ondas moduladas. El segundo se refiere al estudio teórico y experimental de peines de frecuencia generados por lazos de fibra amplificados que incluyen un elemento desplazador de frecuencia, y que permiten la generación de un amplio conjunto de formas de onda ópticas de interés en, entre otros ámbitos, aplicaciones metrológicas. Por una parte, las publicaciones relacionadas con DI-OFDR han consistido en: el desarrollo de un método de interrogación de redes de Bragg en fibra (FBG) mediante una fuente de doble longitud de onda; la minimización del número de puntos de interrogación en frecuencia en sistemas DI-OFDR adaptada a una topología de reflectores equiespaciados, así como el desarrollo de un método basado únicamente en medidas de potencia; y la implementación de un sistema con conversión electroóptica descendente de frecuencia y detección a frecuencia intermedia para la realización de medidas reflectométricas tanto distribuidas como puntuales y, en este caso, con selectividad en longitud de onda. Entre otras ventajas, estos sistemas han permitido la interrogación de FBG con resolución picométrica y alta eficiencia en potencia; velocidades de interrogación de arrays FBG de hasta 10 μs por elemento sensor; y la detección de eventos reflexivos discretos con reflectividades de hasta 90 dB y de retrodispersión Rayleigh en banda C en fibra monomodo estándar; respectivamente. Por otra parte, las publicaciones relacionadas con los FSL se han centrado, respectivamente, en la aplicación de las formas de onda ópticas de tipo chirp generadas por estos dispositivos para la medición de distancia con resolución milimétrica por compresión de digital de pulso, aprovechando para ello su alto producto tiempo-ancho de banda ( 200), y en la descripción teórica de dichas formas de onda. Empleando una descripción del campo generado por los FSL basada en una analogía con la óptica difractiva, se han descrito nuevas propiedades de los pulsos tipo chirp generados por FSL. En concreto, la existencia de captura de fase entre distintos pulsos, la presencia de desviaciones de la linealidad del chirp y la comprobación experimental de la coincidencia entre las fases Talbot generadas en FSL bajo condiciones fraccionales y las secuencias de fase perfecta de Gauss. Los resultados de esta segunda parte de la tesis muestran, además de un buen acuerdo del modelo teórico con los pulsos medidos, la viabilidad del empleo de FSL para medición de distancia láser con compresión digital de pulsos, obteniéndose tasas de compresión y de repetición de 150 y 80 MHz, respectivamente, y un ancho de banda de 20 GHz.This thesis presents several techniques of optical metrology based on Microwave Photonics (MWP), whose incorporation allows for improving some figures of merit with respect to its purely photonic equivalents in the fields of optical metrology and interrogation of optical fiber sensors (OFS). After an introduction where some of the most relevant types of OFSs and the four main methods of optical reflectometry are described, the publications that form the compendium of this thesis are summarized. These have been divided in those which present techniques based on dispersive incoherent optical frequency-domain reflectometry (DI-OFDR), or systems with improvements that can be employed in DI-OFDR, and those which have searched and studied the metrology applications of the frequency shifting loops (FSL). The first group of techniques refers to systems based on incoherent optical frequency-domain reflectometry (I-OFDR) where the inclusion of a dispersive element in the optical circuit allows for incorporating new functionalities. In particular, measuring wavelength shiftings in narrow band reflectors by detecting the differential group delay of modulated waves. The second refers to the theoretical and experimental study of frequency combs generated by amplified fiber loops that include a frequency shifting loop, and that allow for generating a wide group of optical waveforms of interest in, among other fields, metrology applications. On the one hand, the publications related to DI-OFDR have consisted in: the development of an interrogation method of fiber Bragg gratings (FBG) by a dual-wavelength source; the minimization of the number of interrogation points in frequency in DI-OFDR systems adapted to a topology of equally-spaced reflectors, as well as the development of a method based only in power measurements; and the implementation of a system with electro-optic downconversion and detection at intermediate frequency for conducting reflectometric measurements either single-point or distributed and, in this case, with wavelength selectivity. Among other advantages, these systems have allowed for interrogating FBGs with picometric resolution and high power efficiency, interrogating FBG arrays with speeds up to 10 μs per sensing element, and detecting discrete reflective events with reflectivities up to 90 dB and Rayleigh backscattering in C band in standard monomode fiber; respectively. On the other hand, the publications related to FSLs have been focused, respectively, on the application of the chirped optical waveforms generated by these devices for distance measuring with millimetric resolution by digital pulse compression, taking advantage of its high time-bandwidth product ( 200), and the theoretical description of these waveforms. Using a description of the field generated by the FSLs based on an analogy with diffractive optics, new properties of the chirped pulses generated by FSLs have been described. In particular, the existence of phase capture between different pulses, the presence of chirp linearity deviations, and the experimental verification of the coincidence between the Talbot phases generated in FSLs and the Gauss perfect phase sequences. The results of this second part of the thesis show, besides a good agreement between the theoretical model and the measured pulses, the feasibility of using FSLs for laser dis- tance measuring with digital pulse compression, obtaining compression and repetition rates of 150 and 80 MHz, respectively, and a 20 GHz bandwidth

Recent development of respiratory rate measurement technologies

Respiratory rate (RR) is an important physiological parameter whose abnormity has been regarded as an important indicator of serious illness. In order to make RR monitoring simple to do, reliable and accurate, many different methods have been proposed for such automatic monitoring. According to the theory of respiratory rate extraction, methods are categorized into three modalities: extracting RR from other physiological signals, RR measurement based on respiratory movements, and RR measurement based on airflow. The merits and limitations of each method are highlighted and discussed. In addition, current works are summarized to suggest key directions for the development of future RR monitoring methodologies

Long-term displacement measurement of full-scale bridges using computer vision and lidar

Department of Urban and Environmental Engineering (Urban Infrastructure Engineering)Bridge displacement is regarded as a key safety indicator that is widely adopted for structural health monitoring (SHM). Bridge structures deflect in response to applied loads and structural degradation. As extensive vibrations of bridges cause passenger???s discomfort and accelerate structural degradation, modern societies take the bridge displacement into account in their design codes and regular maintenance protocols to ensure serviceability and safety of the bridge structures. The short-term displacement is generally employed in bridge SHM, together with the level of load carrying capacity. Even though the long-term displacement can also provide essential safety information, in addition to the short-term data, the long-term displacement monitoring of bridges is not commonly conducted owing to practical difficulties. The long-term monitoring of displacement using conventional displacement sensors, such as a linear variable differential transformer, laser displacement sensor, and radar, or indirect estimation methods, such as an acceleration-based method or multimetric sensor-based approaches result in errors, which typically accumulate over time. A limited number of research studies have addressed long-term bridge displacement measurementhowever, the sensor drift can still cause errors in those measurements. This paper proposes long-term displacement measurement methods using computer vision and LiDAR, tailored to full-scale bridge structures. The computer vision-based approach compensates for the camera motion-induced errors by using an auxiliary camera and the long-term displacement can be achieved regardless of the camera movement. A LiDAR-based method is also presented, by which the long-term time history of the bridge displacement can be tracked by a temporarily installed LiDAR, thus eliminating the need for a permanent installation in the field. These two long-term measurement approaches were cross validated on a 40 m-long full-scale railway bridge under construction. Over a span of 650 days, these two methods showed a similar trend, thus validating the applicability of each method. Important structural information, such as immediate displacement due to dead load, long-term deflection due to creep, daily fluctuation due to temperature gradient, could potentially provide long-term displacement data in bridge health monitoring.clos

NASA Tech Briefs, Spring 1984

Topics include: NASA TU Services: Technology Utilization services that can assist you in learning about and applying NASA technology. New Product Ideas: A summary of selected innovations of value to manufacturers for the development of new products; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Life Sciences; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences