Static Characterization of Curvature Sensors Based on Plastic Optical Fibers

Sensors able to measure curvature changes are emerging as an effective alternative to the more common strain gauges for structural health monitoring applications. Particularly interesting is the all-optical fiber implementation for its unique properties and the possibility of being embedded. This paper, after a brief description of curvature sensors using plastic optical fibers, focuses on their characterization in applications where high sensitivity is required, and compares their performance with commercial strain sensors based on fiber Bragg gratings. The choice of plastic optical fibers allows the realization of simple, compact and cheap sensors. A characterization setup to test different sensor typologies is proposed and the main uncertainty contributions are investigated

Review of Fiber Optic Displacement Sensors

Displacement Measurements Are of Significant Importance in a Variety of Critical Scientific and Engineering Fields, Such as Gravitational Wave Detection, Geophysical Research, and Manufacturing Industries. Due to the Inherent Advantages Such as Compactness, High Sensitivity, and Immunity to Electromagnetic Interference, in Recent Years, Fiber Optic Sensors Have Been Widely Used in an Expansive Range of Sensing Applications, Ranging from Infrastructural Health Monitoring to Chemical and Biological Sensing. of Particular Interest Here, Fiber Optic Displacement Sensors Have Gained Wide Interest and Have Evolved from Basic Intensity Modulation-Based Configurations to More Advanced Structures, Such as Fiber Bragg Grating (FBG)-Based and Interferometric Configurations. This Article Reviews Specifically the Advanced Fiber Optic Displacement Sensing Techniques that Have Been Developed in the Past Two Decades. Details Regarding the Working Principle, Sensor Design, and Performance Measures of FBG-Based, Interferometers-Based (Including the Fabry-Perot Interferometer, the Michelson Interferometer, and the Multimode Interferometer), Microwave Photonics-Based, and Surface Plasmon Resonance-Based Fiber Optic Displacement Sensors Are Given. Challenges and Perspectives on Future Research in the Development of Practical and High-Temperature Tolerant Displacement Sensors Are Also Discussed

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

Recent Progress in Brillouin Scattering Based Fiber Sensors

Brillouin scattering in optical fiber describes the interaction of an electro-magnetic field (photon) with a characteristic density variation of the fiber. When the electric field amplitude of an optical beam (so-called pump wave), and another wave is introduced at the downshifted Brillouin frequency (namely Stokes wave), the beating between the pump and Stokes waves creates a modified density change via the electrostriction effect, resulting in so-called the stimulated Brillouin scattering. The density variation is associated with a mechanical acoustic wave; and it may be affected by local temperature, strain, and vibration which induce changes in the fiber effective refractive index and sound velocity. Through the measurement of the static or dynamic changes in Brillouin frequency along the fiber one can realize a distributed fiber sensor for local temperature, strain and vibration over tens or hundreds of kilometers. This paper reviews the progress on improving sensing performance parameters like spatial resolution, sensing length limitation and simultaneous temperature and strain measurement. These kinds of sensors can be used in civil structural monitoring of pipelines, bridges, dams, and railroads for disaster prevention. Analogous to the static Bragg grating, one can write a moving Brillouin grating in fibers, with the lifetime of the acoustic wave. The length of the Brillouin grating can be controlled by the writing pulses at any position in fibers. Such gratings can be used to measure changes in birefringence, which is an important parameter in fiber communications. Applications for this kind of sensor can be found in aerospace, material processing and fine structures

Structural Health Monitoring Using Embedded Fiber Optic Strain Sensors

This dissertation involved several research activities with the common goal of developing a structural health monitoring methodology. The research activities encompass: composites fabrication, numerical modeling and a mechanical experimental program using a state-of-the-art sensing technology. The first part of the dissertation presents an analytical and experimental effort to characterize the bending response of sandwich composite plates. A general expression for the shear correction factor for laminated plates was derived using the principle of strain energy equivalence. It was proved that the first-order shear deformation theory (FSDT) can be confidently used in the elastic analysis of sandwich composite plates with lowdensity core using the proposed shear factor. A correlation between the experimental response of sandwich composite plates under transverse load and analytical predictions was conducted. The good agreement obtained provides an experimental validation of the developed analytical method and enhances the test method for simply supported sandwich composite plates subjected to a distributed load (ASTM D6416). The second part of the dissertation deals with structural health monitoring using embedded fiber optic strain sensors. A long-term monitoring of wood structural members of the AEWC Center new office building expansion was conducted using Extrinsic Fabry-Perot Interferometric (EFPI) fiber optic sensors embedded in a FRP laminate. A one-year cyclic variation of the longitudinal strain in the beams was observed, which is attributed to the variation of relative humidity during the year. The major research work of this dissertation presented a fatigue crack monitoring study of composite doubler plate joints using embedded fiber Bragg grating strain sensors was conducted. A methodology for the structural health monitoring and detection of delamination growth of composite joints based on strain measurements was developed. Secondary bonded woven Eglass/ vinyl ester composite doubler plate joints were subjected to fatigue tension loading. The feasibility of monitoring delamination using embedded sensors was investigated. The experimental plan, finite element modeling and the fabrication methodology of the specimens through Vacuum Assisted Resin Transfer Molding (VARTM) processing are presented. The proposed methodology allows detecting a one quarter inch delamination length, which is a common criterion used in marine construction for damage tolerance in service conditions

Impact detection techniques using fibre-optic sensors for aerospace & defence

Impact detection techniques are developed for application in the aerospace and defence industries. Optical fibre sensors hold great promise for structural health monitoring systems and methods of interrogating fibre Bragg gratings (FBG) are investigated given the need for dynamic strain capture and multiplexed sensors. An arrayed waveguide grating based interrogator is developed. The relationships between key performance indicators, such as strain range and linearity of response, and parameters such as the FBG length and spectral width are determined. It was found that the inclusion of a semiconductor optical amplifier could increase the signal-to-noise ratio by ~300% as the system moves to its least sensitive. An alternative interrogator is investigated utilising two wave mixing in erbium-doped fibre in order to create an adaptive system insensitive to quasistatic strain and temperature drifts. Dynamic strain sensing was demonstrated at 200 Hz which remained functional while undergoing a temperature shift of 8.5 °C. In addition, software techniques are investigated for locating impact events on a curved composite structure using both time-of-flight triangulation and neural networks. A feature characteristic of composite damage creation is identified in dynamic signals captured during impact. An algorithm is developed which successfully distinguishes between signals characteristic of a non-damaging impact with those from a damaging impact with a classification accuracy of 93 – 96%. Finally, a demonstrator system is produced to exhibit some of the techniques developed in this thesis

Fiber Bragg Grating Based Sensors and Systems

This book is a collection of papers that originated as a Special Issue, focused on some recent advances related to fiber Bragg grating-based sensors and systems. Conventionally, this book can be divided into three parts: intelligent systems, new types of sensors, and original interrogators. The intelligent systems presented include evaluation of strain transition properties between cast-in FBGs and cast aluminum during uniaxial straining, multi-point strain measurements on a containment vessel, damage detection methods based on long-gauge FBG for highway bridges, evaluation of a coupled sequential approach for rotorcraft landing simulation, wearable hand modules and real-time tracking algorithms for measuring finger joint angles of different hand sizes, and glaze icing detection of 110 kV composite insulators. New types of sensors are reflected in multi-addressed fiber Bragg structures for microwave–photonic sensor systems, its applications in load-sensing wheel hub bearings, and more complex influence in problems of generation of vortex optical beams based on chiral fiber-optic periodic structures. Original interrogators include research in optical designs with curved detectors for FBG interrogation monitors; demonstration of a filterless, multi-point, and temperature-independent FBG dynamical demodulator using pulse-width modulation; and dual wavelength differential detection of FBG sensors with a pulsed DFB laser

Analysis and development of a tunable Fiber Bragg grating filter based on axial tension/compression

Fiber Bragg gratings (FBGs) are key elements in modern telecommunication and sensing applications. In optical communication, with the advancement of the Erbium doped fiber amplifier (EDFA), there is a great demand for devices with wavelength tunability over the Erbium gain bandwidth (in particular, for wavelength division multiplexing (WDM) networks). The center wavelength of a FBG can be shifted by means of change of temperature, pressure or mechanical axial strain. The axial strain approach is the best method among all other techniques because it allows relatively large wavelength shifts with high speed. Axial strain of up to 4% will be required to cover the whole EDFA region (more than 40 nm of central wavelength shift). The formation of Bragg grating results in significant reduction in mechanical strength of optical fibers especially in tension. As a result, axial strain of only about 1% can be achieved by mechanical stretching of FBGs. In order to achieve the remaining 3% strain compression of FBGs has to be applied. In this thesis, the design and analysis of a novel device for achieving central wavelength shift are presented. In particular, the device has achieved, for a fiber with 12 mm FBG, a shifting of 46 nm in compression and 10.5 nm in tension with a reflection power loss of less than 0.25 dB and a FWHM bandwidth variation of approximately 0.1 nm. Both variations are well below the Bellcore standards requirement of 0.5 dB for peak reflectivity variation and 0.1 nm for bandwidth variation. The device consists of two fixed and one guiding ferrules. The difficulties associated with compressing the FBG were handled by carefully selecting tolerances and adjustment procedures. The device allows both tension and compression of FBGs, and the use of different FBG lengths and actuators. The effects of glue deformation and bending of the FBG during compression were analyzed in detail. Further, using the piezoelectric transducer (PZT) actuator as a driver, tuning speed of around 1.5nm/ms was achieved

Modern Applications in Optics and Photonics: From Sensing and Analytics to Communication

Optics and photonics are among the key technologies of the 21st century, and offer potential for novel applications in areas such as sensing and spectroscopy, analytics, monitoring, biomedical imaging/diagnostics, and optical communication technology. The high degree of control over light fields, together with the capabilities of modern processing and integration technology, enables new optical measurement systems with enhanced functionality and sensitivity. They are attractive for a range of applications that were previously inaccessible. This Special Issue aims to provide an overview of some of the most advanced application areas in optics and photonics and indicate the broad potential for the future