A novel fabry-pérot optical sensor for guided wave signal acquisition

In this paper, a novel hybrid damage detection system is proposed, which utilizes piezoelectric actuators for guided wave excitation and a new fibre optic (FO) sensor based on Fabry-Perot (FP) and Fiber Bragg Grating (FBG). By replacing the FBG sensors with FBG-based FP sensors in the hybrid damage detection system, a higher strain resolution is achieved, which results in higher damage sensitivity and higher reliability in diagnosis. To develop the novel sensor, optimum parameters such as reflectivity, a wavelength spectrum, and a sensor length were chosen carefully through an analytical model of the sensor, which has been validated with experiments. The sensitivity of the new FBG-based FP sensors was compared to FBG sensors to emphasize the superiority of the new sensors in measuring micro-strains. Lastly, the new FBG-based FP sensor was utilized for recording guided waves in a hybrid setup and compared to the conventional FBG hybrid sensor network to demonstrate their improved performance for a structural health monitoring (SHM) application

Strain monitoring.

This chapter provides an overview of the use of strain sensors for structural health monitoring. Compared to acceleration-based sensors, strain sensors can measure the deformation of a structure at very low frequencies (up to DC) and enable the measurement of ultrasonic responses. Many existing SHM methods make use of strain measurement data. Furthermore, strain sensors can be easily integrated in (aircraft) structures. This chapter discusses the working principle of traditional strain gauges (Sect. 8.1) and different types of optical fiber sensors (Sect. 8.2). The installation requirements of strain sensors and the required hardware for reading out sensors are provided. We will also give an overview of the advantages and the limitations of commonly used strain sensors. Finally, we will present an overview of the applications of strain sensors for structural health monitoring in the aeronautics field

NOVEL SENSOR PLATFORMS BASED ON FABRY-PEROT RESONATORS FOR APPLICATIONS IN ENVIRONMENTAL GEOPHYSICS

Fabry-Perot resonator sensors have been widely used for various physical and chemical measurements owing to their unique advantages over traditional sensors such as high measurement resolution, mechanically robust, and distributed sensing capabilities. This dissertation focuses on the development of robust fiber optic microwave sensors based on Fabry-Perot resonator mechanism for real-time applications in environmental geophysics. Firstly, a novel Extrinsic Fabry-Perot Interferometer (EFPI)-based fiber-optic sensor for force measurement using a pre-buckled beam was demonstrated. The axial displacement is transferred and amplified to a horizontal deflection at the middle of the buckled beam, leading to a relatively significant change in the Fabry-Perot cavity length. The force measurement range and the size of the sensor can be easily reconfigured by adjusting the size of the beam, enabling force measurement for different scenarios. Secondly, a self-compensated inclinometer with a wide dynamic range and high measurement resolution based on two hollow coaxial cable Fabry-Perot resonators (HCC-FPRs) was reported. By tracking the shift of the resonance wavelength of the HCC-FPR, two HCC-FPRs are used in the inclinometer design, which enables the inclinometer to achieve self-compensation for variations in environmental factors. Thirdly, a Polyvinyl Alcohol (PVA) film-assisted open-ended hollow coaxial cable Fabry-Perot resonator was proposed for highly sensitive embeddable soil moisture measurements. The invented sensor platform could be reconfigured to detect chemical contaminants in soil by changing the functional films in the active zone of the sensor --Abstract, p. i

A backward-mode optical-resolution photoacoustic microscope for 3D imaging using a planar Fabry-Pérot sensor

Optical-resolution photoacoustic microscopy (OR-PAM) combines high spatial resolution and strong absorption-based contrast in tissue, which has enabled structural and spectroscopic imaging of endogenous chromophores, primarily hemoglobin. This makes OR-PAM an important tool for preclinical vascular research. Conventional piezoelectric ultrasound transducers often need to be placed far away from the signal source due to their opacity, which results in reduced acoustic sensitivity. Optical ultrasound sensors are an alternative as their transparency allows them to be positioned close to the sample for minimal source-detector distances. In this work, a backward-mode OR-PAM system based on a planar Fabry-Pérot ultrasound sensor and coaxially aligned excitation and interrogation beams was developed. Two 3D imaging modes, using raster-scanning for enhanced image quality or continuous-scanning for fast imaging, were implemented and tested on a leaf skeleton phantom. In fast imaging mode, a scan-rate of 100,000 A-lines/s could be achieved. In raster-scanning mode, 3D images of a zebrafish embryo were acquired in vivo. The transparency of the FP sensor in the visible and near-infrared wavelength region makes it potentially suitable for combined functional and molecular imaging using OR-PAM and multi-photon fluorescence microscopy

Optical diagnostics for structural health monitoring of inaccessible systems

The work in this thesis is concerned with the development of optical sensors for the structural health monitoring of systems with inaccessible components in the presence of ionising radiation. Two primary approaches to determine structural health are investigated, these being the implementation of distance measurement sensors (to assess, for example, the occurrence of creep/cracking) and gas sensors (to assess the occurrence of outgassing which can be indicative of chemical ageing). Distance measurement sensors are developed using fibre optic Fabry-Pérot interferometry (FPI) measured in reflection. A fast-Fourier transform (FFT) of the reflected interference spectrum is incorporated to demodulate the signal and extract cavity length information. To mitigate noise and enhance measurement accuracy and sensitivity, spectrum reconstruction in the form of a function-fitting algorithm (FFA) is developed, the input of which is fed by the FFT output. The FFA demonstrates measurement improvements of approximately one-and-a-half orders of magnitude. To situate the fibre FPI sensors within spatially confined, closely positioned components, turning mirrors are fabricated on two cores of multi-core fibre (MCF) which redirect the fibre-guided light perpendicular to the fibre axis in opposite directions. This allows for the absolute distance between component parts to be determined as Fabry-Pérot cavities are formed on both sides of the MCF. Three different turning-mirror fabrication processes are developed, the results from each are compared and discussed. The distance measurement capabilities are expanded to that of a two-point measurement system to allow for tilt measurement competencies. This facilitates further comprehensiveness with regards to structural health monitoring. Further, two optical gas sensors are investigated, both based on the principle of absorption spectroscopy. One consists of evanescent wave generation in a tapered fibre and the other of incoherent broadband cavity-enhancement. Modelled and experimental results are presented and limitations are discussed

MINIATURE LOW-COHERENCE FIBER OPTIC ACOUSTIC SENSOR WITH THIN-FILM UV POLYMER DIAPHRAGM

A miniature low-coherence fiber optic acoustic sensor with a thin-film UV polymer diaphragm is developed and studied in this thesis to address the fundamental challenge of miniaturizing acoustic sensors. When miniaturizing an acoustic sensor, there is a critical size limitation at which the transduction mechanism deformation becomes too small for detection. However, a solution to this problem is to utilize a high resolution, low coherence fiber optic interferometric detection system coupled with a soft, thin-film transduction mechanism. A novel fabrication technique was developed to enable the use of elastomers, which inherently exhibit desirably low Young's modulus properties. In addition, the fabrication process enables fabrication of diaphragms at thicknesses on the order of nanometers. The fabrication process also renders highly tunable sensor performance and superior sensing quality at a low cost. The sensor developed exhibits a flat frequency response between 50 Hz and 4 kHz with a useable bandwidth up to 20 kHz, a dynamic range of 117.55 dB SPL, a signal to noise ratio (SNR) of 58 dB, and a sensitivity up to 1200 mV/Pa. In this thesis, it is further demonstrated that by using an array these sensors fabricated from the same batch facilitates accurate directional sound localization by utilizing the interaural phase difference (IPD) exhibited by sensor pairs. Future work is suggested to optimize the sensor performance for a specific application, to carry out studies of more complex array configurations, and to develop algorithms that can help increase the sound localization accuracy

Sensores em fibra ótica baseados em grafeno induzido por laser para potencial aplicação em aquacultura

Cortisol and ammonia are key indicators for the efficient and healthy development of aquaculture cultures. With this in mind, this work proposes the application of a laser-induced graphene (LIG) based sensor for the potential quantification and monitoring of cortisol and ammonia in aquaculture waters. The developed sensors were created from the Fabry-Pérot cavity principle. The cavities were filled with polyimide and in some cases mixed with gold nanoparticles. The interferometers were tested for strain, temperature, and refractive index, being observed an increase in the sensitivity to the refractive index due to the addition of the nanoparticles. The next step was to evaluate the best laser parameters to create a layer of LIG around the interferometers. It was observed that for a power of 10% and an inscription speed of 100 mm/s, the formed LIG was quite porous, homogeneous, and had a leafy texture. Using a pulsed CO₂ laser, the polyimide was then irradiated and transformed into LIG, creating a second cavity within the interferometer. Due to this, the spectral response of the interferometers resembled the Vernier effect. The refractive index behavior was again evaluated, showing improvements in sensitivity up to 153 times due to the creation of the LIG cavity. The interferometers were then functionalized to achieve an affinity to cortisol, this being a novel approach, obtaining sensitivities of -4.1±0.2 nm/log(ng/mL) and -34±5 nm/log(ng/mL) for two interferometers without and with gold nanoparticles mixed into the polyimide, respectively. The increased sensitivity in these interferometers is justified by the presence of these gold nanoparticles in the polyimide. It was also possible to observe that the sensors are immune to interference from substances such as glucose, sucrose, fructose, and ascorbic acid. Regarding ammonia arrest, sensitivities of -80±10 nm/ppm were obtained for sensors where oxazine 170 perchlorate was mixed in the polyimide.O cortisol e amonia são indicadores chave para o desenvolvimento eficiente e saudável de culturas para a aquacultura. Tendo isto em vista, este trabalho propõe a aplicação de um sensor baseado em grafeno induzido por laser (LIG) para a potencial quantificação e monitorização de cortisol e amonia em águas de aquacultura. Os sensores desenvolvidos foram criados a partir do princípio da cavidade de Fabry-Pérot. As cavidades foram preenchidas com poliimida e em alguns casos foi lhes misturadas nano partículas de ouro. Os interferómetros foram testados ao estiramento, temperatura e índice de refração, observando-se um aumento da sensibilidade ao índice de refração devido à adição das nanoparticulas. O próximo passo, passou por avaliar os melhores parâmetros do laser para criar uma camada de LIG à volta dos interferómetros. Observou-se que para uma potência de 10% e uma velocidade de inscrição de 100 mm/s, o LIG formado apresentava bastante porosidade, era bastante homogéneo e tinha uma textura folhosa. Com recurso a um laser de CO₂ pulsado, a poliimida foi então irradiada e transformada em LIG, criando uma segunda cavidade dentro do interferómetro. Devido a isto, a resposta espetral dos interferómetros assemelhou-se ao efeito de Vernier. O comportamento ao índice de refração foi novamente avaliado, mostrando melhorias na sensibilidade até 153 vezes devido à criação da cavidade de LIG. Os interferómetros foram então funcionalizados para terem uma afinidade ao cortisol, sendo que esta é uma abordagem inovadora, obtendo sensibilidades de -4.1±0.2 nm/log(ng/mL) e -34±5 nm/log(ng/mL) para dois interferómetros sem e com nano partículas de ouro misturadas na poliimida, respetivamente. O aumento da sensibilidade nestes interferómetros é justificada com a presença destas nano partículas de ouro na poliimida. Foi também possível observar que os sensores são imunes à interferência de substâncias como a glucose, sacarose, frutose e ácido ascórbico. No que toca à detenção de amónia, obtiveram-se sensibilidades de -80±10 nm/ppm para sensores onde foi misturada oxazina 170 perclorato na poliimida.Mestrado em Engenharia Físic