Bond-slip is an important interaction between steel and concrete in reinforced concrete (RC) structures and other civil engineering constructions. It is essential to understand and to characterize, at local level, this stress transference mechanism. In particular its behavior for monotonic and cyclic demands, the parameters that influence this mechanism, and how it is affected by different deterioration factors. Therefore, characterizing and monitoring the bond-slip mechanism is essential for the safety assessment of RC structures, more specifically determining the reinforcing bars slippage inside the concrete, and therefore the stress and strain distribution in RC members. In this work, two optical fiber sensors are presented, based on silica and polymer fiber Bragg gratings (FBGs), which were implanted inside a concrete block specimen and subjected to a pull-out test. After 6 days of curing, the pull-out test was recorded and the displacement incurred during the test was also monitored with a traditional electric sensor; for comparison with the data acquired with the two optical sensors. The results obtained confirm the viability and advantages of the optical sensors, evidenced by their higher resolution and far lower dimensions (allowing them to be embedded into the concrete) when compared with their electronic counterparts. The straightforward implementation and use of the optical sensors show very promising results when used in civil engineering structures.

This work aimed to develop a fibre optic sensor capable of monitoring the joint movements of the fingers of the hand. This was achieved using multicore optical fibres (MCFs) with fibre Bragg gratings (FBGs) multiplexed along the length of the fibre and in the wavelength. Through the measurement of their spectral wavelength shift, it was possible to reconstruct the directional curvature in multiple points along the length of the MCF. The sensor was characterised in two configurations: using only the MCF and using the MCF encapsulated in a capillary tube. Various materials were tested for the tube, and it was concluded that the most suitable was the nitinol tube due to its superelastic properties, which allow the fibre to return to its original shape after deformation. The sensitivity curves obtained by the radial core-FBGs for different angular positions of the MCF revealed satisfactory results and were similar between each other, demonstrating good performance for the characterisation of the sensor. After obtaining the maximum sensitivities for each FBG inscribed in each MCF core, it was possible to reconstruct the curvature vector at a specific MCF length. Tests of the sensor's performance were carried out using the data obtained for calibrating the sensor, showing that it was capable of reconstructing both the amplitude and direction of the curvature vector with average errors of 5% and 1%, respectively. To validate the robustness of the method, random curvatures and orientations were tested for the four quadrants, obtaining errors lower than those mentioned above. In a second phase of the work, the sensor was used to reconstruct the twodimensional shape of the MCF. It was observed that the reconstructed shape was close to the predicted one, despite localised errors. These were attributed to the distance between the FBGs (≈ 2.5 cm) and the model used for reconstruction which uses the spatial position of the prior FBG to estimate the spatial position of the next, leading consequently to the propagation of errors. In addition, the reconstruction method assumes that the MCF maintains its angular alignment along the MCF length. However, since the MCF length was ≈ 70 cm, small twists can appear along its length, contributing to additional errors. In the final phase of this thesis, the sensor's performance was evaluated in the index finger, using three scenarios: using a 3D printed finger, through a robotic hand, and in the finger of a healthy individual. In each case, the amplitude of the curvature vector was reconstructed and later compared with the predicted one, showing better performance for smoother curvatures. The average errors obtained were in the order of 6% in the case of the tests carried out on the 3D printed finger and 4% in the case of both the healthy individual and the robotic hand. These results showed promising results regarding the use of this technology for the monitoring of the degrees of motor function disability of the hand in rehabilitation contexts, guaranteeing a powerful tool for the occupational therapist.

In this paper we describe the implementation and test of an optical fiber based accelerometer with cross axis insensitivity. The accelerometer uses two matching fiber Bragg gratings which are also responsible by the demodulation of the wavelength encoded information. The accelerometer was tested in a reinforced concrete slab, of a 3-storey building located at the University of Aveiro, Portugal. A maximum error of 0.25% for the slab eigenfrequencies was obtained when compared with the values acquired by an electronic sensor. (C) 2010 Elsevier Ltd. All rights reserved.

A methodology based on optical fiber (OF) sensing was developed to screen single walled carbon nanotubes (SWCNT) in aqueous solutions. This method was validated by the comparison of its analytical performance with that of an ultraviolet-visible (UV-Vis) technique by monitoring the absorbance intensities at 500 nm, and no significant difference (p=0.854) was observed between such two methods. The results obtained by the OF sensor were encouraging in what concerns a new approach for detection and quantification of SWCNT in solutions due to its compact design, less expensive materials and equipment, as well as a requirement of low volume of sample. Additionally, it was concluded that the nonlinear calibration model observed for the analytical response with the OF probe follows the general cumulative symmetric double sigmoidal (SDS) model (R(2)=0.9999), once adapted for the analytical region of interest.

Introduction: due to their excellent aesthetic properties and to the improvement of some mechanical properties, the composites have been acquiring increasing importance in the context of oral rehabilitation. There are, however, some disadvantages that may compromise their clinical success, such as polymerization shrinkage, which could be associated to contraction stress transferred to the tooth, leading to cusp deflection and/or enamel microcracks. On the other hand, stress at the tooth-composite interface has the potential to cause adhesive failure, leading to post-operative sensitivity, microleakage, recurrent caries, and ultimately, to pulp inflammation. Objectives: The aim of this work was to make a pilot study regarding the capability of optical fiber Bragg grating (FBG) sensors to evaluate the linear polymerization shrinkage of composite resins. Materials and methods: Two different composite resins (SDRTM and Esthet X HD – Dentsply DeTrey, Konstanz, Germany) were placed in plastic molds crossed by a fiber Bragg grating sensor and light-cured by a LED unit (Bluephase®, Ivoclar Vivadent, Lichtenstein). Strain in μƐ was plotted as a function of time (seconds), obtaining a representative curve. Besides linear shrinkage evaluation, additional measurements were made concerning the temperature rise induced on the FBG sensor solely due to the LED light curing irradiation and for thermal characterization of both composite resins during light-curing process. Results: it was observed an initial expansion either due to thermal absorption of light curing unit either from the exothermic reaction of composite polymerization immediately followed by linear contraction, which reached 0,43% to Esthet X HD and 0,42% to SDRTM. Conclusions: The optical fiber Bragg grating sensors can be used to evaluate linear polymerization shrinkage of dental composites with clear advantages to assess the evolution of composite shrinkage in real time during all polymerization process. Introdução: devido às suas excelentes propriedades estéticas e à melhoria das suas propriedades mecânicas, as resinas compostas têm vindo a conquistar um papel progressivamente crescente no âmbito da reabilitação oral. Contudo, de entre algumas das suas desvantagens inerentes, a contração de polimerização assume um papel crucial pelas consequências que pode originar como a deflexão cuspídea, propagação de microfracturas e ruptura da interface adesiva da restauração, potenciando a presença de fendas marginais e consequente microinfiltração, sensibilidade pós-operatória, recidiva de cárie e, em última instância, de patologia pulpar. Objectivos: realizar um estudo piloto sobre a capacidade de avaliação da contração de polimerização linear de resinas compostas com recurso a redes de Bragg gravadas em fibra óptica (FBG). Materiais e métodos: introduziu-se uma rede de Bragg num molde de plástico, que posteriormente se preencheu com duas resinas compostas (SDRTM e Esthet X HD – Dentsply DeTrey, Konstanz, Germany) colocadas ao redor da fibra, procedendo-se à sua fotopolimerização com um fotopolimerizador de LEDs (Bluephase®, Ivoclar Vivadent, Lichtenstein). A força da contração, medida em μƐ, foi traçada em função do tempo (em segundos), obtendo uma curva representativa da contração linear. Para além da contração, foram registadas as variações de temperatura provocadas somente pela irradiação do fotopolimerizador de LEDs e durante a polimerização das resinas compostas. Resultados: foi possível observar uma ligeira expansão inicial das resinas compostas imediatamente seguida por contração linear que atingiu valores de 0,43% para o Esthet X HD e 0,42% para o SDRTM. Conclusões: O uso de FBG para avaliação da contração de polimerização linear das resinas compostas é um método viável com vantagens claras na monitorização em tempo real de todo o processo de polimerização.

This thesis investigates how to improve the performance of current designs of optical fiber sensors based on Surface Plasmon Resonance, and how to use a better understanding of the physical and sensing principles behind them to propose new sensing concepts and ideas. We adopt a methodology based on numerical simulations because they provide a better insight onto the operation of these sensors and because they allow an easy and quick way of testing new designs and concepts without the need to fabricate the sensors. We also show that these simulations have a good agreement with experimental results. We adopt a systematic approach to investigate the various parameters that influence the sensor performance, and present different sensors designs, where we study the localization, optical properties, shape and size of the metal components, combined with different type of fibers, resulting in the coupling between the plasmon and optical modes. Furthermore, we verify that choosing the optical modes used in sensing in multimode fibers can also have advantages. We investigate the use of modern artificial materials, such as metamaterials, as well as the inclusion of multiple wires in the fiber to enhance the performance of the SPR sensor. At a more fundamental level, we show that the control of the coupling between multiple plasmon modes in metal components and the optical modes in the fiber constitutes a new way to improve the performance of the sensor and can be inclusively used to develop a new type of SPR sensors capable of measuring simultaneously two variables, such as the external refractive index and temperature

Estimation models for the surrounding refractive index response of EFBGs were developed from computer simulations and tested on experimental data. Numerical simulations were used to investigate the fundamental core mode’s effective refractive index dependence with the fiber radius and the surrounding refractive index. An empirical mathematical model was developed upon the simulation results, and it was used to derive the estimation models. The response curve is estimated from a reference calibration curve using two different approaches:interpolationand extrapolation.The interpolation is performed using the wavelength response of the EFBGs at two different surrounding refractive index values. Then, the response curve is obtained solving a simple pair of linear equations. The extrapolation is performed using the wavelength shift observed during the etching process, which is used to derive a multiplicative factor for the calibration curve. The response curves estimated by the model were compared with the experimental results, showing good agreement within the experimental uncertainties. Considering the surrounding refractive index ranging from 1.333RIU to 1.458RIU, with the sensors’ response ranging from 1544.84nm to 1552.70nm, the maximum deviation between the estimation and the experimental results were 0.26nm and 0.41nm, and the maximum standard deviation were 0.03nm and 0.05nm, respectively for interpolation and extrapolation.

This work reports de development and testing of a low cost plastic optical fiber liquid level sensor for operation within highly flammable or other hazardous environments, such in oil piping or fuel tanks. The sensor consists on a vertical fiber, with engraved grooves, that will be sequentially fulfill when the liquid rises, resulting in an increase of the transmitted optical power. A variation of 2.60% in the transmitted optical power intensity was obtained per measured point. Moreover, the sensor sensitivity and resolution can be tailored accordingly to the application.

The analysis of the quality of food oils is of paramount importance, because the degradation of oils can lead to formation of harmful substances to the human organism. With the increase of the degradation of oils an increase of its refractive index occurs. The objective of this work is to develop and to characterize optical fiber refractometers sensitive to variations of refractive index of food oil samples. The optical fiber refractometers thanks to the intrinsic characteristics make them suitable for monitoring the quality of frying oils. They possess the advantages to require small volumes of sample for analysis, do not contaminate the sample, and supply the response in real time. In this work a long period grating (LPG) as refractometer is used because of their sensitivity to refractive index of the external media: degraded and not degraded frying oil samples. The oil samples had been characterized by the analysis of total polar components. The refractive index of oil is above 1.47, this region the LPG does not show enough sensitivity, a nanolayer of an organic material was coated onto the fiber. Using the Langmuir-Blodgett technique the response of LPG is modified according to the refractive index and thickness of the film. The deposition of the film modifies the rates effective modes of cladding, thus improving the response of the changes in the refractive index of the external media higher than that the refractive index of the cladding (n=1.457).

In this work is studied the response of optical fiber long period grating (LPG) to changes of the refractive index of the external media relatively to variations of wavelength and in transmission. The response of the LPG to refractive index greater and lesser than to cladding is investigated. A nanolayer was deposited onto the fiber to increase the sensitivity of the LPG to refractive index of the external media higher than cladding. The film modifies the rates of effective modes of cladding, thus improving the response of the changes in the refractive index of the external media higher than that in the refractive index of the cladding (ncl ≈ 1.457). The Langmuir–Blodgett technique was used for the deposition of the nanolayer.