Abstract This article presents the development of a liquid-level sensor using fiber optic Bragg gratings (FBG). The sensor comprises a carbon-fiber-reinforced polymers (CFRP) diaphragm packaging an embedded FBG to measure strain. A mathematical analysis determines the FBG response to the pressure exerted by a liquid column over the packaged sensor. The experimental arrangement consists of an acrylic tube with a sensor at the bottom. FBG signals were acquired using a DI 410 optical interrogator manufactured by HBM. A finite element simulation with Ansys was performed to determine the most reposeful position from the FBG sensor. The experimental results show the analysis of eight repetitions of the water column ranging from 10 cm to 60 cm using 10 cm steps. The sensor showed a linear response, with a coefficient of determination of 0.999, and a sensitivity of 4.1 pm/cm; and the maximum standard deviation is 0.64 pm. Although water is used in the calibration process, other liquids could also be used, obtaining a different angular coefficient depending on the density of the liquid used. liquidlevel level FBG. . (FBG) carbonfiberreinforced carbon reinforced CFRP (CFRP strain bottom 41 HBM 1 6 steps 0999 0 999 0.999 4 4. pm/cm pmcm pm 064 64 0.6 process (FBG 099 99 0.99 06 0. 09 9 0.9

Resumo O monitoramento estrutural pode detectar anomalias no tempo, permitindo a implementação de ações de manutenção e reparação mais eficientes em estruturas de obras de arte. Uma alternativa no monitoramento e avaliação de estruturas em engenharia civil é a utilização de fibras óticas inscritas com redes de Bragg (FBG ou Fiber Bragg Grating). Este trabalho visa comparar as técnicas disponíveis para estimar o módulo elástico de compósitos cimentícios, bem como contribuir para a aplicação de sensores de fibra ótica na avaliação da tensão, frequência natural de vibração, coeficiente de amortecimento e módulo elástico de vigas de argamassa. Os sensores FBG fabricados e calibrados pelos autores apresentaram uma boa precisão na medição destes parâmetros em amostras de argamassa. Os resultados mostram que os sensores FBG e strain gauges apresentam resultados semelhantes para a deformação específica e estimativa do módulo elástico (estático e dinâmico), portanto, FBGs nuas poderiam ser investigadas mais detalhadamente em testes não-destrutivos.

Abstract Structural health monitoring can detect anomalies in time, allowing the implementation of more efficient maintenance and repair actions in special engineering structures. An alternative in the monitoring and evaluation of structures in civil engineering is the use of optical fibers inscribed with Bragg gratings (FBG or Fiber Bragg Grating). This paper aims to compare available techniques to estimate the elastic module of cementitious composites, as well as contribute to the application of optical fiber sensors in the monitoring of strain, natural vibration frequency, damping coefficient and elastic modulus of mortar beams. The FBG sensors manufactured and calibrated by the authors presented a good precision for measuring those parameters in mortar samples. The results show that FBG sensors and strain gauge sensors present a similar result for strain and estimation of the elastic module (static and dynamic), therefore, bare FBGs could be further investigated for non-destructive testing.

Resumen: este artículo presenta una propuesta para la compensación de la ganancia no uniforme de los amplificadores ópticos de fibra dopada con Erbio (EDFA) en sistemas multiplexados por longitud de onda (WDM) usando redes de difracción de Bragg (FBG). En esta propuesta las redes neuronales artificiales tipo perceptrón multicapa con propagación hacia atrás (feed-forward backpropagation) se entrenaron bajo el método de la secante (one-step secant) y se seleccionaron según error cuadrático medio. La propuesta optimiza los parámetros del FBG, como la frecuencia central, el nivel de rechazo y la longitud para determinar una respuesta de filtrado basada en un número reducido de FBG que se utilizarán para aplanar la respuesta no lineal de la ganancia del amplificador y evitar el tratamiento por portadora de un filtro de aplanamiento estándar. Mientras que una red neuronal artificial con una estructura de 7-10-6 demostró la viabilidad de igualar la ganancia de un EDFA utilizando tan solo tres FBG, una estructura de 25-18-12 mejoró los resultados cuando la configuración consistió en un arreglo de seis FBG que proporcionaron resultados similares a los que ofrece el filtro estándar de aplanamiento de ganancia. La propuesta se evaluó en un sistema WDM amplificado de ocho portadoras ópticas ubicados entre 195 THz y 196,4 THz.

Abstract: This paper presents a proposal for the non-uniform gain compensation of an Erbiumdoped fiber optic amplifier (EDFA) in a Wave-length Division Multiplexed (WDM) system using Fiber Bragg Gratings (FBG). In this proposal, the multilayer perceptron feed-forward artificial neural network with backpropagation was trained under the secant method (one-step secant) and was selected according to mean square error measurement. The proposal optimizes FBG parameters such as center frequency, rejection level and length in order to determine a filtering response based on a reduced number of FBGs that will be used to flatten the non-linear response of the amplifier gain and avoid the per-carrier treatment of a standard flattening filter. While an artificial neural network with a 7-10-6 structure demonstrated the feasibility of equalizing the gain of an EDFA using as few as three FBGs, a 25-18-12 structure improved the results when the configuration consisted of an FBG array of six resonances that provided similar results to that featured by the standard gain-flattening filter. The proposal was evaluated in an amplified WDM system of eight optical carriers located between 195-196.4 THz.

ABSTRACT This paper presents the conceptual design for a new method for the suppression of OH emission lines at near-infrared (NIR) wavelengths by actively adjusting the aperiodic fiber optic Bragg gratings tension. First, we prepared an experimental study in which we simulated an OH emission line using a semiconductor laser at 1548.43 nm and a commercial FBG, with a Bragg wavelength of 1547.76 nm. We demonstrated that the grating Bragg wavelength can be adjusted by controlling the linear deformation of the fiber with a force in the range of 0 to 53.88 gf (0.528 N) that provides a sensitivity of 0.014 nm g −1. Second, we proposed the design of a system connected to the telescope instrumentation, with the different stages that would allow monitoring the suppression of emission lines.

RESUMEN Este trabajo presenta el diseño conceptual de un nuevo método para la supresión de líneas de emisión de OH en longitudes de onda del infrarrojo cercano (NIR) mediante el ajuste activo de la tensión de rejillas Bragg de fibra óptica aperiódicas. En primer lugar, realizamos un estudio experimental en el que simulamos una línea de emisión de OH utilizando un láser semiconductor a 1548.43 nm y una FBG de uso comercial, con una longitud de onda de Bragg de 1547.76 nm. Demostramos que la longitud de onda de Bragg de la rejilla puede ajustarse controlando la deformación lineal de la fibra con una fuerza en el intervalo de 0 a 53.88 gf (0.528 N) que proporciona una sensibilidad de 0.014 nm g −1. En segundo lugar, proponemos el diseño de un sistema conectado a la instrumentación del telescopio con las distintas etapas que permitirían monitorear la supresión de las líneas de emisión.

Abstract This article discusses procedures for production of tapered fibers in different diameters (tapers) and Bragg gratings writing (tFBG) in such fibers. The reflection spectra in different tFBGs are compared with a FBG written in standard single-mode fiber (SMF28). The gratings were characterized in terms of their thermal sensitivity. Values reasonably close to 11 pm/°C were expected and were obtained in the different gratings in tapers, confirming that the thermo-optical coefficient is predominant in relation to thermal expansion in gratings written in tapered fibers.

Abstract This work shows the experimental validation of a method developed for determining the strain profile applied to a fiber Bragg grating by solving the inverse scattering problem. The non-uniform strain profile is recovered by formulating an optimization problem, solved with an algorithm based on differential evolution. The method has proved to be able of recovering the shape and approximate amplitude of the actual strain profile applied to the FBG, opening new perspectives for optical sensing with fiber Bragg gratings.

To help improve the monitoring of the matte tap-holes on the matte endwall to prevent failures, fibre optic temperature systems were installed on the six-in-line electric furnace at the Polokwane Smelter. This was done with the intent to provide more detailed and timely detection regarding the change in condition of the copper coolers, refractory materials, and associated freeze-lining components at the hot face of the matte endwall and copper coolers. Fibre optics were selected to provide a significantly higher number of sensors and greater spatial coverage across the tap-block compared to conventional temperature measuring systems. Two types of fibre optic technology were installed; type A utilizing Bragg gratings and type B utilizing Raman spectroscopy. Each of these technologies were installed in different tap-holes and compared to the conventional thermocouples and resistance temperature detectors (RTDs) already installed. The characterization of the temperature responses was generated from tapping in terms of magnitude, rates of change, and action of specific events such as drilling, lancing, and tap-hole clay closing. The benefits and limitations of the various systems in place, the analyses done, and the way forward are described.

Abstract Fiber Bragg gratings (FBGs) offer new possibilities to monitor accurately the rotor temperature. Dozens of sensors can be mounted in series in a single fiber and used to measure the temperature in several points of the rotor winding. Such sensors installed directly on the rotor winding surface are thermally isolated from the cooling air by a silicone layer. Because of the temperature gradient in this structure, the sensor is exposed to thermo-mechanical stresses and therefore can be deformed. Since the FBG probes are sensitive to both temperature and strain, the knowledge of each effect separately is necessary to ensure that the temperature readings are not affected by strain. Experimental results obtained in rotor winding mockup tests with thermistors and FBG sensors show that the temperature readings by the FBG are 4.5°C above the temperature defined by the thermistors which were used as references. Multi-physics simulations were carried out to calculate the strain and temperature in the FBG assembly. The theoretical and experimental results are in a good agreement.

Abstract This study describes a localized method – using optical fiber Bragg gratings (FBGs) – to determine the human bite force at the very moment of occlusion, in maximum intercuspidation. The FBG was encapsulated in a silicone splint between uppers and lowers first molars and avoiding pre-contact with the other teeth in a dentition model. Gypsum dental castings of two volunteers are performed in a semi-adjustable articulator in the occluded situation to maintain angulations of the occlusal plane within normal occlusion. The sensor was characterized in vitro for both volunteers and the human bite force is measured in vivo. The orthodontic forces on the sensor are acquired at four moments: before of occlusion (free situation), occlusion, biting and maximum bite. The measured maximum bite forces are 85 N for female (brachyfacial) volunteer and 76 N for male (mesofacial) volunteer, between right uppers and lowers first molars. The difference can be associated to the facial pattern. The results obtained show the possibility to employ the optical encapsulated sensor to perform measurements of orthodontic forces in order to analysis different types of facial patterns with minimum of interference at occlusion.

Abstract Effect of cross-sensitivity between temperature and refractive index (RI) was studied using an optical sensor based on Bragg grating inscribed in an etched multi-mode fiber (MMF). RI sensitivities of different modes were used to obtain a linear response and temperature dependence in water:glycerin solutions. Results pointed out that the absolute average RI sensibility reduces from 0.97 ± 0.05 nm/RIU to 0.76 ± 0.07 nm/RIU while the temperature increases from 15 °C up to 35 °C, respectively. Effect of temperature on the RI sensitivity is related with the increasing difference between the fiber and surrounding RI, which results in lower evanescent field and, consequently, reduced surrounding RI sensitivity. Therefore, in order to improve the precision during the RI measures, the effect of temperature must be considered in the calibration curves, principally in MMF fiber grating sensors that show low RI sensitivity values.

Abstract Results on experimental study on kinetics of fiber Bragg gratings formation in the standard telecom optical fiber SMF-28 at initial stage of irradiation are presented. For low fluence per pulse, it was found that the fast formation of color centers Ge(1) is a dominant mechanism of the refractive index change at initial stage for accumulated fluence of below 0.02 J/cm2 at the wavelength of 266 nm. At low fluence per pulse, the induced change of the refractive index does not depend on UV light intensity and saturates at a level of ∼ 2 × 10 - 6. Dominance of the Ge(1) color center mechanism in the FBG formation at the initial stage makes possible a cost-effective recording of large arrays of weak FBGs for sensors utilizing lasers of low energy per pulse.

Resumen Se presentan resultados de un estudio experimental sobre la cinética inicial de formación de rejillas de Bragg en una fibra óptica estándar de telecomunicaciones SMF-28. Se encontró que para un régimen de irradiación de bajo flujo luminoso por pulso, a una longitud de onda de 266 nm y con una dosis de irradiación acumulada inferior a 0.02 J/cm2, la formación de centros de color tipo Ge(1) es el mecanismo principal de cambio fotoinducido del índice de refracción. Dicho valor de cambio fotoinducido no depende de la intensidad de la luz y se satura rápidamente a ∼ 2 × 10 - 6. La dominancia de este mecanismo de fotosensibilidad en la etapa inicial de irradiación, hace que sea posible grabar, de forma económica, arreglos (redes) extensos de rejillas de Bragg de baja reflexión, utilizando láseres de baja energía por pulso.

Abstract This paper proposes the use of fiber Bragg gratings (FBG) in thermal monitoring of photovoltaic (PV) modules. Results acquired from FBG sensors were compared with two commonly used temperature sensing techniques in PV modules: PT100 sensors and infrared cameras. The experiments were performed using a PV module in actual operating conditions (varying ambient temperature and wind speed). Temperature changes in PV module were monitored with 27 FBG sensors installed on its front surface. During the experiments the ambient temperature, intensity of solar radiation and wind velocity were also monitored. The acquired results showed that FBG sensing technology has a potential for the proposed application.

Abstract Static and dynamic effects on power transmission towers can be evaluated by methodologies available in codes, which suggest the use of linear static analysis. By using numerical simulations, it is possible to observe the strong influence of the geometric nonlinear behavior of transmission cables. Dynamic effects also strongly influence this behavior, with the possibility of resonance between the cables and the structure, but up to the moment, the existent analysis procedures have not been completely validated on an experimental basis. In order to validate a complete analysis methodology, experimental procedures are proposed for a suspension tower of a 138kV transmission line in use. A tridimensional anemometer was installed on this structure in order to measure the values and directions of wind speeds. Simultaneous strain values were collected on the main elements of the tower through optical extensometers. Optical sensor technology with Fiber Bragg Gratings was used, due to the characteristic of immunity to the electromagnetic field occasioned by high electric currents. The string swing angle was evaluated through a high-resolution camera and a tridimensional accelerometer. With this instrumentation, it is possible to create a complete database that correlates wind speeds with the responses of the structural set. At the moment, 5 months of data have been collected and the instrumentation is in the final testing phase and synchronized. After this step, real-time measurements will be performed.

This work shows the utilization of a coaxial cable for the fabrication of a long period Bragg grating. The grating is fabricated removing the dielectric in short pieces of the cable so that the discontinuities account for the variation in the medium refractive index. Simulated and experimental results of the grating resonances are shown as a function of the scattering parameters S, demonstrating the feasibility of the technique. By using the sensor in the MHz frequency range cheaper electronics can be employed, which reduce the overall cost of the sensing system.

Se determinaron los coeficientes característicos para redes de difracción de Bragg simples; usadas como sensores de deformación y temperatura en estructuras metálicas. Diferentes experimentos fueron realizados para establecer la relación entre la longitud de onda filtrada por una red de difracción simple y la temperatura y deformación a las cuales es sometida la fibra óptica. Se compara el desempeño de los sensores ópticos contra sensores eléctricos convencionalmente usados, bajo condiciones normales de operación de las estructuras metálicas. Se obtuvo el valor de la relación constante para valores de temperatura de 20°C a 70°C y de deformación en un rango de cero a 1800 µε . Se obtuvo una mayor velocidad de respuesta en los sensores ópticos FBG frente a galgas extensiométricas o termopares. Los sensores FBG muestran un mayor rango de funcionamiento en pruebas de deformación destructivas.

In this research, the characteristic constants for optical fiber Bragg grating strain sensors were determined for their use in metallic structures. Three different experiments were performed with the purpose of finding the constant relationship between the optical wavelength reflected by the FBG sensor and the strain or temperature incidents in a specific structure. It was established a comparison between the behavior of optical sensors versus electrical sensors used traditionally. From these experiments it was obtained a constant value for temperature values between 20°C and 70°C; and for strain from zero up to 1800 µε. All measurements were taken in 'in-situ' conditions for tests of metallic structures. The results show a higher speed of response for the optical sensors when compared with electrical resistance strain gauges or thermocouples. Also, fiber Bragg grating sensors show a larger operating range in tensile tests.