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To ensure the safety and durability of structures, the constant monitoring of their structural health has been used by engineers to manage these structures. Optical fiber sensors have the advantage of long transducer-to-processing distances, the capacity to work in harsh environments, immunity to electromagnetic noise, and reliability. Therefore, optical fiber sensors are great candidates to monitor structures in-service. Here, we propose the use of a long-period fiber grating (LPG) array to monitor the loading condition of a structure. The identification method was tested in a simply-supported beam model and the estimated loading conditions were compared to the loading conditions applied to the beam. By analyzing different combinations of force intensity and position along the beam length, the results showed the force position could be estimated with 0.901% accuracy and force intensity with 2.51% accuracy using a four-LPG strain sensor array.

This work introduces a new method for long-period fiber grating (LPG) sensors interrogation. This proposal uses a fiber Bragg grating (FBG) array to extract spectral information of the LPG sensor and an Artificial Neural Network to process this information. The information is processed to estimate the LPG resonant wavelength, without prior knowledge on the LPG spectrum. Therefore, the interrogator is LPG-insensitive, can be easily manufactured by optical fiber sensors laboratories at low-cost and is suitable for in-field applications. We demonstrated the filter array and Multilayer Perceptron (MLP) design, which are the proposed interrogator core. Furthermore, we analyzed the interrogation performance by the Mean Squared Error (MSE), the Mean Absolute Error (MAE), and the distribution of the residuals. The results showed our proposal can estimate the LPG resonant wavelength with 2.82 nm uncertainty, considering a 95% confidence interval, over 75 nm dynamic range for several LPGs, with different spectral characteristics. Moreover, our proposal can be easily tailored for different dynamic ranges and resolutions with proper adjustments on the FBG array and MLP.

This paper presents a mathematical model for the interrogation of long period grating (LPG) fiber optical sensor. The mathematical model is simple and robust, and it is based on an electrical harmonic analysis. The model calculates the harmonic amplitude equations in the electrical domain of a wavelength modulated optical signal as a function of all the spectral parameters of the LPG. In addition, this model takes into account any distortions that might have been introduced by the wavelength modulator and by any other source of distortion. The harmonic amplitude equations are obtained by expanding the demodulated signal in Taylor Series and by taking the Gaussian shape of the LPG absorption spectra into consideration. To validate the model, we implemented a simple interrogation setup to compare the measured electrical harmonic amplitudes with those obtained using the calculated harmonic amplitude equations. A dynamic range of 25 nm was achieved by shifting the resonant wavelength of the LPG with an accuracy better than 1 dB.

Made available in DSpace on 2020-12-11T14:56:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-04-15 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) In this paper a new method for the detection of optical phase in passive homodyne interferometers, with high accuracy and wide dynamic range is presented. Unlike the traditional and more recent methods, which use between 3 and 6 harmonic components to calculate the modulation index, the proposed method uses only 2 harmonic components. Because of this, the method has higher accuracy in the presence of noise compared to other methods known in the literature. Further, the method has the advantage of not requiring knowledge of the phase of the excitation signal. The numerical implementation is simple, since it uses a 3rd order polynomial fit for the calculation of the modulation index. For the experimental verification of the method, a configuration based on an unbalanced Mach-Zehnder interferometer (MZI) was used, which resulted in a dynamic range for the modulation index from 8.38 to 16.76 rad. It is important to note that these limits can be easily expanded by means of adaptations of the optical and electronic setup depending on the characteristics of the excitation signal. The proposed method can be used in several applications, such as pressure measurements, vibration, displacement, etc. Fed Fluminense Univ UFF, Telecommun Engn Dept, Postgrad Program Instrumentat & Appl Opt PPGIO, BR-24220900 Niteroi, RJ, Brazil Univ Salgado de Oliveira, Engn Dept, BR-24030060 Niteroi, RJ, Brazil Sao Paulo State Univ, Elect Engn Dept, BR-01001900 Sao Paulo, Brazil Fed Inst Educ Sci & Technol Mato Grosso do Sul, Elect Engn Dept, BR-79641162 Tres Lagoas, Brazil Univ Fed Juiz de Fora, Elect Circuit Dept, BR-36036900 Juiz De Fora, Brazil Sao Paulo State Univ, Elect Engn Dept, BR-01001900 Sao Paulo, Brazil CNPq: 305307/2015-0 CNPq: 420673/2016-4