In this paper, we propose a method that allows to estimate the vertical displacement of a two-dimensional structure by means of strain measurements. Fifteen fiber Bragg gratings (FBGs) 10 mm long have been selected as strain sensors and embedded in a rectangular multilayer panel in order to test the proposed method. Only few optical fibers have been employed avoiding the complex wiring typical of the electrical sensors. The deflection tests have been carried out loading the panel in different configurations. Once the deflection maps were obtained, the values in the center were compared with the displacements directly measured by means of a mechanical comparator finding a very good agreement.
Long-term and continuous health monitoring of structures is of essential interest within the civil engineering communities due to the aging of infrastructures. Detect defects and monitor various physical or chemical parameters related to the health of the structures is vital for safety evaluation and health monitoring allowing for decision-making in terms of maintenance and retrofitting. Among the parameters of interest, the deflection is one of the most important parameters because the deflection limit is usually utilized as a control index of structures when subjected to potential external loads. However, it is not easy to have a real-time structural variations measurement because of the difficulties in measuring the deflection directly. Indeed traditional direct techniques, like dial indicator, level, and total station, have limited discrete points and are suitable only for short term monitoring. While newer whole field techniques, like accelerometer, microwave interferometer, GPS, connected pipe optoelectronic liquid level sensor, are developed for real time deformation measurement, but they need to install additional setup that causes extra costs and measurement data. An alternative approach for monitoring the deflection of the structures is based on an indirect measure such as the strain with attachable sensors and using proper transformation to estimate the displacement field. Along this line of argument, Fiber Bragg Gratings (FBGs) are good alternative to strain gauges and in the last years a demonstration of the deflection monitoring has already been carried out limiting the attention to one dimensional structures. Thus, in this paper, we extend and generalize the deflection estimation to a multilayer bidimensional structures by using FBG strain sensors and a displacement-strain transformation. In particular, several FBGs have been embedded in the structure within only a few optical fibers, thus avoiding the complex wiring typical of strain gauges. Then, from the strain measured by the FBGs, the curvature function has been evaluated as a polynomial function whit the coefficients obtained by least mean square; and the deflection is estimated by integrating twice the curvature function. Experimental results show good agreement with those directly measured by a dial. The proposed technique allows a real-time indirect structural monitoring solving the existing difficulties in measuring the deflection directly, and can be applied to small as well large structure. Furthermore, it is crucial in high energy physics, where particle bidimensional detectors have to measure the position of the incoming radiation with a resolution of few tens of microns, and detectors deflection directly influences this measurements.
We present an optical fiber device involving metal-dielectric colloidal crystal (MDCC) structures as a platform for surface enhanced Raman-scattering (SERS) sensing applications. The MDCC structures received large interest in last years since they permit to combine localized surface plasmonic resonance (LSPR) of noble metallic nanoparticles and photonic bandgap (PBG) of colloidal type photonic crystals (PhCs). Most of the MDCC structures have been provided onto planar substrates and then investigated as SERS platform. However, the integration and/or combination of the advantages MDCC structures with optical fiber technology represent still open challenges. The present paper reports recent results about MDCCs fabricated directly onto fiber optic tip surface by successive depositions of Au-NPs on PSbased CCs. Our goal is the fabrication of miniaturized fiber optic devices for SERS sensing applications. Numerical and experimental results of different samples are here presented and discussed. For the sensing test, the fiber probes were immersed in a 100μM ethanol solution of 4-Mercaptobenzoic acid (4MBA). Then, preliminary SERS results were retrieved by a Labspec Aramis confocal Raman spectrometer.
In this work, we demonstrate an extrinsic pressure sensor realized on single mode fiber tip by means of simple fabrication steps and with low-cost instrumentations. The sensing element consists in a Fabry-Perot cavity: one reflecting surface is the end of the optical fiber, precisely cut, and the other one is a metallic diaphragm. Under the action of the external pressure, the metallic diaphragm bends changing the optical cavity length and, consequently, the characteristics of the reflected signal. The holder structure, which allows the alignment of the fiber tip and reflecting diaphragm, consists in a commercial zirconia ferule with external diameter of Dex = 2.5 mm. Despite its simplicity and cost-effectiveness, the achieved results show performance comparable to more complex and expensive configurations. By using an aluminum plate as reflecting diaphragm. sensitivity ranging in the 70-130pm/mmHg is experimentally.