José Luís Santos

Abstract
A strain gauge sensor based on Fiber Bragg Grating (FBG) for diameter measurement is proposed and experimentally demonstrated. The sensor is easily fabricated inserting the FBG on the strain gauge—it was fabricated using a 3D printer—and fixing the FBG in two points of this structure. The idea is to vary the diameter of the structure. We developed two experimental setups, the first one is used to evaluate the response of the FBG to strain and the second one to assess the possibility of using the structure developed to monitor the desired parameter. The results demonstrated that the structure can be used as a way to monitor the diameter variation in some applications. The sensor presented a sensitivity of 0.5361 nm/mm and a good linear response of 0.9976 using the Strain Gauge with FBG and fused taper. © 2020 Elsevier Inc.

Abstract
This work addresses the role of optical sensing within the new emerging paradigm Industry 4.0. It starts with some thoughts about complex systems and their inherent need of enlarged sensorial tools. Then, the principles of optical sensing are presented with identification of the two principal types. After summarizing what is meant by Industry 4.0, it is detailed how optical sensing can contribute to the raise up of this new industrial concept, focusing on vision, physical sensing, chemical sensing, and sensor multiplexing. Emphasis is given in fiber optic sensing and, when feasible, in fiber Bragg grating sensing technology. Finally, some final remarks are delivered.

Abstract
This paper proposes a scheme to determine the optical dispersion properties of a medium using multiple localized surface plasmon resonances (SPR) in a D-shaped photonic crystal fiber (PCF) whose flat surface is covered by three adjacent gold layers of different thicknesses. Using computational simulations, we show how to customize plasmon resonances at different wavelengths, thus allowing for obtaining the second-order dispersion. The central aspect of this sensing configuration is to balance miniaturization with low coupling between the different localized plasmon modes in adjacent metallic nanostructures. The determination of the optical dispersion over a large spectral range provides information on the concentration of different constituents of a medium, which is of paramount importance when monitoring media with time-varying concentrations, such as fluidic media.

Abstract
Surface plasmon-polaritons are electromagnetic modes that can be excited at a conducting-dielec-tric interface [1]. The engineering of surface plasmon resonance (SPR) based devices is a milestone in the development of optical sensors. The ability to construct an all-optical system to confine lightwave power at subwavelength dimensions with higher levels of sensitivity and resolution in a broad spectral range are the central features that have attracted a rapid-growing interest in SPR sensors [2]. Particularly, minute variations in the refractive index of the surrounding medium (also known as analyte) change significantly the characteristics of the electromagnetic fields of a surface plasmon mode. As a consequence, the spectral shifts in the mode phase and also losses variations in the associated confined power can be used to detect analyte properties that are described in terms of the refractive index [3].

Abstract
Bi-core optical fiber structures are studied for applications in sensing. In this paper, an analysis is performed on the spectral characteristics of light propagating in these fibers with central launching core illumination from a standard single mode fiber. Reflective and transmissive configurations are addressed. The characteristics of a reflective bi-core fiber structure for measurement of strain, temperature and absolute value of torsion are investigated and highlights for further research are presented.