Abstract
The optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry-Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.

Abstract
In this work, 3D printing is explored as a solution for fast prototyping of optical fiber sensors with applications in power transformers. Two different sensing structures were evaluated using finite element method (FEM) analysis and were fabricated using 3D printing. The printed structures are composed by acrylonitrile butadiene styrene (ABS), a common thermoplastic polymer used in 3D printing. Attaching a fiber Bragg grating (FBG) to each structure, frequency measurements were successfully obtained for values between 20 and 250 Hz.

Abstract
Power transformers are at the core of power transmission systems. The occurrence of system failure in power transformers can lead to damage of adjacent equipment and cause service disruptions. Structural and electrical integrity assessment in real time is of utter importance. Conventional techniques, typically electrical sensors or chemical analysis, present major drawbacks for real-time measurements due to high electromagnetic interference or for being time-consuming. Optical fiber sensors can be used in power transformers, as they are compact and immune to electromagnetic interferences. In this work, an optical fiber sensor composed by 2 fiber Bragg gratings, attached in a cantilever structure was explored. The prototype was developed with a 3D printer using a typical filament (ABS) that enable a fast and low-cost prototyping. The response of the sensor to vibration was tested using two different vibration axes for frequencies between 10 and 500 Hz. Oil compatibility was also studied using thermal aging and electrical tests. The studies shown that ABS is compatible with the power transformer mineral oil, but the high working temperatures may lead to material creeping, resulting in permanent structural deformation.

Abstract
We present the design, fabrication and optical characterization of functional metamaterials for optical sensing of Hydrogen based on inexpensive self-assembly processes of metallic nanowires integrated in nanoporous alumina templates([37-42]). The optical properties of these materials strongly depend on the environmental concentration or partial pressure of hydrogen and can be used to develop fully optical sensors that reduce the danger of explosion. Optical metamaterials are artificial media, usually combining metallic and dielectric sub-wavelength structures, that exhibit optical properties that cannot be found in naturally occurring materials. Among these, functional metamaterials offer the added possibility of altering or controlling these properties externally after fabrication, in our case by contact with a hydrogen rich atmosphere. This dependency can be used to design([43-45]) and develop optical sensors that respond to this gas or to chemical compounds that contain or release hydrogen. In this paper we present some designs for hydrogen functional metamaterials and discuss the main parameters relevant in the optimization of their response.

Abstract

Abstract
The use of graphene oxide-based coatings on optical fibers are investigated, aiming to tune the reflectivity of optical fiber surfaces for use in precision sensing devices. Graphene oxide (GO) layers are successfully deposited onto optical fiber ends, either in cleaved or hollow microspheres, by mounting combined bilayers of polyethylenimine (PEI) and GO layers using the Layer-by-Layer (LbL) technique. The reflectivity of optical fibers coated with graphene oxide layers is investigated for the telecom region allowing to both monitor layer growth kinetics and cavity characterization. Tunable reflective surfaces are successfully attained in both cleaved optical fibers and hollow microsphere fiber-based sensors by simply coating them with PEI/GO layers through the LbL film technique.