Abstract
Hollow microsphere fiber sensors are Fabry-Perot interferometers ( FPI) that can be used for lateral loading, temperature, and refractive index sensing. In this work, graphene oxide (GO) is explored as a tunable platform for enhancing the spectral properties of hollow microsphere fiber sensors. GO offers similar mechanical and optical properties as graphene, with the advantage of a wider range of deposition methods and a lower cost. The influence of multilayer coatings of polyethylenimine (PEI) and GO, achieved with the layer-by-layer technique, on the reflectivity of the outer surface, and hence, on the spectrum of the FPI for maximum of 30 bilayers was studied. The obtained results revealed a change of the microsphere outer surface reflectivity and also of visibility of the reflected spectrum when varying the number of bilayers. A maximum signal amplitude of 3.9 dB was attained for the 13th bilayer, allowing to conclude that PEI/GO multilayer coatings can be used for enhancing desired properties of the three-wave FPI for different sensing applications.

Abstract
This work demonstrates the potential of combining a microsphere with a tip for the functionality of the contact sensor. This sensor consists of a tip aligned with the fiber core and a microsphere, which appears during tip formation. This new structure was produced using the electric arc machine. The sensor operation consists of the variation of the tip curvature, which causes a variation of the optical paths and, consequently, a change in the output signal. The study of this micro-cantilever consisted of an exploration of the contact mode. In addition, the sensor was characterized by temperature, which shows very low sensitivity and vibration. This last characterization was performed with two configurations parallel and perpendicular to the oscillating surface. The perpendicular case showed higher sensitivity and has an operating band of 0 Hz to 20 kHz. In this configuration, for frequencies up to 2 Hz, the intensity varies linearly with the frequencies and with a sensitivity of 0.032 +/- 0.001 (Hz(-1)). For the parallel case, the operating band was from 1.5 kHz to 7 kHz.

Abstract
Viscosity measurements of a solution are crucial for many processes involving fluid flows. The current optical fiber viscometers are complex and, in some cases, provide indirect measurements of viscosity through other non-optical effects. We developed a miniaturized optical fiber probe capable of providing an optical interferometric measurement of the viscosity of small volumes of a liquid viscous medium (less than 50 pL). The probe consists of an air cavity with a small access hole for fluids, which resulted from a simple post-processing of a hollow capillary tube. The structure behaves as a two-wave interferometer, where the intensity of the signal is sensible to the position of the air-fluid interface inside the cavity. The fluid displacement over time is obtained by monitoring the signal intensity variations, at 1550 nm, during the process of removing the sensing head from a fluid solution. Multiple sucrose solutions with viscosities ranging from 2.01 to 16.1 mPa.s were used for calibration. The viscosity of the solution is measured through the fluid evacuation velocity in the first 300 ms of resolved oscillations during the evacuation process. Reproducibility measurements, the influence of temperature, and the access hole dimensions are also addressed. The application to biological fluids is important to be considered in future studies.

Abstract
Sensing at small dimensions in biological and medical environments requires miniaturized sensors with high sensitivity and measurement resolution. In this work a small optical fiber probe was developed to apply the Vernier effect, allowing for enhanced temperature sensing. Such effect is an effective way of magnifying the sensitivity of a sensor or measurement system in order to reach higher resolutions. The device is a multimode silica Fabry-Perot interferometer structured at the edge of a tapered multimode fiber by focused ion beam milling. The Vernier effect is generated from the interference between different modes in the Fabry-Perot interferometer. The sensor was characterized in temperature, achieving a sensitivity of -654 pm/degrees C in a temperature range from 30 degrees C to 120 degrees C. The Vernier effect provided a temperature sensitivity over 60-fold higher than the sensitivity of a normal silica Fabry-Perot interferometer without the effect. The temperature resolution obtained was 0.14 degrees C, however this value was limited by the resolution of the OSA and can be improved further to less than 0.015 degrees C.

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.