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 report on the experimental demonstration of an optical-fiber-integrated, nonvolatile transmission switching device. The operating mechanism exploits a cavity resonance spectral shift associated with an induced change in the refractive index of a high-index thin film on the polished side facet of the fiber. In the present case, a thermally induced amorphous-crystalline structural transition in a 500 nm layer of germanium antimony telluride at a distance of 500 nm from the core-cladding interface of an SMF-28 single-mode fiber delivers resonant transmission contrast >0.5 dB/mm at 1315 nm. Contrast is a function of active layer proximity to the core, while operating wavelength is determined by layer thickness-varying thickness by a few tens of nanometers can provide for tuning over the entire near-infrared telecoms spectral range. (C) 2019 Author(s).

Abstract
In this study, an optical signal recording method for optogenetics stimulation of ChR2 channels expressed in human pulp dental (HPD) cells by using a fiber optic refractive index (RI) sensor based on all fiber Mach-Zehnder interferometer was proposed. All-fiber Mach-Zender interferometric biosensor is composed of a specially fabricated twin-core fiber spliced between two pieces of a single-mode fiber which one of the cores was doped with germanium and the other with phosphorous [1]. The interference pattern in the fiber Mach-Zehnder interferometer is occurred by coupling of the propagation lights of both fiber cores. For coupling the light into both cores, a short length of a coreless fiber optic was used. The length of twin-core fiber was 40 cm. Here, one core of the fiber acts as a reference arm and the other cores as sensing arm. For increasing evanescent wave around the sensing arm of the fiber optic biosensor, a short section of the cladding of the twin-core fiber about 2 cm was etched with HF solution. For this propose, after determining the direction of the cores so that the two cores were in the vertical direction, one side of the twin-core fiber was fixed on Plexiglas substrate by using UV glow and the upper side of the sensor was etched. The thickness of remained clad around the upper core was about 1 micrometer. In the experimental setup as is shown in Fig. 1(a), light from an SLD at 1550 nm after passing an isolator arrived at the sensor and output spectrum was monitored with an optical spectrum analyzer which has 10 pm wavelength resolution. The best RI sensitivity of the sensor in the range of 1.39 to 1.43 was obtained to be 675.74 nm/RIU. For detecting of cell signal by using optogenetic stimulation which ChR2 opsin was expressed on HPD cells, it needs that high concentrations of cells were immobilized to the etched fiber surface by PLL biopolymer. Optogenetic stimulation of ChR2 channel was done using a 470 nm laser diode [2] pulse with a frequency of 15 Hz, a number of pulses 120, duty cycle 50 in 60 seconds, and 300 second rest time. As a result of optogenetic stimulation and activation of light-sensitive ion channels, effective RI around the fiber optic biosensor changes [3]. Obtained results were shown in Fig. 1(b). Changes in the RI lead to a wavelength shift of the sensor spectrum. © 2019 IEEE.

Abstract
Optical waveguides directly written in fused silica using a femtosecond laser were characterized from 350 to 1750 nm to gain insight on the waveguide's loss mechanisms and their dependence on processing parameters, such as pulse energy, scan velocity, and annealing temperature. Two major loss mechanisms were identified. In the range of parameters tested, high pulse energy was seen to improve coupling losses at long wavelengths, while high scan velocity has a negative effect in both Rayleigh scattering and coupling losses at long wavelengths. Thermal annealing of the waveguides demonstrated an improvement of the Rayleigh scattering at a cost of higher coupling losses at long wavelengths. Wavelength independent Mie scattering was also observed, evolving negatively with pulse energy. A minimum Rayleigh scattering coefficient of approximate to 0.5 dB.cm(-1).mu m(4) (approximate to 0.08 dB.cm(-1).mu m(4) for thermally treated waveguides) together with a Mie scattering coefficient of approximate to 0.2-0.65 dB/cm are reported.

Abstract
A highly sensitive D-shaped optical fiber refractive index sensor based on surface plasmon resonance is designed and analyzed by using numerical simulations based on the finite element method. The flat surface of the fiber is coated with a gold layer that works as the plasmon active metal, followed by a titanium oxide (TiO2) layer, which is employed to enhance the performance of the sensor. The results demonstrate that the proposed sensor's properties highly depend on the metal and dielectric coating's thickness, enabling the tuning of the resonance wavelength. By supposing the system noise to be 0.1 nm, the theoretical maximum sensitivity was found to be 30000 nm/RIU, with a resolution of 3.33x10(-6) RIU and a figure of merit (FOM) of 312.46 RIU-1, for an analyte with a refractive index of 1.41. The sensor's sensitivity and FOM is improved upon the state of the art, possibly opening new windows of study in the fields of biological and chemical sensing.

Abstract
In this chapter the developments made in femtosecond laser micromachining for applications in the fields of optofluidics and lab-on-a-chip devices are reviewed. This technology can be applied to a wide range of materials (glasses, crystals, polymers) and relies on a non-linear absorption process that leads to a permanent alteration of the material structure. This modification can induce, for instance, a smooth variation of the refractive index or generate etching selectivity, which can be used to form integrated optical circuits and microfluidic systems, respectively. Unlike conventional techniques, fs-laser micromachining offers a way to produce high-resolution three-dimensional components and integrate them in a monolithic approach. Recent advances made in two-photon polymerization have also enabled combination of polymeric structures with microfluidic channels, which can provide additional functionalities, such as fluid transport control. In particular, here it is emphasised the integration ofmicrofluidic systems with optical layers and polymeric structures for the fabrication of miniaturized hybrid devices for chemical synthesis and biosensing.