Abstract
The optical Vernier effect consists of overlapping responses of a sensing and a reference interferometer with slightly shifted interferometric frequencies. The beating modulation thus generated presents high magnified sensitivity and resolution compared to the sensing interferometer, if the two interferometers are slightly out of tune with each other. However, the outcome of such a condition is a large beating modulation, immeasurable by conventional detection systems due to practical limitations of the usable spectral range. We propose a method to surpass this limitation by using a few-mode sensing interferometer instead of a single-mode one. The overlap response of the different modes produces a measurable envelope, whilst preserving an extremely high magnification factor, an order of magnification higher than current state-of-the-art performances. Furthermore, we demonstrate the application of that method in the development of a giant sensitivity fibre refractometer with a sensitivity of around 500 mu m/RIU (refractive index unit) and with a magnification factor over 850.

Abstract
Based on the characteristics of ferrofluids, a monolithic optofluidic device for magnetic field sensing is proposed and demonstrated. The device consists of a Fabry-Perot interferometer, composed by an optical waveguide orthogonal to a microfluidic channel, which was fabricated inside a fused silica substrate through femtosecond laser micromachining. The interferometer was first optimized by studying the influence of the waveguide writing parameters on its spectral properties. Waveguides written at higher pulse energies led to a decrease of the signal-to-noise ratio, due to an enhancement of micrometer sized defects associated with Mie scattering. Fringe visibility was also maximized for waveguides written at lower scanning speeds. Making use of the tunable refractive index property exhibited by magnetic fluids, the interferometer was then tested as a magnetic field sensor by injecting a ferrofluid inside the microfluidic channel. A linear sensitivity of -0.25 nm/mT was obtained in the 9.0-30.5 mT range with the external field parallel to the waveguide axis.

Abstract
The use of optical sensors inside the needle can improve targeting precision and can bring real-time information about the location of the needle tip if necessary, since a needle bends through insertion into the tissue. Therefore, the precise location of the needle tip is so important in percutaneous treatments. In the current experiment, a fiber sensor based on a Fabry-Perot (FP) cavity is described to measure the needle curvature. The sensor is fabricated by producing an air bubble between two sections of multimode fiber. The needle with the sensor therein was attached at one end and deformed by the application of movements. The sensor presents a sensitivity of -0.152 dB/m(-1) to the curvature measurements, with a resolution of 0.089 m(-1). The sensory structure revealed to be stable, obtaining a cross-sensitivity to be 0.03 m(-1)/degrees C.

Abstract
The proposed technique demonstrates a fiber ring resonator interrogated by an optical time domain reflectometer (OTDR), for intensity sensing. By using this methodology, a cavity round trip time of 2.85 mu s was obtained. For a proof of concept, a long-period grating was inserted in the resonant cavity operating as a curvature sensing device. A novel signal processing approach was outlined, regarding to the logarithmic behavior of the OTDR. Through analyzing the experimental results, an increase in the measured sensitivities was obtained by increasing applied bending. With curvatures performed from 1.8 m(-1) to 4.5 m(-1), the sensitivity values ranged from 2.94 dB center dot km(-1) to 5.15 dB center dot km(-1). In its turn, the sensitivities obtained presented a linear behavior when studied as a function of the applied curvature, following a slope of 0.86x10(-3) dB. The advantages of applying this technique were also discussed, demonstrating two similar fiber rings multiplexed in a series of configurations.

Abstract
The fabrication of Turn Around Point Long Period Fiber Grating in standard SMF-28e fibers through femtosecond laser direct writing is demonstrated and its sensing sensitivity is improved by coating the fiber with titanium dioxide through physical vapor deposition. © 2021 The Author(s).

Abstract
A fiber sensor composed by a graphene oxide membrane at the tip of a capillary is presented. The graphene oxide membrane acts as a low-reflectivity mirror, distanced from a single mode fiber forming a low finesse Fabry-Perot interferometer. The response of the sensor to acoustic pressure with varying frequency is studied in the range between 5 and 45 kHz, attaining a minimum signal to noise ratio of 14 dB. © 2021 The Author(s).