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

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).

Abstract
We discuss the novel concept of harmonics of the Vernier effect for optical fiber sensors as a tool to break the limits of conventional optical Vernier effect currently used. The new effect provides enhancements scalable with the harmonic order. © 2021 The Author(s).

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
Near-surface optical waveguides were fabricated in alkaline earth boro-aluminosilicate glass (Eagle2000), by femtosecond laser direct writing, using two distinct approaches. First, the capability of directly inscribing optical waveguides close to the surface was tested, and then, compared to the adoption of post writing wet etching to bring to the surface waveguides inscribed at greater depths. Laser ablation was found to limit the minimum surface to core center distance to 6.5 mu m in the first method, with anisotropic wet etching limiting the latter to 3 mu m without any surface deformation; smaller separations can be achieved at the cost of the planar surface topography. Furthermore, the waveguide's cross-section was seen to vary for laser inscription nearing the surface, observations that were also corroborated by its distinct guiding characteristics when compared to the adoption of post writing wet etching. The spectral analysis (in the 500-1700 nm range) also evidenced an increase in insertion loss for longer wavelengths and smaller surface to core center separations, caused, most likely, by coupling loss due to the interaction between the propagating mode and the surface. Different lengths of waveguide exposed to the surface were also tested, revealing that scattering loss due to surface roughness is not an issue at the centimeter scale.