Orlando Frazão

Abstract
The decoupling of temperature and refractive index measurements was achieved by exploiting the properties of the asymmetric spectrum generated by Fano resonance, resulting from the interference between the Bragg reflection of the grating and the Fresnel reflection at the fiber tip. This spectral asymmetry enabled the implementation of a combined wavelength-based and intensity-based interrogation scheme. By separating the influence of each parameter in the spectral response, it was possible to measure the refractive index independently, without interference from temperature variations. A refractive index sensor with a minimum detectable change of delta = 1.2 & times; 10(-4) RIU was demonstrated. In addition to introducing a novel structure that leverages Fano resonance, the sensor was also applied as an evaporation rate sensor. The results demonstrate its potential for a wide range of applications, serving as a foundation for the development of future optical sensing technologies.

Abstract
This paper presents the conditions required for effective sensitivity amplification in the optical harmonic Vernier effect. Two distinct cases are analyzed: in the first, the sensor cavity is the shortest, while in the second, it is the longest. Based on the proposed theoretical model, supported by experimental results, it is concluded that, in the first case, the sensitivity associated with the spectral extremes increases with the order of the harmonic states. In contrast, in the second case, the sensitivity at the spectral extremes remains constant, regardless of the harmonic order. To evaluate the effectiveness of applying the optical Vernier effect and to differentiate between the two cases, a new formulation of the magnification factor (M-factor) is introduced. This leads to the definition of a novel figure of merit for the optical Vernier effect, denoted as (FoM(Vernier)). In Case 1, where harmonics are generated by increasing the reference cavity, the figure of merit assumes a value of (m + 1). In Case 2, where harmonics are generated by increasing the sensor cavity, the figure of merit remains constant at 1, regardless of the state order (whether fundamental or harmonic). This study also concludes that the observed increase in sensitivity is apparent rather than intrinsic, as the sensitivity curve produced by the optical Vernier effect mirrors that of a conventional interferometer.

Abstract
The present letter proposes the implementation of Vernier-effect harmonics through the virtualization of different reference cavities. A Fabry-Perot interferometer (FPI), actuated by a piezoelectric transducer (PZT), was employed as the sensing element. Subsequently, the sensitivity of the dynamic range was investigated for both the individual interferometer and the implementation of the Virtual Vernier effect. A sensitivity of (8 +/- 0.05)x10(-3) nm/nm was achieved for the single sensor measurement. Considering the implementation of the Vernier effect, the following sensitivities were obtained: (65.6 +/- 0.08)x10(-3) nm/nm for the fundamental, (132 +/- 1)x10-3 nm/nm for the first harmonic, and (192 +/- 1)x10(-3) nm/nm for the second harmonic. Furthermore, a maximum dynamic range of 11.25 mu m and a maximum resolution of 5 pm were achieved. This study highlights the advantages of simultaneously measuring both a single sensor cavity and a harmonic of the Virtual Vernier effect, in order to achieve large dynamic ranges along with high resolution.

Abstract
Fano resonances emerge from the coherent interference between a discrete resonant state and a continuum of propagating modes, giving rise to a characteristically asymmetric spectral line shape with heightened sensitivity to minute variations in the underlying physical parameters. This study provides a chronological perspective on the development and increasing implementation of Fano-type effects in fiber-optic technology. Their implementation in fiber Bragg gratings (FBGs) through numerical simulations of resonant Fano behavior. Specifically, a Fano-like response in an FBG can be designed by introducing a tailored phase shift into the grating structure; the magnitude of this phase discontinuity constitutes an effective control parameter for tuning the interference condition and, consequently, the resulting spectral asymmetry. The resonance produces a distinctive, asymmetrical spectral response that is attractive for a broad range of photonic functionalities. Key applications include fiber-integrated sensing - where the enhanced spectral sensitivity supports the detection of changes in refractive index, temperature, pressure, or biomolecular interactions - as well as narrowband filtering and spectral shaping for telecommunications and optical signal-processing systems.

Abstract
This work presents an experimental framework for offshore seismic monitoring that combines Distributed Acoustic Sensing (DAS) with ocean-bottom seismometers (OBS). The study was conducted in the Azores region - Faial, where an HDAS interrogator prototype was connected to dark fiber submarine fiber-optic cable, complemented by the installation of two Ocean Bottom Seismometers (OBS) for calibration and validation of DAS technology. The main objective is to demonstrate that seismic observations obtained by DAS from seafloor cables can provide essential information similar to OBS and particularly in areas where land-based monitoring stations are limited.