Abstract
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Abstract
The present study investigates the impact of a virtual Vernier effect in the fundamental state and first harmonic to enhance the sensitivity of a strain sensor. A fibre loop mirror (FLM) combined with an internal elliptical cladding (IEC) fibre section was used as the sensor, while a virtual reference spectrum derived from theoretical equations enabled the Vernier effect. For the individual sensor, a sensitivity of 15.39 ± 0.03 pmµe?¹ and a free spectral range (FSR) of 4.22 ± 0.01 nm are obtained. For the virtual Vernier effect, a detuning of 0.15 m is used in both states, resulting in an FSR of 30.2 ± 0.1 nm. A sensitivity of 109.8 ± 0.7 pmµe?¹ is achieved for the fundamental state, associated with a figure of merit (FoM) of 1.01 ± 0.03, and a sensitivity of 230 ± 2 pmµe?¹ for the first harmonic, associated with a figure of merit (FoM) of 2.1 ± 0.1. This work demonstrates the feasibility of implementing the virtual Vernier effect, not only enabling Vernier effect amplification but also reducing implementation complexity and increasing system robustness under harsh conditions.

Abstract
Regional sea-level change in the semi-enclosed Baltic Sea is strongly influenced by atmospheric forcing and wind-driven redistribution of water masses, leading to significant spatial variability in absolute sea level trends across the different sub-basins. This study focusses on absolute sea level trends in the Baltic Sea using satellite gridded sea level anomalies (0.0625º) from the European Seas Gridded L4 product provided by the E.U. Copernicus Marine Service (https://doi.org/10.48670/moi-00141). The daily time series (from January 1993 to the end of December 2023) are first deseasoned by removing the average annual cycle at each point. Then robust linear trends are estimated at each grid point by computing median slopes. In contrast to ordinary least-squares slopes characterising linear trends in the mean, these median slopes are calculated by minimising the mean absolute deviation of a linear trend model from the observations instead of the mean quadratic deviation, which makes them more robust to outliers and sensitive to the typical tendency of changes rather than to large deviations. Uncertainty is computed assuming non-independence by the Huber sandwich robust estimator for the covariance matrix.The derived median slopes are in general higher than ordinary linear trends in the mean, except in the northern and easternmost areas of the Baltic. In the Bay of Bothnia ordinary linear trends and median trends are very similar, while in the eastern end of the Gulf of Finland median trends are similar or even slightly lower than ordinary linear trends. In the remaining areas, median trends are significantly larger than ordinary linear trends, the largest difference occurring in the Bothnian Sea. Coastal areas exhibit trends that differ from those in the adjacent basins. In the Gulf of Finland, median trends are higher than ordinary linear trends along the Finnish coast, whereas along the Roslagen coast (northern Stockholm Archipelago) the two slope estimates are in good agreement. Along the southern coastline of the Bothnian Sea, median sea-level trends reach the highest values, exceeding 6 mm/year.The present study is financed within the scope of the Recovery and Resilience Mechanism (MRR) of the European Union (EU), framed in the Next Generation EU, for the period 2021 - 2026, within project NewSpacePortugal, with reference 11.

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.

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Abstract
This study experimentally investigates the impact of EDFA pump power on the State of Polarization (SOP) in optical fiber systems at 1550 nm, with particular relevance for distributed sensing applications. Using a tunable laser and polarimeter, three power levels were tested:-18 dBm,-20 dBm, and-23 dBm. Results show that polarization stability is strongly affected by power: while-18 dBm and-20 dBm provided repeatable SOP and phase behavior,-23 dBm caused significant phase shifts and Stokes parameter drift. Furthermore, for EDFA output powers greater than 0 dBm, the polarization state exhibits a strong dependence on optical power. Despite these effects, Polarization Dependent Gain (PDG) remained low (~ 0.46 dB), confirming the EDFA meets commercial specifications. The study highlights a trade-off where lower input powers, though avoiding saturation, can worsen polarization instability in short fiber systems, which is critical for optical communication and distributed sensing design.