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
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
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.