Abstract
This work analyzes the sensitivity of an optical system consisting of two fiber Fabry-Perot ( FP) interferometers and the apparent increase in sensitivity due to the harmonics of the Vernier effect. Two scenarios are examined: (1) when the larger FP cavity acts as the sensor, and (2) when the smaller FP cavity acts as the sensor. The computation analysis reveals that in the first scenario, higher-order spectral harmonics yield greater sensitivity for maxima and minima of the same order. In the second scenario, however, the sensitivity remains constant and does not depend on the harmonic order. Moreover, it is demonstrated that the sensitivity curve is identical for both scenarios, regardless of the harmonic order. This outcome occurs because the use of spectral harmonics simply reduces the free-spectral range in certain situations, bringing the extrema closer to the maximum sensitivity condition (i.e., Delta L = 0) and thereby increasing sensitivity. Consequently, if points on the envelope other than maxima or minima are used, the sensitivity achieved is the same for both scenarios.

Abstract
In this letter, we propose a method for utilizing the internal cavities of optical circulator devices-commonly referred to as parasitic cavities-as optical reference cavities. The method involves using an optical circulator operating at 1550 nm, illuminated by a light source at 1330 nm, thereby enhancing the amplitude of the interferometric signals generated by the internal optical cavities. The system was characterized by using both an Optical Spectrum Analyzer (OSA) and the Low-Coherence Interferometry (LCI) technique. Experimental results indicate that the Optical Path Difference (OPD) remains constant with varying aperture sizes, thereby confirming the feasibility of employing the optical circulator as a reference sensor. Finally, its performance as a reference sensor is demonstrated through its integration with an external cavity that functions as a displacement sensor.

Abstract
In this study, we demonstrate the measurement of electric power using an optical ground wire ( OPGW). The tests were conducted on an OPGW cable from a high-voltage transmission line in Sines, Portugal, operating at 400 kV. A buried fiber position, free of 50 Hz and 100 Hz frequency interference, was selected to confirm that the 50 Hz frequency is not due to mechanical perturbation or electronic noise. Additionally, two suspended fiber positions (at 2500 m and 8500 m), where these frequencies were clearly observed, were analyzed. This study also examined the positioning of poles and splice detection between cables.

Abstract
Azobenzenes are a class of compounds which allow the writing and erasure of linear birefringence along any desired direction, through their ability to photoisomerize. This property enables applications requiring polarization control, which, despite extensive exploration in the visible spectrum, have yet to be fully capitalized in the infrared region. This study aims to systematically characterize the creation and relaxation of induced linear birefringence dynamics in azopolymers thin films for the 1550 nm region. Maximum birefringence values as high as 6.02 x 10(-2) were attained during the recording phase with a 445 nm pump laser, that stabilized at 5.40 x 10(-2) during the relaxation phase, achieved for a 2.4 mu m sample. In addition, a maximum phase shift of Delta Phi = 0.54 pi stabilizing at Delta Phi = 0.50 pi, was observed for a 9.7 mu m sample with a 532 nm writing laser. Accordingly, this shows the promising potential of azopolymers for many applications.

Abstract
The fabrication of Mach-Zehnder and Fabry-Perot interferometers in SMF-28e fibers by femtosecond laser direct writing is demonstrated. The feasibility and effectiveness of this technique in fabricating high-sensitivity fiber optic interferometers is highlighted. TiO2 coated Mach-Zehnder interferometers exhibit improved refractive index sensitivity compared to uncoated interferometers, while the dual-cavity intrinsic Fabry-Perot interferometers shows enhanced spectral response and sensitivity for measurement of gas pressure.

Abstract
Optical resonant structures, such as circular disks and optical microbubble resonators (OMBRs), are crucial for highresolution chemical and biochemical sensing. Both can be integrated into microfluidic systems: resonant disks can be fabricated within microfluidic channels, while OMBRs use thin silica capillary walls to confine fluid samples in a hollowcore cavity. Optical modes are typically excited using tapered optical fibers, which offer efficiency but lack robustness for functional devices. This work presents two femtosecond laser-written waveguide designs for exciting whispering gallery modes (WGMs) in these resonant structures. For resonant disks, suspended waveguides are fabricated tangentially between the microfluidic channel walls. For OMBRs, integrated waveguides are written on fused silica substrates to excite resonant modes. Both configurations provide stable and robust optical sensing solutions. The OMBR platform achieved a sensitivity of 45 nm/RIU with a resolution of 4.4x10(-5) RIU, while monolithically integrated disks reached 80 nm/RIU with a resolution of 7.0x10(-4) RIU. In both cases, the Q-factor exceeded 10(4) across the measurement range. These results confirm that femtosecond laser-written waveguides can efficiently excite resonant modes, offering promising platforms for chemical and biochemical sensing applications.