Abstract
Distributed Acoustic Sensing (DAS) leverages the sensitivity of optical fibers to detect environmental vibrations. This study demonstrates the capability of DAS to identify and characterize the acoustic signatures of passing vessels, highlighting its potential to enhance maritime surveillance and monitoring. © 2025 Elsevier B.V., All rights reserved.

Abstract
Distributed acoustic sensing (DAS) is an instrumental approach that allows fiber optic cables to be turned into dense arrays of acoustic sensors. This technology, based on an optical time-domain reflectometer (OTDR) technique, is attractive in marine environments where instrumentation is difficult to implement. Its main applications lie in seismology, oceanography, and bioacoustics.Current technology limits the range of DAS to ca. 150 km making it very useful in the Azores, where seismic stations only exist on the Islands with a strong E-W alignment, as shown by Matias et al. (2021). The Azores have been suffering an increase in extreme wave conditions that affect navigation and coastal infrastructures. DAS can provide proxies for significant wave height, period, and surface currents on the shallow sections of the cable, complementing existing monitoring networks.The Azores region is part of the migration routes for fin and blue whales, which are known to produce acoustic signals during certain parts of the year. These vocalizations provide crucial data for Passive Acoustic Monitoring that can be used to support the establishment and update of mitigation measures addressing their preservation. DAS has already demonstrated its usefulness in detecting and tracking baleen whales using acoustic records.One issue that needs to be addressed in using DAS data is calibration. It is well demonstrated that strain or strain rate as measured by DAS can be converted to ground motion along the direction of the submarine cable section, if the apparent phase velocity is known. Similarity between DAS converted signals and co-located seismograms is well demonstrated but the absolute value is likely to vary with the cable coupling to the seafloor.This work reports on the recent operation of a DAS interrogator deployed at the Faial landing site to monitor the first 50 km of the telecommunication cable between Faial and Flores islands operated by Fibroglobal. The instrument used, HDAS developed by the IO-CSIC, recorded at 50 Hz for 11 months starting on the 15th of December 2023 with 10 m gauge length. For calibration purposes two 4C OBS were deployed close to the cable at ~10 km and ~30 km distance from the landing point. The OBSs were deployed in July 2024 and recovered in November 2024, providing 5 months of simultaneous recordings with the DAS.As expected, both earthquakes and whale vocalizations were identified on the DAS and OBS. We show that DAS can contribute to an improved localization of local offshore earthquake parameters due to its high density of sensors, particularly for the events occurring NW of Faial Island, with locations North of the cable. Clear landward and seaward oceanic waves are identified on the cable's shallow section. In all the applications the main question to address is the variable coupling of the cable to the seafloor in the Azores plateau of volcanic origin.This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), by the MODAS project 2022.02359.PTDC, and by EC project SUBMERSE project HORIZON-INFRA-2022-TECH-01-101095055.

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.

Abstract
This work studies the influence of an Erbium-Doped Fiber Amplifier (EDFA) on the phase variation of light in an optical fiber. To this end, the state of polarization (SOP) was measured as a function of optical power by adjusting the EDFA amplification, for two different laser output powers (2 dBm and 5 dBm). Results show that phase variation correlates with changes in optical power in both cases.

Abstract
Topological photonics, leveraging concepts from condensed matter physics, offers transformative potential in the design of robust optical systems. This study investigates the integration of topologically protected edge states into plasmonic nanostructures for enhanced optical sensing. We propose a toy model comprising two chains of metallic filaments forming a one-dimensional plasmonic crystal with diatomic-like unit cells, positioned on a waveguide. The system exhibits edge states localized at the boundaries and a central defect, supported by the Su-Schrieffer-Heeger (SSH) model. These edge states, characterized by significant electric field enhancement and topological robustness, are shown to overcome key limitations in traditional plasmonic sensors, including sensitivity to noise and fabrication inconsistencies. Through coupled mode theory, we demonstrate the potential for strong coupling between plasmonic and guided optical modes, offering pathways for improved interferometric sensing schemes. This work highlights the applicability of topological photonics in advancing optical sensors.