The employability of photonics technology in the modern era’s highly demanding and sophisticated domain of aerospace and submarines has been an appealing challenge for the scientific communities. In this paper, we review our main results achieved so far on the use of optical fiber sensors for safety and security in innovative aerospace and submarine applications. In particular, recent results of in-field applications of optical fiber sensors in aircraft monitoring, from a weight and balance analysis to vehicle Structural Health Monitoring (SHM) and Landing Gear (LG) monitoring, are presented and discussed. Moreover, underwater fiber-optic hydrophones are presented from the design to marine application.

In order to complete this set of three companion papers, in this last, we focus our attention on environmental monitoring by taking advantage of photonic technologies. After reporting on some configurations useful for high precision agriculture, we explore the problems connected with soil water content measurement and landslide early warning. Then, we concentrate on a new generation of seismic sensors useful in both terrestrial and under water contests. Finally, we discuss a number of optical fiber sensors for use in radiation environments.

The use of in-fiber core-to-cladding coupling components for thermal heating purposes has been well assessed in the last decades within the development of fiber optic devices for flow measurements and water thermal conductivity calculation. In these devices, light travelling in the fiber core is transferred into the cladding and absorbed by a metallic layer surrounding the fiber, with the consequent resistive heating generation. Here we demonstrate the use of a Turn-Around-Point (TAP) Long Period Grating (LPG) as resonant core-to-cladding light coupling mechanism for the fabrication of a highly efficient heating device based on metallic coated Fiber Bragg Grating (FBG). A properly designed TAP LPG was fabricated by means of point-to-point UV laser and spliced to a 150 nm thick Au-coated FBG. The heating efficiency characterization of the final device was analyzed, in both air and water, by evaluating the temperature increase in the gold layer surrounding the FBG at incremental values of the injected power. Collected results confirm that the use of LPGs involving the excitation of higher order cladding modes provides an excellent transferring mechanism of the fiber core light into the cladding, which in turn guarantees very high thermal heating efficiency to the final device. Moreover, by comparing such results with the performance of other in-fiber core-to-cladding coupling components already presented in literature, it was found that the TAP LPG-based device exhibits an actuation efficiency 2.5 times greater, thus resulting the most effective and highly performing solution for energy transfer to the metallic overcoat.

Our group, involving researchers from different universities in Campania, Italy, has been working for the last twenty years in the field of photonic sensors for safety and security in healthcare, industrial and environment applications. This is the first in a series of three companion papers. In this paper, we introduce the main concepts of the technologies employed for the realization of our photonic sensors. Then, we review our main results concerning the innovative applications for infrastructural and transportation monitoring.

The determination of the amount of moisture in soil is crucial in several application fields such as agricultural, geotechnical, hydrological, and environmental engineering. In this paper, we propose the use of an advanced all-optical self-heated sensing platform for highly sensitive and repeatable monitoring of the soil volumetric water content (VWC). The proposed platform is realized by integrating, inside a standard metallic needle, a fiber optic heating device integrated with a fiber Bragg grating sensor (FBG) acting as thermal monitor. The all optical active heating device consists of two main components: i) a core offset fusion splice between two single mode optical fibers to spread light from the core to the cladding and ii) a metal-coated region of the fiber where the FBG is inscribed. A full characterization of the proposed device has been performed for different VWC values from 9 to 36 %VWC in the soil temperature range between 5 and 40 °C. Collected results demonstrate that the proposed platform is able to perform highly stable, fast, robust and energy efficient measurements of the soil water content, with a repeatability error of 1.5 %VWC, time response ≤ 10 s and resolution ≤ 0.25 %VWC. The possibility of tuning the value of the injected power according to the application makes the proposed sensor extremely flexible and versatile for its exploitation in large areas and/or long-distance monitoring applications with relevant advantages in terms of energy savings, costs and system complexity.

Monitoring in radiation-intensive environments poses major challenges for conventional sensing technologies, which often suffer from performance degradation, high maintenance demands and limited long-term stability. In this context, optical fiber (OF) sensors represent a promising alternative. However, their deployment in such applications requires a systematic qualification of their radiation response under realistic operating conditions. In this work we report on results collected during the first neutron irradiation campaign of both bare and titanium dioxide-coated UV-written Long Period Gratings (LPGs), as well as Fiber Bragg Gratings (FBGs) inscribed by UV and femtosecond laser techniques, in photosensitive B-Ge doped fibers. The LPGs were exposed to a cumulative neutron fluence of ∼3.4 × 10¹⁵ n/cm², while the FBGs to a higher fluence of ∼8.6 × 10¹⁵ n/cm². The spectral response of the investigated samples under neutron irradiation was analyzed with respect to wavelength shifts and visibility variations. Collected results clearly demonstrate the robustness of OF devices in neutron environments and highlight the potential of nanoscale coatings to design radiation-hardened optical sensors. Moreover, such advances enable strategies for the development of enhanced OF sensing platforms for environmental monitoring in nuclear and high-energy physics applications.