Fiber Bragg Grating (FBG) sensors facilitate compact, multiplexed, and electromagnetic interference-immune monitoring in embedded and harsh environments. The removal of the polymer jacket, a measure taken to withstand elevated temperatures or facilitate integration, exposes the fragile glass. This underscores the necessity of functional coatings, which are critical for enhancing durability, calibrating sensitivity, and improving compatibility with host materials. This review methodically compares coating materials and deposition routes for FBGs, encompassing a range of techniques including top-down physical-vapor deposition (sputtering, thermal/e-beam evaporation, cathodic arc), bottom-up chemical vapor deposition (CVD)/atomic layer deposition (ALD), wet-chemical methods (sensitization/activation, electroless plating (EL), electrodeposition (ED)), fusion-based processes (casting and melt coating), and hybrid stacks (e.g., physical vapor deposition (PVD) seed → electrodeposition; gradient interlayers). The consolidation of surface-preparation best practices and quantitative trends reveals a comprehensive understanding of the interrelationships between coating material/stack, thickness/microstructure, adhesion, and sensitivity across a range of temperatures, extending from approximately 300 K to cryogenic regimes. Practical process windows and design rules are distilled to guide method selection and reliable operation across cryogenic and high-temperature regimes.

Operational monitoring of helicopter landing gear can benefit from direct strain measurements able to resolve transient contact and roll-out dynamics. This work presents a field demonstration of a fiber Bragg grating (FBG) measurement and processing workflow applied to the main landing gear of an Erickson S-64 Skycrane operated by the Italian National Fire Brigade. Four gratings were bonded to the landing-gear structure, and wavelength data were acquired at 1 kHz during operational activities. The study does not aim to perform damage detection, health-state assessment, or hard-landing classification; rather, it evaluates whether field-acquired FBG signals affected by realistic acquisition imperfections can be transformed into interpretable time–frequency descriptors. Raw wavelength records were screened for dropouts, saturation artifacts, and spikes, then converted to normalized strain and high-pass filtered. Event-centered analyses based on Hann-window short-time Fourier transform (STFT), Thomson multitaper estimates, and ridge tracking revealed recurrent transient signatures and sensor-dependent frequency families in two representative datasets. The results support FBG-derived landing-event characterization under field conditions, while showing that synchronized avionics references, temperature compensation, and larger repeated datasets are required before validated Health and Usage Monitoring System (HUMS) or structural-integrity decisions can be made.

Adaptive systems modify their configuration in response to the environment, an idea informed by biomimicry. This study reports early-stage development of metal-coated fiber Bragg grating (FBG) sensors for embedding in adaptive structures. FBGs offer a small size, spectral sensitivity, and operation from cryogenic to high temperatures, but bare silica fibers become brittle after polymer removal. To improve robustness and compatibility, we investigated electroless deposition of Cu, Ni, and Ag on 9/125 single-mode fiber. The workflow comprised coating removal, gold sputtering, sensitization/activation, optional polyimide etching, and metal deposition. Copper required Sn/Pd activation to achieve uniform coverage; nickel deposited without activation but produced islanded, weakly adherent films. Tollens’ chemistry plated silver on bare silica and acrylate; etched polyimide supported deposition only within a narrow window (∼2 M KOH, 20 min, 60°C) and remained morphologically non-uniform. Attempts to form an Ag/Ni bilayer failed because the Tollens’-derived Ag underlayer thinned in the alkaline electroless nickel bath, preventing coalescence of a continuous Ni film. This outcome is consistent with Ag–Ni immiscibility, poor catalytic initiation on unactivated Ag, and the chemical/mechanical fragility of thin Ag in ammoniacal media. The results delineate viable metallization routes for resilient, embedded optical sensors.