Bragg wavelength monitoring during temperature protocol (WP3, LNE-Cnam, january 2026)

Fiber Bragg Gratings (FBGs) inscribed in silica fibers using laser techniques are wavelength-multiplexed, electromagnetic-immune, small size and weight sensors. FBGs provide a power-independent spectral signature that may be locked to a reference gas cell to provide absolute wavelength measurements. An uncertainty in temperature measurement of 0.1 K is typically achieved with most commercial FBG interrogation units. Among all FBG types, type-III FBGs inscribed by the Point-by-Point (PbP, i.e. voids defects) technique using femtosecond (fs) lasers are among the most temperature stable. However, although ideally suited to high-temperature monitoring in harsh environments, FBGs sensors still exhibit poor long-term temperature stability at elevated temperatures on account on several mechanisms, ranked in terms of increasing temperature: stress relaxation (of frozen stress during fiber drawing), dopant diffusion and glass devitrification. Devitrification also has detrimental impact on fiber mechanical reliability as the fiber becomes fragile at elevated temperatures (beyond 900°C). All of these phenomena lead to Bragg Wavelength Shifts (BWSs) and result in typical drifts of several picometers per hour (corresponding to ~0.1 K/h) at temperatures above 900°C. This work summarizes the results obtained with regenerated (type-R) gratings and type-III gratings within WP3 of the INFOTherm Project that targets long-term monitoring in the range [25°C-700°C]. A focus is made on three kinds of Single Mode Fibers (SMFs): conventional telecom-grade SMF-28e (Corning), SM1500 (FiberCore) and radiation-hardened fluorine (F)-doped/Pure Silica Core (PSC) fibers (Exail).