Regeneration of Fiber Bragg Gratings and Their Responses Under X-Rays

Fiber Bragg gratings (FBGs)-based temperature sensors present numerous advantages such as small packaging, fast acquisition rate, and accuracy for structural health-monitoring applications in nuclear environments. Among the various classes of FBGs, Type I FBGs are inscribed with a UV continuous or pulsed laser on a photosensitive fiber or with femtosecond laser even in nonphotosensitive fibers. These gratings, however, cannot survive at temperatures exceeding 400 °C. Regenerated FBG (RFBG) gratings, instead, are derivative from type I grating: when a high thermal treatment (> 650 °C) is applied after the inscription on a prehydrogenated fiber, a new grating (RFBG) appears with different thermal properties. It withstands temperatures as high as 1000 °C, opening the way to new application fields such as the temperature monitoring of the nuclear reactor cores. This work investigates the radiation response of RFBGs originated from type I seed FBGs inscribed with an argon laser (244 nm) in two different fibers (SMF-28e fiber or in a B/Ge co-doped fiber), loaded with either H ₂ or D ₂ before the inscription to enhance the fiber photosensitivity. Regeneration was achieved at 650 °C or 900 °C depending on the fiber type. After this, the RFBGs were irradiated under 40-keV X-rays at two different dose rates [1 Gy(SiO ₂ )/s or 10 Gy/s] and at two temperatures of irradiation-25 °C or 250 °C. At room-temperature (RT) irradiation, a Bragg Wavelength Shift (BWS) of about 35 pm was observed for the SMF-28e and more than 130 pm for the B/Ge fiber at 400-kGy dose, and the combined temperature and radiation constraints reveal that the RFBGs are radiation-tolerant. Indeed, no BWS is observed anymore, at the highest temperature of irradiation.