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 degrees C. Regenerated FBG (RFBG) gratings, instead, are derivative from type I grating: when a high thermal treatment (> 650 degrees 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 degrees 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 degrees C or 900 degrees C depending on the fiber type. After this, the RFBGs were irradiated under 40-keV X-rays at two different dose rates [1 Gy(SiO2)/s or 10 Gy/s] and at two temperatures of irradiation-25 degrees C or 250 degrees 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.