Radiation Effects on Type I Fiber Bragg Gratings: Influence of Recoating and Irradiation Conditions

Fiber Bragg gratings (FBGs) present strong advantages for temperature or strain sensing in harsh radiation environments even if their properties are affected by radiations. The amplitudes and kinetics of these radiation induced changes depend on numerous parameters, intrinsic or extrinsic to the FBGs themselves. In this paper, we characterized 40 keV X-ray radiation effects on type I FBGs inscribed in prehydrogenated SMF-28 from Corning through an ultraviolet laser exposure at 244 nm (cw). We performed a systematic study of the influence of several FBG manufacturing parameters on their radiation response up to 100 kGy (SiO2) highlighting radiation-induced Bragg wavelength shifts (RI-BWS) up to 130 pm (similar to 13 degrees C error for temperature measurements) but no decrease of those FBG reflectivity. Among the investigated parameters are the duration and temperature (100 degrees C and 300 degrees C) of the thermal treatments applied post-inscription to stabilize the FBG and to complete the H-2 outgassing. For such type of FBG, the device has to be recoated after inscription; we then characterize the impact of this manufacturing step on the FBG response showing that its recoating with NOA-81 acrylate slightly degrades its radiation resistance. In addition to this study, the influence of two other parameters have also been characterized: RI-BWS increases with the dose rate in the range 1-50 Gy/s and a pre-irradiation at 1.5 MGy does not stabilize type I FBG response to a second irradiation.