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Fiber Bragg grating (FBG) sensors are expected to provide valuable data in extreme radiation environments associated with nuclear research reactors. However, when the fast neutron fluence reaches 1018–1019 n/cm2, the radiation-induced changes in the material density and refractive index may drastically bias the measurements. This paper evaluates the radiation effect on the FBG performances by comparing their properties before and after their exposure to fast neutron fluences exceeding 1019 n/cm2 ($E > 1$ MeV). We studied the responses of FBGs manufactured by three different laboratories in the same single-mode optical fiber but using different inscription conditions. The Bragg wavelength and the reflectivity were measured before and after irradiation thanks to a dedicated mounting. For nearly all FBGs, the Bragg peak remains visible after the irradiation while the radiation-induced Bragg wavelength shifts (RI-BWSs) vary from a few picometers (equivalent temperature error <1 °C) to nearly 1 nm (~100 °C error) depending of the FBG inscription conditions. Such high RI-BWSs can be explained by the huge refractive-index variation and compaction observed for bare fiber samples through other experimental techniques. Our results show that by using specific hardening techniques, the FBG-based temperature measurements in a nuclear research reactor experiment may become feasible. [ABSTRACT FROM AUTHOR]

We have investigated radiation induced absorption in Al-doped, P-doped and Ge-doped optical fibres under ^60Co source gamma-radiation up to a total dose of 71 kGy at two temperatures 30 and 80^\circC. The Al-doped and P-doped fibres demonstrated high radiation sensitivity required for the optical fibre dosimetry. The RIA response to temperature increase from 30 to 80^\circC depended on the dopant. In Al-doped fibres the absorption level decreased by 25% whereas in P-doped fibres it increased by at least 10%. For comparison we also tested standard telecom-grade Ge-doped fibres. Such fibres demonstrated a monotonous rise of the RIA during the whole irradiation with a small decrease of sensitivity at the higher temperature. [ABSTRACT FROM AUTHOR]

We have studied Co^60 gamma-irradiation effect on the characteristics of Type IA fiber Bragg gratings. A record Bragg peak shift of 190 pm was observed for a grating written in Fibercore PS-1250/1500 photosensitive fiber at a radiation dose of 116 kGy. Type IA and Type I gratings show different kinetics under radiation and during post-radiation annealing, which can be used for the design of a grating based dosimetry system. [ABSTRACT FROM AUTHOR]

We have studied the gamma-radiation induced effect on the Bragg peak position in fiber gratings. For the first time, to the best of our knowledge, a short-wavelength shift was observed for the Type I gratings written in an intrinsically photosensitive fiber. Based on the kinetics of the peak shift observed under irradiation and during post-irradiation annealing, we propose that two different types of defects are generated by ionizing radiation. One type is responsible for the refractive index increase during irradiation, while the creation of the defects of another type results in the index decrease. The concurrence of the two types of defects can be a way to obtain high stability of the gratings under irradiation. [ABSTRACT FROM AUTHOR]