Radiation (X, gamma, protons and electrons) and high temperature influences on fiber Bragg grating measurements

Optical fiber temperature sensors have numerous advantages such as their small sizes, low weights and their immunity to a large band of the electromagnetic spectrum. The fiber Bragg gratings technology has the same advantages than the other optical sensors and is also characterized with an ultra-fast time response with a temperature accuracy better than 0.5 C. For nuclear environments such as near-Earth space or a nuclear reactor core the radiation - and the temperature - influence the performances of Bragg gratings. In this manuscript, we studied the behaviors of three gratings types: type I gratings are photo-inscribed with continuum or pulsed laser. They cannot resist to temperature higher than 400 C. Type R - Regenerated - gratings are done as type I with a further high-temperature treatment (≥ 600 C) to create a new grating resistant to temperatures exceeding 1000 C. Type II gratings are inscribed with femtosecond laser and are well-known to have a thermal stability as good as type R gratings. Regarding their responses to radiation and thermal constraints, type I gratings are unstable under radiations even with pre-thermal annealing (300 C). In addition, larger is the dose-rate or the accumulated dose larger the grating degradation is. Type R gratings are unstable under radiations at room temperature. However for the irradiation temperature above 150 C these gratings present an equivalent temperature error due to radiations of less than 1 C. After a thermal treatment at a temperature above 450 C, type II gratings are stable under radiations at room temperature (less than 1 C of radiation induced error). (author)