Study of Fiber Bragg Grating Samples Exposed to High Fast Neutron Fluences

Abstract

Abstract-Fibre 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 10 18 to 10 19 n/cm², the radiation induced changes in the material density and refractive index may drastically bias the measurements. The present study evaluates the radiation effect on the FBG performances by comparing their properties before and after their exposure to fast neutron fluences exceeding 10 19 n/cm² (E > 1 MeV). We studied responses of FBGs manufactured by three different laboratories in the same single-mode optical fibre 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-BWS) vary from a few pm (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 fibre 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.

Numerical Analysis of Radiation Effects on Fiber Optic Sensors

“…Figure 5 reveals that the length-change effect on the Bragg wavelength compensates for the RI effect, producing a zero Bragg wavelength shift up to 102 kGy. At higher doses (1049-1540 kGy), the compaction effect on the Bragg wavelength dominates; as a result, the Bragg wavelength shifts towards the shorter wavelengths, thus supporting the experimental results reported in [17,26,36]. Please note that in [17,26,36], Nuetron-irradiation-induced shift was reported.…”

“…While our simulation results for FBG demonstrate a redshift of the Bragg wavelength (λ B ) due to the radiation-induced RI change, experimental results reported in [17,26,36] showed a radiation-induced blueshift of λ B . Comparing the simulation results to the experimental work reported in [17,26,36] suggests that additional parameters must be considered for simulating our designs.…”

“…If the attenuation effects are not considered, Figure 4 would indicate that an increasing RI change resulting from the radiation dose is accompanied by an increase in the amplitude of the Bragg peak, since we are independently analyzing the RIC effect. While our preliminary simulation results showed a redshift of the Bragg wavelength as a function of increasing doses, this contradicted the experimental findings reported in [17,26,36]. In the next section, we discuss another key aspect-one that involves RIC effects and considers the length change-for accurately predicting the resonance shift behavior in FBG.…”

“…FBG inscription in the core of the fiber using femtosecond (fs) laser radiation and changing the refractive index of the core have found potential applications in nuclear environments, as reported in [9,10,17,25,26]. An FBG is created by generating a periodic RI modulation in the fiber core.…”

“…These defects are primarily a function of fiber composition, and a significant amount of work has been performed to mitigate such defects, including studying fiber composition and irradiation conditions, choosing an appropriate wavelength of light, and optimizing manufacturing conditions. [3,[5][6][7][8][9]17]. The fiber darkening associated with RIA modifies the complex RI of the material in accordance with a Kramer-Kronig relation [18], and this change in RI is uniform throughout the sensor.…”