Radiation resistance of fibre Bragg gratings under intense reactor irradiation

We study radiation-induced absorption (RIA) of light and radiation-induced shift (RIS) of a resonant wavelength in fibre Bragg gratings (FBGs) during reactor irradiation up to a neutron fluence of 4.8×10²⁰ neutron·cm^–2 (a flux density of 2.87×10¹⁴ neutron·cm^–2s^–1) and gamma radiation doses of 2.3×10⁹ Gy (a dose rate of 1.4 kGy·s^–1). The FBGs are fabricated by femtosecond writing in radiation-resistant optical fibres with an undoped silica glass core, as well as by UV writing in a standard germanosilicate fibre (SMF-28). The RIS of the resonant wavelength of the grating is 2.5–3.4 nm for all the studied samples (with the exception of FBGs with a polyimide coating), which, apparently, is due to radiation densification of the silica glass network. The gratings written through the polyimide coating exhibit an anomalously large RIS in the short-wavelength region of the spectrum (by ∼1.5%), which depends linearly on the neutron fluence. Such a large shift is apparently due to the shrinkage of the polyimide coating in the case of exposure to intense gamma-neutron radiation at high temperature and in vacuum, which, even at a small coating thickness (∼10 μm), leads to a significant compression of the fibre region with the grating. The performed experiments demonstrate that, under such intense radiation exposure, FBGs written in radiation-resistant optical fibres with a protective copper coating are preferable for practical use, FBGs with a polyimide coating can presumably be considered as dosimeters, and germanosilicate fibres with FBGs are unsuitable due to significant RIA (∼2 dB·cm^–1).