Photobleaching of Neutron Radiation Induced Attenuation of Optical Fibers at Liquid Nitrogen Temperature

In this work, the effect of neutron radiation on optical fibers at cryogenic temperature was investigated and the effect of photobleaching as a way to mitigate the damage caused by this type of radiation was demonstrated. A neutron beam supplying a neutron flux of around 1.5 x 10¹⁰ neutrons/cm² s corresponding to an absorbed dose rate of 0.04 Gy(SiO2)/s was used, which enabled reaching fluences as high as 10¹⁵ neutrons/cm² (equivalent absorbed doses of 2.3 kGy(SiO²) while maintaining the optical fibers at 77 K with a liquid nitrogen bath. A comparison was made between the radiation-induced attenuation (RIA) dynamics of standard (Ge-doped), photosensitive (high Ge-doped), and rad hard (F-doped and pure silica core) fibers. The effectiveness of four power levels (4.5, 1, 0.5, and 0.1 mW) of photobleaching light at 1050 nm at mitigating RIA at 1550 nm was also studied. The recovery of the fibers upon warm-up from 77 K to room temperature was recorded as well as a comparison between RIA of pristine fibers and pre-irradiated fibers.