Deduced Refractive Index Profile Changes of Type I and Type II Gratings When Subjected to Ionizing Radiation

Type I and type II Bragg grating arrays in silica fiber have been irradiated at the Ohio State University Nuclear Reactor Laboratory in the gamma facility and the pool-type reactor, to receive a relatively small gamma dose (2.82 Mrad) and a relatively large gamma-neutron dose (638 Mrad, 6×10
¹⁷
(n/cm
²
)), respectively. The Bragg grating fibers were first irradiated in the gamma facility and those same fibers were then subsequently irradiated in the reactor, while being monitored in situ by optical frequency domain reflectometry. The response of the Bragg gratings, in terms of reflected amplitude, was analyzed, both in the frequency and spatial domains. A Bragg grating modeling code, written in MATLAB, was then used to generate modeled fiber Bragg grating (FBG) responses to match the experimental responses, based on some fundamental assumptions. With the modeled responses, we were able to deduce the alterations to the refractive index profile of the Bragg grating inscribed fibers as a result of the irradiation.