Comparative study of long period and fiber Bragg gratings under gamma irradiation

We evaluated comparatively the radiation sensitivity of two long period gratings (LPGs) manufactured in a SMF28™ optical fiber through the melting-drawing method based on CO2 laser assisted by a micro-flame, of a commercial fiber Bragg grating (FBG), and of a commercially available Draw Tower Grating (DTG), all exposed to gamma radiation. The tests were run on-line using the optical frequency domain reflectometry (OFDR) technique, in order to provide unequal sensitivity, the gratings being wavelength multiplexed. The dose rate dependence of the LPGs response was evaluated for dose rates of 0.37 kGy/h and 0.24 kGy/h, up to the total dose of (33.9 ± 0.8) kGy and (21.5 ± 0.5) kGy. The FBG and DTG proved to be radiation hardened having a radiation sensitivity of 12 pm/kGy, while the LPGs radiation sensitivity was as high as 1.2 nm/kGy under the same irradiation conditions. A recovery of the radiation induced wavelength shift for LPGs was noticed at room temperature, at a rate of 2.2 pm/h, over a 211 h period. The effect of LPG wavelength change after exposure to gamma-ray can be exploited in the development of radiation dosimeters, up to the dose of 10 kGy, when saturation appears. The temperature sensitivity of the LPG2 changed from 48 pm/°C before the irradiation, to 50 pm/ °C after the irradiation for a total dose of 21 kGy.