Influence of the coating type on the radiation sensitivity of FBGs

The ionizing radiation response of fiber Bragg gratings (FBGs) as a function of grating fabrication parameters and fiber characteristics has been investigated in a number of studies. In the present work we analyze a particular aspect of the problem, which was not considered up to now: the influence of the fiber coating on the radiation sensitivity of FBGs. The FBGs used in our study are draw tower gratings written before applying the coating in a fiber with a photosensitive 18 mol.% GeO₂-doped core. We irradiated polyimide, acrylate and ormocer coated gratings as well as mechanically stripped ormocer coated FBGs using a Co60 radiation source up to a 40 kGy dose. This total dose level corresponds to a near-Earth space radiation environment mission. We show that the coating type must be taken into account for correctly interpreting ionizing radiation effects on FBGs. Our analysis also shows that a properly designed coating can enhance or decrease the grating radiation sensitivity. Therefore, the results are of interest for both dosymetry and space communication applications of the FBGs.