Photosensitivity of optical fibers with extremely high germanium concentration

Abstract

Writing and thermal annealing of fiber Bragg gratings (FBGs) in an optical fiber containing 75 mol.% GeO2 in the core have been studied by analyzing the first three diffraction orders of the FBGs. Double oscillations of the grating reflectivity has been observed during the FBG formation in an H2-loaded fiber, and the corresponding three grating types revealed have been labeled as type I(H2), type IIa(H2)-, and type IIa(H2)+. The results obtained have shown that the negative index change related to the type IIa photosensitivity cannot be described by the accumulated UV-dose only, but strongly depends on the UV-radiation intensity for both pristine and H2-loaded fibers, unlike the type I and type I(H2) photosensitivities.