Ultraviolet-induced absorption during very short continuous exposure in Ge-doped optical fiber

We report an experimental study on the photoinduced optical absorption (OA) changes in Ge-doped optical fibers, in the 360–435 nm spectral range. An original set up was used allowing very short continuous (cw) exposure (6–30 ms), where the intrinsic 400 nm photoluminescence (PL), assigned to the two-fold coordinated Ge, is used as probe source. We investigated, in both unloaded and H2-loaded fibers, the dependence of this photoinduced OA on the fluence, in the 0.1–4.8 J/cm2 range. Four kinds of uncoated optical fibers with different Ge content, 0, 4, 13, 17 mol%, were transversally exposed to a cw frequency-doubled Ar+ laser operating at 244 nm. Increasing the fluence up to ∼0.6 J/cm2, on both unloaded and H2-loaded fibers, the photoinduced OA increases and reaches a maximum. For larger fluences, the photoinduced OA decreases monotonically in the unloaded fibers whereas it remains constant in the H2-loaded ones. This OA decreases monotonically when the wavelength increases, indicating that it is the low energy tail of an absorption band located at larger energies. These photoinduced OA increase on increasing the Ge content. Based on our experimental results and on the literature data, we propose that the Ge(1) formation, via a consecutive two-step process, may be the source for the measured photoinduced OA.