Luminescence spectroscopy of point defects in silica-based optical fibers

We have studied by confocal microscopy the ultraviolet (5 eV, ∼27 J/cm2) and γ-ray (∼1 MeV, ∼1.2 kGy, ∼0.3 Gy/s) radiation-induced photoluminescence changes in a germanium (∼22 wt%) and phosphorus-co-doped (∼1.2 wt%) silica-core optical fiber. We have compared the results obtained by this technique to those obtained when the probe laser light is injected into the core (diameter 62.5 μm) of a few-meter-length multimode fiber. We showed that the confocal microscopy set-up reduces the photobleaching effect occurring with the second set-up. So, we used this technique (514 nm excitation, P = ∼5 mW) to compare the emitting defects created by both types of irradiation. We showed that the two radiative environments generate globally the same defects. In the fiber core, at least four different radiation-induced defects have luminescence bands in this spectral range (520-850 nm) with particular radial distributions. From our results, we assume that two of these bands peaking around 550 and 650 nm are related to defects intrinsic to a-SiO2, whereas another one (710 nm) is probably related to the fiber fabrication process. Concerning the defect emitting around 600 nm, we assigned it to a phosphorus related defect.