Structures and generation mechanisms of paramagnetic centers and absorption bands responsible for Ge-doped optical-fiber gratings

Paramagnetic centers and absorption bands induced by ultraviolet photons in Ge-doped (Formula presented) glass are investigated. Four kinds of samples with different Ge contents were exposed to ultraviolet photons from a KrF excimer laser (5.0 eV), a XeCl excimer lamp (4.0 eV), and a KrCl excimer lamp (5.6 eV). Irradiation with the KrF excimer laser induces two paramagnetic centers, named Ge(1) and Ge(2), in proportion with a decrease in the absorption at 5.1 eV and with an increase in absorption at 4.5 and 5.8 eV. The total density of the induced paramagnetic centers is linearly proportional to each induced change of the three absorption components and their proportionality constants are independent of the Ge content of the samples. The 4.0-eV photons from the XeCl excimer lamp induce only a Ge (Formula presented) center, while the 5.6-eV photons from the KrCl excimer lamp induce a Ge(1) besides a Ge (Formula presented) center. From these results, Ge(1) and Ge(2) are, respectively, assigned to the Ge electron center (GEC) and the positively charged Ge oxygen-deficient center (Formula presented) which donated an electron to the GEC. The oscillator strength of the GODC for the absorption at 5.1 eV was found to be 0.1. From this, it is considered that the GODC that acts as the electron donor is the Ge lone pair center (GLPC). Thermally stimulated luminescence (TSL) is also examined in Ge-doped (Formula presented) glass that was exposed to photons from the KrF excimer laser. The TSL spectrum is very similar to the photoluminescence spectrum that is known to be due to the GLPC’s. It was found that the absorption, which was induced by the KrF excimer laser photons, decreases during the TSL measurement and that this decrement of the absorption is proportional to the TSL intensity. As mentioned above, the electrons that are to be trapped to generate the GEC’s are released from the GLPC’s during the photon irradiation. Then, in its reverse reaction, these electrons are thermally detrapped from the GEC’s to regenerate the GLPC’s, and the TSL is caused by an electronic de-excitation in such formed GLPC’s. To conclude, the TSL phenomenon further validates the assumption that the GLPC is the electron donor to generate the GEC’s.