STRUCTURES AND GENERATION MECHANISMS OF PARAMAGNETIC CENTERS AND ABSORPTION-BANDS RESPONSIBLE FOR GE-DOPED SIO2 OPTICAL-FIBER GRATINGS

Paramagnetic centers and absorption bands induced by ultraviolet photo ns in Ge-doped SiO2 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 excim er lamp (5.6 eV). Irradiation with the KrF excimer laser induces two p aramagnetic centers, named Ge(1) and Ge(2), in proportion with a decre ase 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 absorptio n components and their proportionality constants are independent of th e Ge content of the samples. The 4.0-eV photons from the XeCl excimer lamp induce only a Ge E' center, while the 5.6-eV photons from the KrC l excimer lamp induce a Ge(1) besides a Ge E' center. From these resul ts, Ge(1) and Ge(2) are, respectively, assigned to the Ge electron cen ter (GEC) and the positively charged Ge oxygen-deficient center (GODC) (+), 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 SiO2 glass that was exposed to photons from the KrF excimer laser. The TSL spectrum is very similar to the photol uminescence spectrum that is known to be due to the GLPC's. It was fou nd that the absorption, which was induced by the KrF excimer laser pho tons, decreases during the TSL measurement and that this decrement of the absorption is proportional to the TSL intensity. As mentioned abov e, the electrons that are to be trapped to generate the GEC's are rele ased from the GLPC's during the photon irradiation. Then, in its rever se reaction, these electrons are thermally detrapped from the GEC's to regenerate the GLPC's, and the TSL is caused by an electronic de-exci tation in such formed GLPC's. To conclude, the TSL phenomenon further validates the assumption that the GLPC is the electron donor to genera te the GEC's. [S0163-1829(98)05407-1].