RESUMO
A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950°C for more than 24 h without degradation in reflectivity.
RESUMO
The kinetics of the temperature dependent redox reaction between chromium and manganese (Cr(6+) + 3Mn(2+)âCr(3+) + 3Mn(3+)) in highly viscous silicate melts were studied by UV-vis-NIR spectroscopy at temperatures in the range from 25 to 800 °C. At high temperatures, the reaction is in equilibrium. During cooling, it is continuously shifted to the right. During cooling from Tg+50 K to Tg (Tg = glass transition temperature), a further decrease in the Cr(6+) concentration was obtained which, however, was less pronounced if larger cooling rates were applied. In this temperature range, the kinetics plays an important part. Finally, at a certain temperatures below Tg, the equilibrium was frozen. The temperature, the equilibrium is frozen in decreases with decreasing cooling rate. It also decreases with the glass transition temperature of the respective composition. The activation energies increase with the activation energies of the viscosity of the respective melt. The redox reaction is controlled by the viscosity, i.e., the rearrangement of the glass network and not by diffusion. The reason is a drastic change in the coordination spheres during the reaction which leads to a high inner reorganization energy according to Marcus' Theory.
