| Popis: |
Photobleachable defects are often formed in gemnanosilicate glasses during preform fabrication and fiber drawing. Rapid cooling, stress gradients, and exposure to a chemical reducing agent, such as hydrogen, or an oxygen deficient atmosphere increase the photosensitivity. Of particular interest is the presence of an ultraviolet absorption band at 242 nm, commonly assigned to the occurrence of an oxygen vacancy in the glass network. These sites are precursors of photoinduced paramagnetic Ge E' centers and other, yet not clearly defined, color centers that provide a permanent change in the refractive index with negligible additional fiber absorption in the infrared. |
