(1) OH group molecular vibration
OH ion exists in optical fiber. OH ion is impurity and causes loss in optical fiber. OH ion has an absorption by the fundamental vibration. The absorption peak is approximately at 2.73 μm. The remarkable peak at 1.38 μm and the small peak at 0.95 μm shown in Fig.2.12 are due to the second-harmonic and third-harmonic overtones of OH absorption, respectively.
(2) Transition metal ion
Light is absorbed by transition metal ions, such as ions of Cu, Fe, and Cr. However, these transition metal ions are usually not mixed in optical fiber, since the fiber manufacturing process has been improved. Light absorption by transition metal ions is negligible.