Absorption losses in optical fibers

Absorption of signal is a serious loss mechanism in an optical fiber. Absorption occurs in optical fibers due to the presence of imperfections in the atomic structure of the fiber material, due to some basic inherent intrinsic material properties and due to some extrinsic material properties. Imperfections may appear in atomic structure due to oxygen deficiencies and missing of certain molecules. Diffusion of hydrogen molecules may also induce absorption. But the contribution from imperfections is relatively small in fiber optic absorption losses. Inherent intrinsic absorption is caused by basic fiber material properties. If a material is free from impurities and imperfections, then entire absorption is due to intrinsic absorption. Silica fibers possess very low intrinsic material absorption. Here absorption is caused by the vibration of silicon-oxygen bonds. The interaction between these bonds and the electromagnetic field of the optical signal is responsible for intrinsic absorption. Presence of impurities in the fiber material leads to extrinsic absorption. This is caused by the electronic transition of metal impurity ions from one energy level to another. Another reason for extrinsic absorption is the presence of hydroxyl ions in the fiber.