Light energy is absorbed by ions present in the silica glass. A certain portion of attenuation in optical fiber is caused from the conversion of optical power to heat. This happens due to the presence of impurities in the fiber such as hydroxyl ions. Due to impurities inherent to the fiber’s manufacture and internal molecular structure, a small percentage of the signal strength is absorbed and released as heat within the optical fiber. Hydroxyl ions are known as one the main contributors of this effect.