Absorption is a process in an optical fiber that results in the conversion of optical power to heat. This is caused by impurities, such as hydroxyl ions, that are present in the fiber. Absorption is one of the many factors that contribute to optical attenuation in optical fiber, which is the decrease in optical power as the light passes through the fiber.

Absorption occurs when the hydroxyl ions in the optical fiber absorb photons of light, converting the optical energy into heat. This process reduces the amount of light that is able to move through the fiber. The amount of light that is absorbed depends on the concentration of impurities in the optical fiber, with higher concentrations leading to more absorption.

Absorption can also be caused by light-induced structures in the fiber, such as color centers. Color centers are created by the interaction of photons with the material structure of the fiber, resulting in the formation of defects within the material. These defects can absorb and scatter light, resulting in additional absorption.

Absorption is an important factor in determining the performance of an optical fiber. It reduces the optical power that is able to travel through the fiber and can limit the distance that light is able to travel. To counter this, optical fiber manufacturers may use special techniques to reduce the absorption of light in the fiber, such as purifying the glass or coating the fiber with materials that prevent absorption.