Optical fiber consists of a and a layer, selected for due to the difference in the between the two. In practical fibers, the cladding is usually coated with a layer of or. This coating protects the fiber from damage but does not contribute to its properties. Individual coated fibers (or fibers formed into ribbons or bundles) then ha. Glass optical fibers are almost always made from, but some other materials, such as,, and as well as crystalline materials like, are used for longer-wavelength infrared or other specialized applications. Silica and fluoride glasses usually have refractive indices of about 1.5, but some materials such as the can have indices as high as 3. Typically th.