Ultraviolet (UV)-curable resins generate less hydrogen than does silicone. Therefore, the recommended covering materials for optical fibers are a UV-curable resin only or a combination of a UV-curable resin and nylon. The research into and experimental manufacture of graded index fibers were mainly conducted in the early developmental stage of optical fibers. The first consideration in the selection of fiber materials is that they have a low optical loss; that is, they must have a high degree of transparency. Silica is one of light-passing materials with the lowest refractive index. The speed of light traveling in a medium is inversely proportional to its refractive index. The speed of light propagating in the fiber is inversely proportional to the refractive index of the fiber material, which generally changes with wavelength. Only one mode is transmitted in the single-mode fiber. Therefore, the bandwidth of the single-mode fiber is considered inversely proportional to the length of the cable.