Fiber is a core refractive index higher than that of the cladding of the coaxial cylindrical dielectric waveguide. The main component of the core material is doped silica or plastic. Core diameter of about 8μm ~ 100μm. Generally cladding material is silica, an outer diameter of 125μm, the effect is to limit the intensity of light in the core. In order to enhance the flexibility of the optical fiber, the mechanical strength and resistance to aging characteristics, still clad outer layer of the coating layer increases, its main component is an epoxy resin and silicone rubber and other polymer materials. Light energy is concentrated in the core transmission. Cladding propagating light to provide a reflective surface and an optical isolator, and play a role as mechanical protection.