Optical fiber is made of transmissive dielectric materials such as quartz glass (fused silica glass) and plastic. The optical fiber confines and transmits light by utilizing total internal reflection. The structure of typical optical fiber is double-layer: the first layer is core where light is confined and propagates, the other layer is clad which surrounds the core, as shown in Fig.2.1, where nair, n1, and n2 are the refractive index of air, core, and clad, respectively. The refractive index of core is a little bit higher than that of the clad, since an additive such as GeO2 and P2O5 is doped in the core. Therefore, light exhibits the total internal reflection at the boundary between the core and the clad repeatedly, and propagates in the fiber.