The transparent glass or plastic section with a high refractive index. The center of the optical fiber through which light is transmitted. The core is the part of the fiber that carries the light signal that has information impressed upon it. The fiber optic core is made of very pure glass and its diameter is measured in microns. The central region about the longitudinal axis of an optical fiber, which region supports guiding of the optical signal. For the fiber to guide the optical signal, the refractive Index of the core must be slightly higher than that of the cladding. In different types of fibers, the core and core cladding boundary function slightly differently in guiding the signal. Especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding.