Plastic optical fiber is a step-index multimode optical fiber, composed of a cylindrical "core" surrounded by a "clad" layer. The light refraction index of the core is higher than that of the clad. Multi-mode links can be used for data rates up to 800 Gbit/s. Multi-mode fiber has a fairly large core diameter that enables multiple light modes to be. Plastic optical fiber (POF) has always been "lurking in the background" in fiber optics; a specialty fiber useful for illumination and low speed short data links. This large core. We'll cover single mode, multimode, and armored fiber cables below. The cladding has a diameter of 125 µm and consists of a material with dopants that change the refractive index.