44 Fundamentals of Optical Devices 1.3 OPTICAL FIBERS Optical fiber is the most important component in fiber-optic communication systems as well as in many fiber-based optical measurement setups. The basic structure of an optical fiber is shown in Figure 1.3.1, which has a central core, a cladding, and an external coating to protect and strengthen the fiber. Cladding Core Coating Figure 1.3.1 Basic structure of an optical fiber. Light guiding and propagation in an optical fiber is based on total internal reflection between the core and the cladding. Because of the extremely low attenuation in silica, optical signals can transmit for very long distance along an optical fiber without significant loss in optical power. To understand the mechanism of optical propagation in fiber, we start by discussing fundamental concept of light wave reflection and refraction at an optical interface.