Fiber Optic Cable The world of telecommunications is rapidly moving from copper wire networks to fiber optics. Optical fiber is a very thin strand of pure glass which acts as a wave guide for light over long distances. It uses a principle known as total internal reflection. Fiber optic cable is actually composed of two layers of glass: The core, which carries the actual light signal, and the cladding, which is a layer of glass surrounding the core. The cladding has a lower refractive index than the core. This causes Total Internal Reflection within the core. Most fibers operate in duplex pairs: one fiber is used to transmit and the other is used to receive. But it is possible to send both signals over a single strand. There are two main types of fiber optic cables: Single Mode Fiber (SMF) and Multi-Mode Fiber (MMF). The difference is basically in the size of the core. MMF has a much wider core, allowing multiple modes (or “rays”) of light to propagate.