A fiber strand consists of three layers, each with a specific function.

Starting from the inside, the “core” of the fiber is where the light signal travels. If the fiber is a “multimode” type, the core will be either 62.5 or 50 microns (millionths of a meter) in diameter. “Singlemode” fiber has a core size that is normally 9 microns across. The core is made of the highest quality optical glass.

The second layer from the inside is the “cladding”. The cladding is also made of optical glass, but of a distinctly different composition than the core. Because of its different composition, light rays that contact the core/cladding boundary at a less than severe angle will be reflected back towards the center of the core. Think of the cladding as a round mirror that helps keep the light signals within the core. This property of retaining the light within the core is called total internal reflection. The cladding diameter is 125 microns, whether the fiber is singlemode or multimode.

The third or outside layer is the “buffer” or “primary” coating. This layer is plastic, and is a “strength member” that provides flexibility and protection for the glass cladding and core underneath. Buffer coatings can be either 500 or 250 microns. The overall diameter of the three layers is approximately four times as thick as a human hair.

When a fiber is stripped for splicing or connector installation, the buffer coating is removed with the stripping tool.