Fiber optic cables have a glass core, an optical cladding layer (shown in blue in the diagram) and a primary coating. The core uses photos to transmit digital signals. The cladding is also of glass and has a diameter of 125 μm regardless of the type of fiber.

The material is selected to ensure full low-angle reflection of the light signal at the point between the cladding and the core, and thus keep the light within the core. The primary coating is a protective acrylate layer generally with a thickness of 60 μm. It is applied to the cladding as protection for the fiber core immediately after the core is drawn, and shields the optical fiber from contamination from external influences.

Attenuation of the light beam at the fiber core closely depends on the wavelength. Transmission technology uses specific wavelengths or “optical windows”. These are 850 and 1300 nanometers for multimode fibers and 1310 and 1550 nanometers for single-mode fibers.