The cladding is the layer of dielectric material that immediately surrounds the core of an optical fiber and completes the composite structure that is fundamental to the fiber’s ability to guide light. The cladding of telecommunications grade optical fiber is also made from silica glass, and is as critical in achieving the desired optical performance properties as the core itself.
For optical fiber to work, the core must have a higher index of refraction than the cladding or the light will refract out of the fiber and be lost. Initially multiple cladding diameters were available, but the industry swiftly arrived at a consensus standard cladding diameter of 125 μm, because it was recognized that a common size was needed for intermateability.
A cladding diameter of 125 μm is still the most common, although other fiber core and cladding size combinations exist for other applications. Because of their similar physical properties it is possible, and in fact highly desirable, to manufacture the core and cladding as a single piece of glass which cannot be physically separated into the two separate components.
It is the refractive index characteristic of the composite core-clad structure that guide the light as it travels down the fiber. The specific materials, design, and construction of these types of optical fibers make them ideally suited for use in transmitting large amounts of data over the considerable distances seen in today’s modern telecommunications systems.
The third section of an optical fiber is the outer protective coating. The typical diameter of an uncolored coated fiber is 245 μm, but, as with the core and cladding, other sizes are available for certain applications.
Coloring fibers for identification increases the final diameter to around 255 μm. The protective coating typically consists of two layers of an ultraviolet (UV) light cured acrylate that is applied during the fiber draw process, by the fiber manufacturer.
The inner coating layer is softer to cushion the fiber from stresses that could degrade its performance, while the outer layer is made much harder to improve the fiber’s mechanical robustness. This composite coating provides the primary line of physical and environmental protection for the fiber.
It protects the fiber surface to preserve the inherent strength of the glass, protects the fiber from bending effects, and simplifies fiber handling. The colored ink layer has properties similar to the outer coating, and is thin enough that its presence does not significantly affect the fiber’s mechanical or optical properties.