While it may appear state-of-the-art on the surface, fiber optic technology is a fairly simple method of conducting light that has been around for some time. The principle of guiding light by refraction was first demonstrated in the mid 19th century, and has been developing for use in practical applications ever since.
Cores, Cladding and Coatings
The core of optical fibers consists of a flexible light guide usually produced from glass, quartz or plastic. These guides are very thin, typically 9μm to 1 mm. Light entering the fiber will, ideally, be transmitted down the length of the core. As light travels down the core, it typically does so in some sort of a zigzag pattern. The bouncing waves of light interfere constructively with each other, creating light amplification by superposition. Every configuration of zigzag pattern can be identified by the pair of angles the bouncing light makes while it travels, referred to as a mode. Some types of fibers encourage the propagation of only one mode, while others encourage several modes during transmission. These are known as single mode and multimode optical fibers.
The method in which the light is conducted down the core depends on the type of fiber, either step index or gradient index. These different types of fibers are constructed in ways that emphasize different qualities of light conductivity. Step index fibers have a constant index profile while gradient index fibers have a non-linear, rotationally symmetric index profile. The different indexes affect the method in which light rays travel down the fiber core. In step index fibers the index of refraction is constant, and rays travel the length of the media in a straight line. Gradient index fibers reduce the refraction from the middle outwards, resulting in light traveling in a spiral form around the optical axis.
Material surrounding the core, called cladding, is required for transmitting the light for any length beyond a short distance. If no cladding exists, the environment around the core absorbs the light traveling within, drastically reducing the transmission. Cladding material surrounding the core must possess a low refractive index to contain the core light, while protecting against surface contaminant scattering. In all-glass fibers, the cladding is typically made of glass. For other fibers, the cladding can be made from plastic.
Outside the cladding, a coating or buffer is typically applied that protects the core material. Depending on the environment in which the fiber is to be used, coatings must also possess a certain level of resistance to temperatures and tensile strength to allow the fiber to continue to function. Most optical fibers are manufactured with a polymer buffer or coating that protects the fiber from scratches that would severely reduce the inherent strength of the fiber. Polymer coatings have been used for more than 25 years in the telecommunications and specialty fiber optic markets. However, some specialty fiber optic applications require a fiber that can operate in an environment that would damage or destroy the polymer coating. For example, extreme temperatures, acidic chemicals, and mechanical stress can damage the fiber coating severely and cause the fiber to fail.
While most polymer coatings can operate at a maximum temperature of 125 degrees Celsius, applications in a severe environment can require operation in temperatures of up to 700 degrees Celsius. Many types of acids will dissolve polymers, and when a fiber with a polymer coating is subjected to continuous tensile stress, it will break because the coating allows atmospheric moisture to penetrate and attack the fiber surface. These environmental attacks can be eliminated by coating the fiber with either gold or aluminum.