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.
Originally developed for ultra high reliability telecommunications applications, metal coated fibers were discovered to allow fibers to operate in extreme conditions. The metal coating around the fiber preserves the high initial mechanical strength, provides resistance against static fatigue and creates a hermetic seal. When fibers with metal coatings are placed under high tensile stress, the hermetic seal prevents moisture from penetrating the coating and attacking the fiber surface. In addition to high strength, metal coated fibers can be used at very high temperatures. For example, aluminum coated fibers can withstand temperatures between --269 and +400 degrees Celsius, and gold-coated fibers between --269 and +700 degrees Celsius.
Typical applications that require metal coatings include monitoring radiation in nuclear power plants, oil exploration, oil/gas burner (furnaces) monitoring, and chemical processing. Fibers used to monitor nuclear radiation in power plants must be able to withstand incredibly high heat. When fibers are heated to 400 degrees Celsius, an aluminum coating can reduce damage from nuclear radiation by a factor of 100.
In oil exploration, a fiber with a sensor is lowered into a deep hole with hot water, oil, acids and other corrosive materials. Using fiber optics for data relay as opposed to the traditional wire cables provides a number of advantages. The fiber optic communication system features a lighter cable that is immune to electromagnetic interference. There is also no risk of fire hazard due to electrical shorting with a fiber optic system.
In oil/ gas burners, the fiber is inserted directly into, or very near to, the flame. Flame temperatures exceed the maximum that polymer coatings can survive, but gold fibers can withstand such temperatures. Use of optical fibers in this application enables spectral analysis of the flame — information that allows users to control parameters that can increase efficiency and lower fuel cost.
While processing very acidic chemicals, monitors used to ensure proper mixing are regularly exposed to highly acidic conditions. Gold fibers can survive this environment, where polymer-coated fibers will fail.
Additional applications for gold fibers have been found in high temperature vacuum and pressure feedthroughs, which allow fiber signals to be passed through an airtight seal without breaking the seal. Vacuum feedthroughs with gold fibers have been used in manufacturing solar cells and semiconductors, and in high temperature spectroscopic sensing for power generation.
Aluminum and gold coatings can be applied to a wide variety of fibers, including step index multimode, graded-index and single-mode varieties. However, the process of applying a molten metal coating to a tiny strand of glass is a complex task requiring a high level of expertise.
Temperature and other process controls are critical for both gold and aluminum coatings. Applying the gold-coating to fibers requires an extremely high temperature of ~1100 degrees Celsius. Such high temperatures require special compatible materials, handling tools, draw rates, and methods for applying the metals. Aluminum oxidizes rapidly in the presence of oxygen. Therefore, when applying an aluminum coating, it is imperative to keep the application point oxygen-free.
For many applications, the metal fibers are assembled into cables or bundles, in addition to the feedthroughs discussed earlier. Additional manufacturing processes ensure the quality of the finished fibers, both for metal coated as well as polymer jacketed fiber. Surface interferometers, for example, can be used to verify the quality of end face polishes. In certain applications where fibers are assembled into precision 2D arrays, fiber locations can be measured to sub-micron accuracies. For high power laser delivery applications, special epoxy-free, air gapped termination designs, as well as laser polishing methods, are utilized.
As the number of practical applications for optical fibers grows, so does the need to continue to develop the manufacturing processes surrounding this technology. Specialty coatings available today enable applications for fiber optics previously thought impossible.