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.