Metal coated fibers
- For high temperatures
- High strength
- For harsh environments
Available types
- Step-index UV
- Step-index IR
- Graded index
- Singlemode
Applications of metal-coated fibers
Optical fibers are usually made with a silica core that may be doped silica or pure silica and a cladding layer that is usually silica with a lower refractive index than the core. Such a fiber will be unusable in practice without an additional layer or coating. The coating is added to prevent the ingress of moisture and other contaminants that would lead to very early failure of the fiber. Coatings are usually polymers of various types, the most common being acrylate. Acrylate has the advantage of being easy to apply and is low cost.High strength
Metal-coated fibers were originally developed for ultra high reliability telecommunications applications. The metal coating imparts high strength and gives a hermetic coating to the fiber.High temperature
In addition to high strength, metal-coated fibers can be used at very high temperatures. The fact that most fibers are limited to use below 100°C is because of the coating that is applied to the fiber not the fiber itself. Aluminum coated optical fibers can be used up to 400°C. The new CuBALL fibers offer even higher temperatures.Hermeticity
Metal coatings can give excellent results in hostile atmospheres. Gases such as hydrogen or hydrogen sulphide that can be found in oil wells can permeate conventional fiber coatings leading to increased attenuation and premature failure.Radiation resistance
Pure silica core optical fibers have been found to give good results when used in an environment where nuclear radiation is present. Metal-coated pure silica fibers, however, have an additional advantage in some cases. It has been found that the damage that occurs with neutron radiation can be reduced by a factor of 100 times using metal coated fibers and heating them to 400°C.Lack of fluorescence
In spectroscopy the fluorescence produced by most polymer coatings can be a serious problem that is overcome by switching to metal coatings.Absence of 'creep'
Optical fiber sensors are becoming more common and metal-coated fibers can be used with advantage in a wide variety of sensor types. For example strain gauges based on optical fibers usually require good contact between the fiber and the surrounding medium. With plastic coated fibers there is usually a degree of 'creep' when a longitudinal force applied to the fiber can stretch the coating without imparting the force to the fiber.Medical applications
Optical fibers with metal coatings can be autoclaved. The high strength and small bend radius of the fiber is an additional advantage.Integrated optics
In integrated optics devices it is often necessary to hermetically seal a fiber as it exits the package. Copper coated fibers can be brazed or soldered to substrates making a hermetic seal.Vacuum applications
For many UHV applications epoxy and even some plastic coatings used on fibers are undesirable due to 'outgassing'. Metal-coated fibers allow fibers to be brazed in connectors or ferrules thus avoiding the use of epoxy. When it is desired to take an optical fiber through the wall of a vacuum chamber, metal-coated fibers allow an epoxy free feedthrough to be made.Specifications of fibers
CuBALL coated large core step index fibers are available in core diameters of 200 µm, 400 µm, 600 µm, and 1000 µm. The attenuation performance is similar to the plastic coated types (see HPSUV and HPSIR).Singlemode fibers are available in 800 and 1300 nm versions with CuBALL coating. A graded index (50/125 µm) fiber is available with CuBALL coating. The compressive stress imparted to the singlemode and graded index fibers by the metal coat can cause an increase in attenuation in these types of fibers. The magnitude of the additional attenuation depends on the temperature of use. An application note explaining the effect in more detail is available from Oxford Electronics.
