Fiber Optic Cable

About the fiber optic cable

Fiber optic cable consists of an inner cylindrical core surrounded by an outer cylindrical cladding. The refractive index of the inner material is greater than that of the outer material, and the relationship between the two refractive indices affects the transmission characteristics of light along the cable. The amount of attenuation of light as it travels along the cable varies with the wavelength of the light transmitted. This characteristic is very non-linear and a graph of attenuation against wavelength shows a number of peaks and troughs. The position of these peaks and troughs varies according to the material used for the fibers. It should be noted that fiber manufacturers rarely mention these non-linear attenuation characteristics and quote the value of attenuation which occurs at the most favorable wavelength.

Two forms of cable exist, known as monomode and multimode. Monomode cables have a small diameter core, typically 6 Ám, whereas multimode cables have a much larger core, typically between 50 Ám and 200 Ám in diameter. Both glass and plastic in different combinations are used in various forms of cable. One option is to use different types of glass fiber for both the core and the cladding. A second, and cheaper, option is to have a glass fiber core and a plastic cladding. This has the additional advantage of being less brittle than the all-glass version. Finally, all-plastic cables also exist, where two types of plastic fiber with different refractive indices are used. This is the cheapest form of all but it has the disadvantage of having high attenuation characteristics, making it unsuitable for the transmission of light over medium to large distances.

Protection is normally given to the cable by enclosing it in the same types of insulating and armoring materials that are used for copper cables. This protects the cable against various hostile operating environments and also against mechanical damage. When suitably protected, fiber optic cables can even withstand being engulfed in flames.

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