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 µm, whereas multimode
cables have a much larger core, typically between 50 µm and 200 µm 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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