Fiber Optic Cables

. The fiber optic cables are to be used under variety of situation such as underground, outdoor poles or submerged under water. The structure of cable depends on the situation where it is to be used, but the basic cable design principle remains same.
. Mechanical property of cable is one of the important factor for using any specific cable. Maximum allowable axial load on cable decides the length of the cable can be reliability installed.
. Also the fiber cables must be able to absorb energy from impact loads. The outer sheath must be designed to protect glass fibers from impact loads and from corrosive environmental elements.
1. Fiber Arrangements
. Several arrangements of fiber cables are done to use it for different applications. The most basic form is two fiber cable design. Fig. 1 shows basic two fiber cable design. It is also known as basic building block of fiber cable.
. For providing strength to the core several coatings of different materials  are applied as shown in Fig. 1.

. Multiple fiber cables can be combined together using similar techniques Fig. 2 shows commonly used six fiber cable.
. The basic fiber building blocks are used to form large cable. These units are bound on a buffer material which acts as strength element along with insulated copper conductor. The fiber building blocks are surrounded by paper tape, PVC jacket, yarn and outer sheath.

2. Fiber Optic Cable Ducts
. Number of cores are bundled in plastic ducts. To ease identification, individual fibers are colour coded Table 1 shows an example of the colour coding  used by manufactures.

If there are more than 12 fibers in a tube they are usually bundled together in quantities of 12 and held together with a coloured binding yarn.
3. Cable Jacket 
The cable jacket, the final outer layer of the cable, may use a number of materials depending on the required mechanical properties, attenuation, environmental stress and flammability. Table 2 lists the properties of common cable jacket materials.

4. Plastic Fiber Optic Cables
. Fibers can also be manufactured from transparent plastic which offers advantages of large diameter (1 mm), increased flexibility, can be cut using a hot razor blade, ease of termination. But because of high intrinsic loss use of plastic fibers is normally restricted to only few metres.
. Plastic optic fiber (POF) offers noise immunity and low cable weight and volume and is competitive with shielded copper wire making it suitable for industrial applications.
. Silica (glass) optical fiber has better transmission characteristics (Low loss) than POF. Also, silica fiber can tolerate higher temperature than plastic fiber. On the other hand, POF is more flexible, less prove to breakage, easier to fabricate and cost is low than glass fibers.
. Another advantages of glass/glass is that very clean fracture surface can be obtained which ensures that fiber cladding inside the connector retains its optical characteristics right upto the end face of fiber. Whereas in plastic glass/plastic fiber some additional losses exists due to fracture zone of plastic which even after grinding and polishing still have microscopic end face absorption areas. These advantages and disadvantages are summarized in table 3.


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