In this section the basic properties of optical fibers are discussed in some detail with emphasis placed on concepts that are important in optical fiber sensor technology. The structure of optical fibers is quite simple, as shown in Figure. Basically they consist of layered cylinders of glass or plastic with small diameters. There is a central cylinder called the core, which is made up of one type of glass or plastic. Surrounding the core is a cylindrical shell called the cladding that is made of a slightly different type of glass or plastic. The difference between the core and cladding materials will be discussed later. Finally, this layered cylinder is usually surrounded by a Protective jacket. The light-guiding capability of the fiber is dependent upon the properties of the core and cladding while the mechanical strength of the fiber is maintained by the jacket that is usually made of plastic.
the structure of an optical fiber
Some of the design objectives considered in the development of a good optical fiber are illustrated by the simple system shown in Figure 2. The system consists of a pulse-modulated optical source. The input signal at the left represents the intelligence (information) that is impressed on (modulates) the light beam that, after emerging from the source, is focused with a lens into one end of an optical fiber. The light travels through the fiber and emerges from the opposite end, where it is directed into an optical detector (photodetector), possibly focused again with a second lens.
Four major optical fiber design objectives will be discussed. The first major objective is the desirability of maximizing the amount of available light that is transferred (coupled) into the core of the fiber. It is only the light in the core that is propagated along the length of the fiber with relatively low optical power loss. In order to maximize the amount of light transferred (coupled) into the core, it is necessary to maximize the numerical aperture (NA) of the fiber. This is one of four important fiber parameters that strongly affect the behavior of the simple system shown in Fig. 2.2. After introducing the other three important parameters, each will be discussed in some detail.
basic optical fiber link
The third optical fiber design objective is the desirability of maximizing the information carrying capacity of the fiber. The input to the fiber may be a light beam of continuously-varying intensity or a group of well defined pulses of light as shown in Fig. 2.2. As the light pulses propagate through the fiber, their amplitude will decrease due to attenuation. In addition, due to a number of other effects to be discussed, the individual pulses also may broaden (spread). If they become too broad they will overlap or coincide with one another in both time and space. If this OCcurs, the intelligence (information) originally impressed on the light beam would be lost. Pulse broadening that occurs in the fiber is called dispersion. This
parameter places a limit on a fiber’s information carrying capacity (signaling rate). The fourth design objective is the desirability of maximizing the strength of fibers when they are Initially drawn and maintaining this strength when the fibers are formed into cables or are used in sensors and other applications. Before considering these and other fiber design objectives in some detail, the basic theory of light propagation in optical fibers will be discussed, beginning with light-ray propagation in layered media.
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