Fiber Optic Basics


MSPP systems use optical fibers to carry traffic through the network. In this section, you explore the basic construction of optical fiber and learn how the reflection and refraction of light affect its propagation through a fiber cable. This section also discusses the differences between the two major classes of optical fibers, multimode and single mode.

Optical Fiber

Two fundamental components of optical fiber allow it to transport light: the core and the cladding. Most of the light travels from the launch point to the end of the fiber segment in the core. The cladding is around the core to confine the light. Figure 2-1 illustrates the typical construction of an optical fiber.

Figure 2-1. Typical Construction of an Optical Fiber


The diameters of the core and cladding are shown, but the core diameter might vary for different fiber types. In this case, the core diameter of 9 μm is very small, considering that the diameter of a human hair is about 50 μm.

The cladding's outer diameter is a standard size of 125 μm. The uniformity in cladding diameter allows for ease of manufacturing and ubiquity among optical component manufacturers.

The core and the cladding are made of solid glass. The only difference between the two is the method by which the glass was constructed. Each has different impurities, by design, which change the speed of light in the glass. These speed differences confine the light to the core.

The final element in this picture is the buffer/coating, which protects the fiber from scratches and moisture. Just as a glass pane can be scratched and easily broken, fiber-optic cable exhibits similar physical properties. If the fiber were scratched, in the worst case, the scratch could propagate, resulting in a severed optical fiber.

Light Propagation in Fiber

Reflection and refraction are the two phenomena responsible for making optical fiber work.

Reflection and Refraction

The phenomenon that must occur for light to be confined within the core is called reflection. Light in the core remains in the core because it is reflected by the cladding as it traverses the optical fiber. Thus, reflection is a light ray bouncing off the interface of two materials. It is most familiarly illustrated as light from an object being returned (reflected) as your image in a mirror.

Refraction, on the other hand, occurs when light strikes the cladding at a different angle (compared to the angle of reflection). Light undergoes refraction when it exits the core and proceeds into the cladding. This degrades optical transmission effects because key parts of the optical signal pulse are lost in the fiber cladding. Thus, refraction is the bending of the light ray while going from one material to another.

Refraction is probably less familiar, but you can see its effect in day-to-day living. For example, a drinking straw placed in a glass of clear liquid looks as if it is bent. You know that it is not bent, but the refraction properties of the liquid cause the straw to appear as bent.

Reflection and refraction are described mathematically by relating the angle at which they intersect the material surface and the angle of the resultant ray. In the case of reflection, the angles are equal.

Figure 2-2 illustrates a typical refraction/reflection scenario. Where light is predominantly reflected and not refracted, the index of refraction for the core is greater than the index of refraction for the cladding. The index of refraction is a material property of the glass and is discussed in the next section.

Figure 2-2. Refraction and Reflection


In the case of refraction, Snell's Law relates the angles as detailed in the next section.

Index of Refraction (Snell's Law)

The speed of light varies depending upon the type of material. For example, in glass, the speed of light is about two-thirds the speed of light in a vacuum. The relationship shown in Figure 2-3 defines a quantity known as the index of refraction.

Figure 2-3. Index of Refraction


The index of refraction is used to relate the speed of light in a material substance to the speed of light in a vacuum. Glass has an index of refraction of around 1.5, although the actual number varies slightly from one type of glass to another. In comparison, air has an index of refraction of about 1, and water has an index of refraction of about 1.33.

The index of refraction (n) is a constant of the material at a specific temperature and pressure. The index of refraction is a fixed value. In another material at those same conditions, n would be different.

In glass, n is controlled for the glass by adding various dopant elements during the fiber-manufacturing process. Adding controlled amounts of dopants enables fiber manufacturers to design glass for different applications, such as single-mode or multimode fibers. For optical fiber, n is engineered slightly differently for the core and the cladding.

Types of Optical Fiber: Multimode and Single Mode

Two basic types of optical fibers existmultimode fiber (MMF) and single-mode fiber (SMF). The most significant difference lies in their capabilities to transmit light long distances at high bit rates. In general, MMF is used for shorter distances and lower bit rates than SMF. For long-distance communications, SMF is preferred.

Figure 2-4 shows the basic difference between MMF and SMF.

Figure 2-4. Optical FiberMultimode and Single Mode


Notice the physical difference in the sizes of the cores. This is the key factor responsible for the distance/bit rate disparity between the two fiber types. Figure 2-5 illustrates the large core effect of MMF.

Figure 2-5. Light Propagation


In a nutshell, the larger core diameter allows for multiple entry paths for an optical pulse into the fiber. Each path is referred to as a modehence, the designation multimode fiber. Because MMF has the possibility of many paths through the fiber, it exhibits the problem of modal dispersion. Some of the paths in the MMF are physically longer than other paths. The light moves at the same speed for all the paths, but because some of the paths are longer, the light arrives at different times. Consequently, optical pulses arrive at the receiver with a spread-out shape. Not only is the shape affected, but the overall duration of the pulse is increased. When the pulses are too close together in time, they can overlap. This leads to confusion by the receiver and is interpreted as unintelligible data.

For SMF, only a single entrance mode is available for the optical signal to traverse the fiber. Thus, the fiber is appropriately referred to as single-mode fiber.




Building Multiservice Transport Networks
Building Multiservice Transport Networks
ISBN: 1587052202
EAN: 2147483647
Year: 2004
Pages: 140

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