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This chapter is from the book

This chapter is from the book

The Fiber Cable

Let's try an analogy. Consider water flowing through a hose, as illustrated in Figure 1-6. The water is guided through the hose walls. There is no conductor in the center because its presence would cause friction and slow the flow.

Figure 1-6 Water Hose Analogy.

Whether the walls are straight, as in Figure 1-6(a), or curved, as in Figure 1-6(b), the water flow experiences little resistance to the flow. In the same way a fiber optic cable guides light along the cable. You might say the guidance of light in a fiber is more structured than the water-hose analogy. The light is guided by a changing refractive index in the fiber. Two types of fiber are shown in Figure 1-7. Figure 1-7(a) shows a step (refractive) index fiber, and Figure 1-7(b) shows a graded index (GRIN) fiber.

Figure 1-7 Step Index and Graded Index Fibers.

In the step index fiber, the light ray bounces from wall to wall, whereas in the graded index the ray's direction changes more slowly. A typical single optical fiber consists of a tiny strand of silica mixed with dopants to vary the refractive index. Surrounding this glass core is a layer of cladding. The refractive index of the core is greater than that of the cladding, so the light ray will remain in the core. Of course, if the fiber/cladding combination is the graded index (GRIN) type, the refractive index varies slowly (decreases) from core to cladding. In addition to the cladding, two or three or more layers of protective material cover the fiber. Some examples are shown later in the text.

Some fibers are made from plastic. Plastic is more durable and bendable but has higher attenuation than glass fiber. Another advantage of plastic is cost. Since plastic fibers are typically of larger diameter, the connectors and splices are larger, making them easier to fabricate and utilize. The higher precision and cost involved with the glass fibers are a disadvantage when they are used for short runs where attenuation doesn't matter.

There exists also a combination plastic and glass fiber where the core is glass (silica) and the cladding is plastic. These combination fibers have higher losses than the all-glass fibers, but their higher numerical aperture (NA) makes coupling more efficient. The combination fibers are used for moderate path lengths. These path lengths are longer than the all-plastic, which is a high-loss fiber, but shorter than the all-glass fiber. Typical characteristics for commercial fibers are shown in Table 1-3.

Table 1-3 Typical Fiber Characteristics

 

NA

Loss (dB/km)

Core Diameter (10-6m)

Glass

0.24

5

50

Plastic/glass

0.41

8

200

Plastic

0.48

200

103


Note that the numerical aperture increases with the losses. Naturally, the larger the NA, the easier it is to couple into a fiber. The larger the core diameter, the easier it is to splice.

In the next section we discuss a communication system.

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