3.3 OPTICAL FIBER

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3.3 OPTICAL FIBER

Optical fiber is now being deployed extensively and is the most preferred medium for all types of networks because of the high data rates that can be supported. Light in a glass medium can carry more information over large distances, as compared to electrical signals in a copper cable or a coaxial cable.

Optical fiber is the most attractive transmission medium because of its support for very high data rates and low attenuation.

The challenge in the initial days of research on fiber was to develop glass so pure that at least 1% of the light would be retained at the end of 1 kilometer. This feat was achieved in 1970. The recent advances in fiber have been phenomenal; light can traverse 100km without any amplification, thanks to research in making purer glass. With the state of the art, the loss will be about 0.35dB/km for 1310 nanometers and 0.25dB/ km for 1550nm.

Light transmission in the fiber works on the principle that the light waves are reflected within the core and guided to the end of the fiber, provided the angle at which the light waves are transmitted is controlled. Note that if the angle is not proper, the light is refracted and not reflected. The fiber medium has a core and cladding, both pure solid glass and protected by acrylate coating that surrounds the cladding.

As shown in Figure 3.2, there are two types of fiber: single mode and multimode. Single mode fiber has a small core and allows only one ray (or mode) of light to propagate at a time. Multimode fiber, the first to be commercialized, has a much larger core than single mode fiber and allows hundreds of rays of light to be transmitted through the fiber simultaneously. The larger core diameter allows low-cost optical transmitters and connectors and hence is cheaper.

Figure 3.2: Optical fiber.

Gigabits and even terabits of data can be transmitted through the fibers, and the future lies in optical fiber networks. Currently twisted copper wire is being used for providing telephones to our homes, but soon fiber to the home will be a reality.

The speed of transmission is 2 × 10⁸ meters/second in optical fiber.

We will study optical communication systems in detail in Chapter 14, "Optical Fibre Communication Systems."

The two types of optical fiber are single mode and multimode fiber. Single-mode fiber allows only one ray (or mode) of light to propagate at a time whereas multimode fiber allows multiple rays (or modes).

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