Network Sales and Services Handbook (Cisco Press Networking Technology)
The following topics are covered in this chapter:
Optic Bands and Transmission Windows
SONET
DWDM
The foundation of any optical system is the fiber optic cabling. Basic fiber optic cables consist of the following components:
Silica core This is the center of the cable.
Cladding Cladding is also at the center of the cable; this and the silica core are used for carrying the optic, or lightwave (l), signal
Coating, strength members, and a plastic jacket These enclose the fiber, providing the necessary tinsel and scratch resistance to protect the fibers.
Transceivers These are attached to both ends of the core for emitting and receiving the light pulses that form the information bits in the optical network. The capability of clear glass to contain light is the key behind optical transmissions and is based around the principle of total internal reflection.
Fiber optic transmission operates by injecting light at a specific angle. The glass cladding acts as a mirror, reflecting light within the fiber optic silica core. Significant in this light transmission is the Refractive Index (RI), or the change in the speed of light in a substance (in this case, silica) relative to the speed of light in a vacuum.
Light travels at about 300,000 kilometers (km) per second in a vacuum. When light moves from a substance of lower density to one of higher density such as light moving from air to water the light changes and is refracted. Refraction is the phenomenon that makes a stick appear to bend when one half is placed in water.
The "bending" of the light beam depends on two things:
The angle at which light strikes the water
The RI
At some point an angle is formed so that the light reflects off the water like a mirror. This angle is the critical angle, and the reflection of light is the total internal reflection.
This reflection principle determines how light propagates down a clear fiber. Fiber optic strands are manufactured so that the core contains a higher RI than the surrounding cladding. When light travels through the core and hits the cladding at a particular angle, it stays in the fiber. The exact size of the angle depends on the difference in RI; however, if a typical RI difference of 1 percent is assumed, all light striking the cladding at eight degrees or under will continue on in the fiber.