Optical fibers carry information as signals of light. The fibers (see illustration) comprise two layers of the highest quality glass. The inner, glass core (1) of a fiber optic cable is surrounded by an outer cladding of glass (2) of a different refractive index. The cladding contains the light pulses within the core. The light signal cannot leave the core because it always hits the edge of the core at too shallow an angle to escape – an effect known as total internal reflection. A sheath (3) provides physical protection, and bundles of the sheathed cores (trunk cables) are given strength by a central steel wire (4). Narrow-core fibers (A) are increasingly used because they allow signals to be sent over greater distances without blurring. In a wide-core cable (B) more reflections occur, causing the pulse to spread out and merge with adjacent pulses. To prevent this, more space must be left between pulses in wider cables and that limits the volume of data that can be transmitted.
Related category TECHNOLOGY
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