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How Fiber Optics Work

By: Craig Freudenrich, Ph.D. & Chris Pollette  | 

How Does an Optical Fiber Transmit Light?

fiber optics light signal
Diagram of total internal reflection in an optical fiber Online2Design.com/HowStuffWorks

Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straight down the hallway — light travels in straight lines, so it is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber.

The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances.

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However, some of the signal within the fiber is lost as it travels longer distances. The extent that the signal degrades depends on the purity of the glass, the number of bends in the fiber or splices that connect sections of fiber and the wavelength of the transmitted light.

For example, with multimode fiber, diameters of 850 nm = 3 dB/km; 1,300 nm = 1 dB/km. For single mode cable, 1,310 nm = 0.5 dB/km; 1,550 nm = 0.4 dB/km).

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