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Index of refraction is a measurement of speed of light in material. By definition, vacuum has refractive index of 1(c in vacuum = 1). The higher the index, the slower the light will travel in the material. Typical value for fiber core is 1,62 and for the cladding 1,52.
Total internal refraction:
When light travelling inside material hits a boundary, it is partially refracted and partially reflected. However if the angle is greater than the critical angle of the materials them it is reflected back in its entirety. For this to occur the material in which the light is travelling in must have higher refractive index than the material outside.
Multi-mode fiber:
In optical fiber technology, multimode fiber is optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core. Multimode fiber transmission is used for relatively short distances because the modes tend to disperse over longer lengths.
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Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit to 275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or 1300nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet 914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fiber in new applications using Gigabit and beyond.
Single-mode fiber:
In optical fiber technology, single mode fiber is optical fiber that is designed for the transmission of a single ray or mode of light as a carrier and is used for long-distance signal transmission.
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