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Dispersion, Scattering and other unfavourable physical phenomena that may take place in optical fiber
Optical fiber as a medium got many virtues. Unlike copper media (e.g cat6e), WiFi (e.g 2,4 GHz, 5 GHz or Bluetooth) optical fiber is not exposed to EMI (electromagnetic pulse), interferences, signal interceptions. Usually is not limited by weather conditions, lay of the land (wifi), short length of a segment (cat5e). But even so sophisticated medium as optical fiber has got downsides caused by laws of physics.
You can’t cheat physical laws especially if you deal with optical fibre technology, then you have to rely on physical laws which describe how photon of light (the smallest part of the light) behaves in the glass (the silica to be more precise). I’ve collected all unfavourable phenomena and described them precisely but using plain language (believe me I tried hard).
Attenuation
Attenuation means a loss of optical power. Te attenuation of optical fiber is expressed by the coefficient which is defined as the loss of fiber per unit length in dB/km. Fiber attenuation varies significantly with the wavelength of light. The lower the wavelength is, the higher attenuation we get. The attenuation of the optical fiber is a result of two factors : Absorption and Scattering
Absorption is caused by the absorption of the light and conversion into heat by molecules in the glass. The most involved factor in absorption are OH- ions (just the water) which are just impurities which modify the refractive index of the glass. The smallest affects of OH- in silica glass has been noticed in transmission windows 850nm, 1300nm, and 1550nm. That’s the reason why these frequencies are being used in optical fiber technology. Unfortunately impurities are inevitable, manufacturers have only limited influence on this factor and can prevent partly by keeping high standards of production and by using high quality of raw materials.
Scattering occurs when light collides with individual atoms in the glass and is reflected in accidental directions. Light rays which are scattered with different angles outside the numerical aperture of the fiber will be absorbed by a cladding or transmitted back, toward the source (reverse reflection)
Dispersion
To find out how dispersion affects modes in optical fiber, first we have to recall in short how is build the core of optical fiber. We distinguish two types of multimode fibers step index and graded index. In step index fiber, the core has got the same density with the same index of refraction alongside whole fiber, in graded index fiber the core is not homogenic and is build from “layers”, every “layer” is doped differently, what means that index of refraction changes steadily from the axis of the core to the border with cladding. The work of both solutions you can see below on the graphic. In step index fiber, modes travel incoherently, in graded index fiber fluently. In single mode fiber dispersion doesn’t exists.
Modal dispersion
Every impulse of the light consists of many modes, almost each of them moves on different lane accordingly what I mentioned above . In graded index fiber modal dispersion has been reduced significantly, but in step index fiber it is a huge problem. In single mode fiber modal dispersion takes place in narrow scope and only in very long links.
Chromatic dispersion
Chromatic dispersion is linked with the color of the light and broad of spectral output. Light travels at different speeds accordingly to its color and therefore is refracted with different angles in different doped layers. Besides the LED light has much more broad of spectral output than laser, which concentrates most of its light in narrow spectral range. These two factors cause that the components of light (modes) travel in optical fiber with different speeds and on different paths.
PMD
Polarisation Mode Dispersion
This dispersion occurs because many external factors like wind, temperature, tension, stretching, which affect optic fiber. These factors cause that optic fiber is getting oval instead of staying circular. What influence does it have on modes ? Modes travel with different speed . PMD takes place in multimode and singlemode optical fibers and is very hard to diagnose, test and measure.