Dispersion occurs when a pulse of light is spread out during transmission on the fiber. A short pulse becomes longer and ultimately joins with the pulse behind, making recovery of a reliable bit stream impossible. (In most communications systems bits of information are sent as pulses of light. 1 = light, 0 = dark. But even in analogue transmission systems where information is sent as a continuous series of changes in the signal, dispersion causes distortion.)
1. Types of dispersion
There are many kinds of dispersion, each of which works in a different way, but the most important three are discussed below:
1.Material dispersion (chromatic dispersion):-
Both lasers and LEDs produce a range of optical wavelengths (a band Of light) rather than a single narrow wavelength. The fiber has different refractive index characteristics at different wavelengths and therefore each wavelength will travel at a different speed in the fiber. Thus, some wavelengths arrive before others and a signal pulse disperses (or smears out).
2. Intermodal dispersion (Mode Dispersion):-
When using multimode fiber, the light is able to take many different paths or “modes” as it travels within the fiber. The distance traveled by light in each mode is different from the distance travelled in other modes. When a pulse is sent, parts of that pulse (rays or quanta) take many different modes (usually all available modes). Therefore, some components of the pulse will arrive before others. The difference between the arrival times of light taking the fastest mode versus the slowest obviously gets greater as the distance gets greater.
3. Waveguide dispersion:-
Waveguide dispersion is a very complex effect and is caused by the shape and index profile of the fiber core. However, this can be controlled by careful design and, in fact; waveguide dispersion can be used to counteract material dispersion.
Dispersion in different fibers:
Mode dispersion > .material dispersion > waveguide dispersion