Polarization Maintaining optical fibers
Dispersion is the fact that light pulses spread out while they travel along a fiber. This fact occurs because the speed of light in the fiber depends on its wavelength (color of the light) and the propagation mode.
The light pulses in optical fibers are actually composed of a small range of wavelengths (colors). Practically, no light source can generate a pure single colored light. They always generate light in either narrow wavelength range (such as a semiconductor laser) or relative broad range (such as a LED).
Different wavelength of light travel in optical fibers at different speed. This means some light arrive at the receiver a bit later then others. This fact makes the received light pulses broader than at the transmitter side. This pulse broadening (spread out) is called dispersion.
Dispersion can also be caused by multimode transmission (different mode travels at different speed), the dependence of refractive index on wavelength, variations in waveguide (optical fiber) properties with wavelength, and transmission of two different polarizations of light (PMD) through single mode fibers.
:: Dispersion’s impact on bit rate in fiber optic digital communication system
Like power loss in a fiber optic link (attenuation), dispersion can limit the distance a lightwave signal can travel through an optical fiber. But different than attenuation, dispersion does not weaken a signal, it makes the signal blurry.
For example, if you send out a 1 millisecond width pulse but the pulse spreads to 10 milliseconds at the end of the fiber, then signals blur together in time that the signal becomes unintelligible.
The degree of signal blurry (signal overlap) at which pulse dispersion causes problems in digital systems depends on the design. But one rough guideline for estimating maximum bit rate is that the interval between pulses should be four times the dispersion (signal delay). It can be given as Maximum Bit Rate = 1/(4 x Dispersion).
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