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)

:: What is polarization mode dispersion then?

1. Polarization mode and polarization mode dispersion (PMD)

In single mode fibers, light pulses are actually composed of two distinct polarization modes. The electric field vector of the two modes are perpendicular to one another, or called orthogonal. Normally the two polarization modes behave just the same in the fiber which means they can not be distinguished.

But that is only the theory with a perfect symmetrical fiber and no outside force on the fiber. Since the world is not perfect and neither is the fiber, these two polarization modes do behave differently in real world fibers.

Stresses within the fiber, and outside forces applied to the fiber cause the refractive index of glass to differ slightly for these two polarization modes. This phenomenon is called birefringence.

Birefringence makes these two polarization modes travel at slightly different speed. This speed difference broadens lightwave signal just as other dispersions and this fact is called Polarization Mode Dispersion (PMD).

2. PMD and its impact on single mode fiber optic systems

The potential effects of polarization mode dispersion became significant only a few years ago when high speed fiber optic digital communication systems came to play, such as the 40Gbit/s systems.

Polarization mode dispersion (PMD) is smaller in magnitude than other types of dispersions, but it is more difficult to compensate for, at least until now. PMD becomes a problem in systems with data rates higher than 2.5Gbit/s. PMD makes more challenge to sending higher data rates over long distance.