: What is dispersion in optical fibers?  

Dispersion is a simple fact that light pulses distribute whenever they travel over a fiber.  This simple fact happens because the speed of light from the fiber is dependent upon its wavelength (color of the light) and also the propagation mode.

The light pulses in optical fibers are in fact made up of a small assortment of wavelengths (colors). You can even ask the optical design consultant via https://izakscientific.com/optical-design-consultant/  to know more about optical illusions. 

Virtually, no light source may create a pure only colored light. They constantly create light in a narrow wavelength range (like a semiconductor laser) or a comparatively broad range (for instance, a LED).

: Dispersion's effect on bit speed in fiber optic electronic communication system

Like electricity loss in a fiber optic connection (attenuation), dispersion could restrict the space a lightwave signal could travel via an optical fiber.  

But distinct from attenuation, dispersion doesn't weaken a sign, it creates the sign blurry. By way of instance, if you send out a 1-minute diameter pulse, however, the pulse spreads into ten milliseconds at the end of the fiber, then indicates blur together in time which the sign becomes unintelligible.

: What's polarization mode dispersion then?  

1.   The electrical field vector of both manners are perpendicular to one another, or known as orthogonal.  Normally both polarization modes act exactly the same from the fiber that means they can't be distinguished.

But that's merely the concept with great symmetrical fiber and no external force on the fiber.  Since the entire world isn't ideal and neither is that the fiber, both of these polarization modes do act differently in actual world fibers.

Stresses inside the fiber and external forces applied to the fiber trigger the refractive index of glass to fluctuate marginally for those two polarization modes.