Solution:

Speckle Patterns:

A speckle pattern is a map of the energy pattern at the end of a multimode fiber. The pattern is a characteristic of laser transmission and does not happen with LED sources.

It appears as a number of almost randomly distributed areas of high energy with spaces in between. This is illustrated in Figure,

When the observation is made close (within a few meters) to the transmitter you get a very clear pattern where the spots are well-defined.

When you get further away from the transmitter the spots become less and less well-defined and finally, the fiber end face becomes almost uniformly illuminated. This change is caused by mode dispersion over the transmission distance.

Light from an LED produces almost uniform end-face illumination close to the transmitter.

Speckle patterns are caused by interference between modes as they emerge from the end of the fiber.

They are a good way to tell how much coherence exists between light in various modes.

This is of course not true. The speckle pattern is just an easy way of looking at what is happening within the fiber.