In reply to Jorge Olenewa.

One way to think about modal dispersion is using the ray model of light, since each mode has a characteristic velocity in step index multi-mode fiber, it’s like different modes are distinct rays of light entering the fiber at different angles. Since each ray is traveling a different path this causes the instantaneous pulse to spread out over time. A good analogy is that of runners in a marathon race. At the start of the race the runners are tightly packed together crossing the start line. By the end of the race runners are widely dispersed and the gap between the winner and the last runner can be an hour or more. Here is a wikipedia article on modal dispersion. https://en.wikipedia.org/wiki/Modal_dispersion