Fiber Optic Multimode principle I'm trying to figure out how the multimode fibre optic really work, I looked in many sites but didn't really found an answer to my question; let's take an example this picture: !enter image description here in the multimode, are the 3 lights, the green, blue and red, are sent SIMULTANEOUSLY or these are the paths that could be taken?

That's not "three lights" (that's DWDM.) The colors are there to delineate _three photons_. In single-mode fiber, the photons flow perfectly parallel down the center of the fiber. In multi-mode, the light isn't as "perfect" and the fiber diameter is much wider. As a result, the photons don't flow perfectly parallel -- nor are they necessarily fired straight into the center of the fiber. Thus, the path of some photons are more oblique than others. The net result is a pulse of light entering in one end does not all reach the other end at the same time -- some paths are longer. This is known as "mode dispersion"; the longer the cable, the worse it gets.