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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?

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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.

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  • well that did not answer my question .. are they like sent by 3 source of laser? like parallel vs serial ?? Mar 13, 2015 at 2:31
  • You still don't get it. It's ONE laser, one stream of light. For MM, the laser (usually cheaper LED source) is many photon "lines" wide, not perfectly parallel or fired perfectly into the fiber.
    – Ricky
    Mar 13, 2015 at 2:35
  • Fiber optic communication isn't done on a per-photon basis.
    – Ricky
    Mar 13, 2015 at 2:36
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    Copper wire and fiber are completely different. The number of photons doesn't matter, there isn't any direct correlation to the amount of information. The picture is showing the way the light moves through the fiber, that's it. Mar 13, 2015 at 3:56
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    When you turn on a flashlight, does only one photon come out of it? No. Lots of them are generated in lots of directions. Laser's generate lots of photons, mostly in one direction. (the better the laser, the more parallel they are.)
    – Ricky
    Mar 13, 2015 at 4:55
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"Multimode fiber is like shooting a shotgun down a conduit - you get a lot of pellets into it, but they tend to bounce of the walls so they don't go very far and they aren't very accurate.

Singlemode fiber is like firing a rifle down the conduit - it's just a single bullet but it will go straighter, faster, and further than the pellets. " - Jim Jachette

This is the explanation that made sense to me. Both modes are one shot of light.

As for why the two modes exist.

Singlemode equipment use to be very expensive. Multimode equipment was alot cheaper than Singlemode. So it made sense to use multimode on the short distances.

Today the equipment prices and fiber prices themselves are almost equal. People are just hung up on multimode or need to maintain existing multimode networks.

If you are building a new fiber network, just go for Singlemode.

Hope this helps.

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