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Single versus multi mode?

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Videogamer555

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What does this mean? I have heard lasers (usually diodes) described as being single or multi mode. I have also heard this terminology used to describe fiber optic cables. What does it mean?
 





If you consider a singlemode diode with it's single spot, there are physical limitations to the amount of power that can be produced within the small area of active material that produces that single spot. In order to get more power, more active area is added. This results in multiple spots, as if several singlemode diodes were stacked side by side, making a line instead of a spot.

The terms singlemode and multimode can be applied to ANY laser, with the end result being the same: You can only get so much power from a given singlemode cavity before you have to increase the cross-section of the cavity, which often results in more than one mode (or spot) in the beam, a cluster or line of spots.

There is much more to this, some of which I'll touch on later once I'm home from work.. the science behind emission modes is highly complex and interesting.
 
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^That's a good link.. pullbangdead's education revolves around laser diodes and their design/manufacture.. in fact he has studied directly under the inventor of GaN (blu-ray, blue, green) laser diodes.
 
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Singlemode fiber can only conduct one ray of light, so only one signal can be transmitted/received. Multimode allows multiple rays of light to travel at once, allowing simultaneous travel of multiple signals.
 
ElektroFreak said:
Singlemode fiber can only conduct one ray of light, so only one signal can be transmitted/received. Multimode allows multiple rays of light to travel at once, allowing simultaneous travel of multiple signals.
Click to expand...

What prevents multiple rays from entering? If I shine in a red and a green laser, a yellow beam will emerge from the other end.
 
I've never tried using fiber to combine colors, but it does stand to reason that you would get yellow out if you somehow put green and red in. I don't use fiber at all really except ready made data fiber, so there are limits to my working knowledge. Fiber coupling can be difficult, I know that much.
 
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I always thought multimode meant like wideband and would work with any wavelength, but singlemode meant it was tuned for a specific wavelength, allowing that wavelength to travel very long distances compared to other wavelengths (and other wavelengths got attenuated) much like a resonant waveguide.
 
You may well be at least partially correct. At this point I'd have to defer your questions to fiber experts, as I am not one.
 
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