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I've read at wikipedia about "Single-mode optical fiber" ( https://en.wikipedia.org/wiki/Single-mode_optical_fiber ) but maybe i have a misunderstanding at this point :

"Waves can have the same mode but have different frequencies. This is the case in single-mode fibers, where we can have waves with different frequencies, but of the same mode, which means that they are distributed in space in the same way, and that gives us a single ray of light".

I know that a mode have a specific frequency, is this disagree with the words above ?

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No, a mode does not have a specific frequency, but a specific field distribution.

For each mode there is a minimum frequency that can be propagated (cut-off frequency) but above the cut-off frequency any frequency can be transmitted.

In a single-mode fiber, the diameter of the core is sufficiently small to have just one mode above cut-off in the range of frequencies of interest.

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  • $\begingroup$ So mode is it something different from the one that is referred to this : "A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation" (en.wikipedia.org/wiki/Normal_mode)?? $\endgroup$ – chaviaras michalis Mar 21 '17 at 10:05
  • $\begingroup$ @chaviarasmichalis No, it's not different, but pay attention that in that definition it is not said that the frequency is a property of the mode. $\endgroup$ – Massimo Ortolano Mar 21 '17 at 10:09
  • $\begingroup$ @Massimo_Ortolano Sorry but i can't see this , "These fixed frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies" $\endgroup$ – chaviaras michalis Mar 21 '17 at 10:16
  • $\begingroup$ @chaviarasmichalis An optical waveguide is not a resonant system and does not have resonant frequencies. The normal modes in a waveguide are normal modes of the field, which means that the (transverse) field distribution is the same everywhere. I suggest you to avoid Wikipedia and have a look at a book on optics ;-) $\endgroup$ – Massimo Ortolano Mar 21 '17 at 10:21
  • $\begingroup$ Thanks , would you like to suggest an introductory book which speaks about these things ? $\endgroup$ – chaviaras michalis Mar 21 '17 at 10:23

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