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I am looking into some possible applications of side glow optical fibers (plastic). Will side glow fibers "drive" adjacent fibers? That effect is undesired.

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    $\begingroup$ Qualified "yes" : it depends on how close the other fibers are and how much crosstalk you can tolerate. A detailed calculation is quite complicated due to the circular cross-section (and thus rapidly varying angles of incidence and exitance). Why do you want to use high-loss fibers in the first place? <-- What I'm saying is, "Tell me the problem you're trying to solve, not how you want to solve it" $\endgroup$ Nov 28, 2014 at 18:45
  • $\begingroup$ The problem is related to the guitar. Fretboards have markers on the side that the player can see (3,5,7,9, etc). But they can be hard to see in low light. If one uses a capo to change the key, then the markers are useless. I could use one side glow fiber in a channel below the markers to illuminate the marker positions. I want to use up to 4-5 more to illuminate "shifted" positions relative to the capo. Each fiber would be "coded" with some kind of cladding to produce the shift. But I worry about crosstalk. $\endgroup$
    – user38780
    Nov 28, 2014 at 20:08
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    $\begingroup$ I'd strongly recommend against dealing with side glow, and going with a separate fiber mounted inside the fretboard so its open end (probably with a small lens) replaces the existing painted dots. BTW, the dots are not "useless", as they continue to indicate the actual note played. You're confusing the open string note with and without capo, and the resultant desired chord fingering positions here. But may I recommend you hop over to music.stackexchange? You may find folks who have done stuff like this. $\endgroup$ Nov 28, 2014 at 20:14
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    $\begingroup$ postscript: you should learn to play without the need for such visual clues. Just ask any violinist or cellist! $\endgroup$ Nov 28, 2014 at 20:15
  • $\begingroup$ I will check out that exchange, thanks. I can play without clues. I just got off on a tangent thinking about this. Shifted markers are indeed useful. But we may be talking about different things. $\endgroup$
    – user38780
    Nov 28, 2014 at 20:25

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I started answering this assuming that you were talking about scintillating fiber. I'm not so sure that this is the case anymore, but I'm leaving the answer none-the-less


Short answer: On straight runs the answer is no on geometric grounds, but any curvature can allow some degree of cross-talk.

Explanation

Any photon going into a right cylindrical geometry will hit the outward boundary at the opposite geometry that it had coming in (this is easy to see in a right-rectangular context but also true in the cylindrical context), so will pass out of the fiber again by time-reversal symmetry.

However, if there is any curvature the above consideration does not apply and some photons entering the fiber on the outside of a curve can be internal reflected and be transmitted longitudinally.

That said, I've seen these types of fibers deploy in unwrapped bundles on more than one occasion. As long as the radii of curvature are large the amount of cross-talk is likely to be pretty small and can probably be empirically measured.

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  • $\begingroup$ I can do this with straight runs only. I guess I'll purchase some and do some tests. I did consider end glow, but that would require up to 22 fibers. Also, space is limited. 1/4" thickness and 3/4" wide space between fret board edge and truss bar channel. And I really want to use as little of this as possible. Fiber is not scintillating, but soft side glow. There are no surface imperfections. Impurities cause the glow. (neon tube effect) $\endgroup$
    – user38780
    Nov 28, 2014 at 20:13

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