I am working with fiber tapers and I noticed that some light escapes from the tapering region and I am quite curious. I am using a 500 micron plastic optical fiber and I managed to stretch the fiber enough that the PMMA core is in contact with air and I can see some light escaping.

Supposing that the core is $n=1,49$ and the resin around it is $n=1,45$, according to the critical angle, all beams inside the core are at a incidence angle of $76,7°$.

Since the air is approx $n=1$, when changing the medium around the core from resin ($n=1,45$ - critical angle $76,7°$) to air ($n=1$ - critical angle $42,1°$) I believe that there should not be any light escaping from the taper. So why does it?

  • 2
    $\begingroup$ I can't make total sense of your post, but how thick is the resin? The reflection inside these tapers does not occur according to geometric optics but an exponentially dampened wavefront enters the material with the lower index of refraction. If it's too thin, that wave will "leak", so to speak. $\endgroup$
    – CuriousOne
    Commented Jun 10, 2015 at 19:09
  • $\begingroup$ It might be useful to include a diagram of your set-up. $\endgroup$
    – Kyle Kanos
    Commented Jun 10, 2015 at 19:38
  • $\begingroup$ My problem is that i have limited knowledge in this area, so looking at it from the geometrical optics point of view kind of helps. Searching on the internet i found some specifications: 500 micron fiber, 480 microns core diameter. Does it help? -----> postimg.org/image/g2pxssynr (Comparison of a regular fiber and a fiber taper with a 532nm laser) s23.postimg.org/r4v0r8sqj/IMG_20141118_151745206.jpg (fiber sticked to the table, huge loss of light) postimg.org/image/ytrv2yt87 (fiber taper profile) $\endgroup$
    – user83349
    Commented Jun 19, 2015 at 15:24

1 Answer 1


It is possible that in your fiber there is not only guided mode, but also a leaking one - with a larger angle, due to some coupling between modes or due to injecting the leaking mode from the source.


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