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?