This is an interesting question, to me anyway…
Critical angle issues, is mostly determined by the frequency being generated by the Laser being used…
The fiber itself has an inner-coating surrounding the fiber core itself, called Glass Cladding. It's designed to reflect light onto the fiber strand, or maybe keep the light onto the fiber strand…Light, as you may know, wants to move in all directions possible at once. But like a garden hose, you want to control it, move it around some, and keep it contained, which is what the cladding is designed to do…
The angle issues come into play, at the connector ends, if the glass ends that meet and touch each other, are too flat, some of the light may be reflected back, as a return loss measurement, maybe call it a glare, but it has a tendency to degrade the signal..
A lot of fibers being used to help with glare issues, and angle issues, are the APC type..(angle polished connector) The end of the fiber connector isn’t flat anymore, its actually cut at a 7-degree angle, and it needs to mate with a fiber cut the same way only in the opposite direction. This allows for a better transference of frequencies, and helps with reflected return loss issues…
Most or maybe all the frequencies used at AT&T, and probably telecommunications, in general, are in the C-Band range.
So, maybe fiber designs don’t have to be so crazy if you know the frequency range being used…Another reason for using C-Band frequencies is the Erbium-doped fiber, and Erbium-doped amplifiers that are used for long haul stuff…
