Optical fiber transmitting light gained lengthwise?

December 30, 2015 1:06 PM

Having a brainfart about bare optical fiber that is really bothering me. Tried to google this and while I think the answer is yes my google-fu is failing in a big way.

Light travels down an optical fiber by way of total internal reflection, right? Usually that light enters through one of the ends of the fiber. If you stripped off the protective coating along its length would light pass through the "top" of the fiber and would it then be subject to total internal reflection if it entered at the right angle (or rather if it hit the other side of the fiber at the right angle)?

posted by Slackermagee to Science & Nature (5 answers total)

i think you are asking if light entering through the side is then "trapped"? if so (it's not completely clear), then no, not necessarily. the animation here shows why, if you reverse the direction of the light (which is ok to do - the yellow lines describe how light works in either the direction of the arrow, or the opposite). you can see (i hope) that no incoming ray is at the angle such that it will be internally reflected from a parallel side.

it's tricky to explain well without a diagram.... (sorry).
posted by andrewcooke at 1:15 PM on December 30, 2015

Best answer: It's not the best source, but there's a Wikibook said to be by somebody named Faraaz Damji (Frazzydee) that says "Cladding also ensures that outside light does not get into the cable and interfer with the light transmitted." https://en.wikibooks.org/wiki/Optics/Fibre_optics

Cladding is the first layer of protective sheath, and it's reflective. The second is called the buffer coating, and protects against physical damage.

http://computer.howstuffworks.com/fiber-optic1.htm
posted by Sleeper at 1:24 PM on December 30, 2015 [1 favorite]

Best answer: just to clarify a little, there's no real conflict between these two answers (i hope).

what i was saying is that if you have a perfectly uniform, perfectly smooth piece of glass, with perfectly parallel sides, then the light moving from outside to inside can never be at the right angle to be perfectly internally reflected once it is inside. in other words, i gave the "theoretical" answer, using simple geometry (and snell's law).

in practice, because of imperfections, bends, and maybe other details i don't know about, you're going to want to protect the fibre anyway.
posted by andrewcooke at 2:57 PM on December 30, 2015 [1 favorite]

Response by poster: So, my thought was on a bent piece of optical fiber, so I guess in that case the answer is yes-but-only-for-light-coming-in-at-the-right-angles?
posted by Slackermagee at 3:39 PM on December 30, 2015

Best answer: it's easier to think backwards. if light can get in, then light can get out (you can "reverse time" in physics, roughly, and things still work correctly). and yes, light leakage from bent cables is a problem, so light can also enter bent cables.
posted by andrewcooke at 4:00 PM on December 30, 2015 [2 favorites]