From: gapdrr@gsusgi2.gsu.edu (Daniel R T Rothman)
Subject: Re: Help on Fiber-optics
Organization: Georgia State University
Date: Sat, 12 Sep 1992 15:40:38 GMT

lth@nevada.edu  writes:


>Hello Everyone,

>I am looking for information and/or references on how fiber-optic network
>works.  If you do have any source(s), please reply to my account.  Thanks
>you very much in advance.
>						Lam.
>					

the basic mechanism of fiber optics is a phenomenon known as total internal
reflection. Any time a beam of light passes from one substance (or vacuum,
or whatever..) to another that has a different index of refraction, a certain
amount is reflected, and a certain amount is reflected. The proportion of
reflection:transmission is a direct function of 1) differenct (or maybe
ratio, i forget) in indeces of refraction and 2) angle of incidence. 

The principle thing we're interested in is the angle of incidence. as the
beam of light intersects the surface more obliquely, a greater fraction
is reflected. If you have a glass coffee table at home (i think you can
do it with a framed picture too....) you can demonstrate this effect by 
turning off all the lights and shining a flashlight (preferably a good tight-
beamed on..) and noting that some of the light is transmitted throught
the coffee table, and some reflected to the far wall. if you've got a good
light (like a laser) you'll actually see twin spots on the far wall. Some
of the light is bouncing off of the glass/air interface at the BOTTOM of the
table, and getting bounced back through the glass. As you tilt the flashlight
farther and farther, the light spot on the far wall should get brighter. I'm
not sure if you can actually perceive it... You can if you use a laser - no
special quality to the laser light for this purpose, it just makes the
effect more marked, because it's more tightly focused.

Now the trick with fiber optics is that light is shot down the MIDDLE of a
very thin piece of glass (special glass, but still essentially glass).
In fact, this glass is so thin that ANY light that went into it in the first
place hits a glass/air interface at a VERY oblique angle. How oblique? and
well you might ask. This angle is SO oblique, that all of the light reflects
back into the glass (remember the double-spot from the bottom side of the
coffee table?)  The next great trick here is that when it bounces from one
side of the glass tube, when it hits the other side, the same thing happens.

How, you ask, does this work when you wrap a fiber optic cable around a 
corner? This one may not be so terribly easy to conceptualize, but here
it goes. Remember please that the fiber optic cable is VERY thin. It's 
ALWAYS very thin, you don't build large ones because they don't "corner" very
well...  What happens is that the cable gets wrapped around the corner. The
light wants to go straight. But the light, in going straight will eventually
hit a glass/air interface. The key word in the preceeding sentence was
"eventually". The cable is thin enough that any time the cable is not
straight, the light can't hit the interface with a steep enough angle to
escape. Even if the light starts to "take the curve" on the steep inside,
the path is too narrow for it to hit the far wall at anything other than
a very shallow angle. Now you ask, what happens if you KEEP crimping the
cable tighter? It breaks. The thing is glass you know. I beleive that 
fiber optic cable is designed to maintain total internal reflection at
ANY curvature that can be achieved before material failure.


daniel rothman                        | gapdrr@gsusgi2.gsu.edu  Internet
computer specialist - GAP             | gapdrr%gsusgi2@gsuvm1   Bitnet
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