How Optical Fibers Work
Fiber optics is one of the newer buzzwords these days. Optical fiber has a
number of advantages over the copper wire used to make connections
electrically. For example, optical fiber, being made of glass (or sometimes
plastic), is immune to electromagnetic interference, such as is caused by
thunderstorms. Also, because light has a much higher frequency than any radio
signal we can generate, fiber has a wider bandwidth and can therefore carry
more information at one time.
But just how does it work? We're talking about a thin, flexible "string" of
glass. Looking sideways at it, we can see right through it. How can we keep
light that's inside the fiber from getting out all along the length of the fiber?
Consider an ordinary glass of water. We know that if we look through the
water at an angle, images will appear distorted. This happens because light
actually slows down a little bit when it enters the water, and speeds
up again when it moves back into the air again.
Since the light has a slight but measurable width, if it hits the
water at an angle, the part of the light that hits the water first will
slow down first. The result is that the direction the light is traveling
changes, and the path of the light actually bends at the surface of
the water.
No matter what angle the light is traveling as it approaches the
water, it will take a steeper angle once it actually enters the water.
You can see this at any time by looking at a picture or newspaper
through a glass of water, and by looking at different angles. Even a
straw in a glass of water looks bent, although it really isn't. This
phenomenon is called refraction.
The same phenomenon happens with glass, although we don't
usually notice it when looking through a window. Nevertheless, light
striking the glass at an angle bends as it slows down within the glass,
and then bends again as it speeds up when leaving the glass. You can
see this phenomenon clearly if you slide a piece of flat glass over the
print in a book or newspaper.
Any substance that light can travel through will exhibit this
phenomenon to some extent. Glass happens to be a very practical choice for
optical fiber because it is reasonably strong, flexible, and has good light
transmission characteristics.
The question to be answered now is, "How can we use this phenomenon to
keep the light inside the glass, especially if we want to bend the glass (with the
light still inside) around corners?"
Now, consider looking into a glass of water from below the surface of the
water. If you look up through the bottom of the glass, you will see a somewhat
distorted view of the ceiling or whatever is above the glass. However, if you
look in from the side of the glass and observe the underside of the top surface,
you will begin to note an interesting and useful effect.
If you are looking up from a steep angle, the light you see entered the top
surface of the water at a shallower angle, as shown on the left. However, as you
look at the underside of the top surface from a shallower angle, as shown on the
right, you will find a point at which light can't enter the top surface at a yet
shallower angle. At this point, the top surface of the water looks like a perfect
mirror, even though you know it isn't.
Now, the light you see is
reflected from the surface,
rather than being refracted
through it. This effect
persists for all angles
shallower than the critical
angle at which the
phenomenon first appears.
As you might expect, the
same phenomenon is
exhibited by glass or any
other material through
which light might pass.
Consider a single glass fiber, such as the one shown in an enlarged view here.
The actual fiber is so thin that light entering one end will experience the
"mirror effect" described earlier in this discussion every time it touches the wall
of the fiber. As a result, the light will travel from one end of the fiber to the
other, bouncing back and forth between the walls of the fiber.
This is the basic concept of optical fibers, and it correctly describes the
fundamental operation of all such fibers. Unfortunately, it is not possible to use
fibers of this basic construction for any practical application. The reason for
this has to do with the physical realities of the phenomenon of reflection within
the fiber, and how the parameters involved will change under different
conditions.
The basic fact governing the reflection of light within the fiber has to do
with the speed of light inside the fiber, and the speed of light in the medium
just outside the fiber. Every possible material through which light can pass has
a characteristic called the refractive index, which is a measure of the speed of
light through that material as compared to the speed of light in open space. We
won't get into the mathematics in this demonstration; it is only necessary for
you to understand this concept.
One of the requirements of an optical fiber is that its diameter remain
constant throughout its length. Any change in the thickness of the fiber will
affect the way light reflects from the inner walls of the fiber. In some cases, this
could even mean that the reflected light could exceed the critical angle required
for total reflection, and so be lost through the walls of the fiber.
Unfortunately, the same effect will be noticed if the characteristics of the
medium outside the fiber should change. For example, if the fiber gets wet (as
it would in rain, fog, or some underground situations), the characteristics of the
boundary between the inside and the outside of the fiber will change, and hence
the effective shape of the fiber will change, and will keep changing as drops of
water move along the surface of the fiber.
The question now is, "How can we make the fiber so the boundary layer is
permanently fixed and precisely predictable?"
The easiest way to ensure that the boundary between the inside of the fiber
and the outside of the fiber remains constant and unchanging no matter what is
to create a permanent boundary of known characteristics. The practical
approach is to surround the glass fiber with another layer of glass, while
making sure that the speed of light in the outer layer remains faster than the
speed of light in the inner fiber. The result is shown here.
In this figure, the original fiber is now the core of a two-layer construct. The
diameter of the core is kept constant, at approximately 50 to 60 µm
(micrometers, at one time designated "microns") and its surface is kept as
perfectly smooth as possible. The outer layer, known as cladding, is bonded at
all points to the surface of the core.
To the outside world, this construction is effectively one solid piece of glass,
even though it is constructed of two different types of glass. Thus, it is
impervious to water, dirt, and other materials. If the outer surface gets wet, that
makes no difference because it still doesn't affect the boundary between the
core and the cladding. The whole composite fiber may be covered with rubber
or plastic for easier handling and visibility.
This type of optical fiber is known as a multi-mode step-index fiber, because
of the fixed and definite boundary, or step, between the core and the cladding,
as well as the fact that light traveling through the fiber may assume any of
several possible electromagnetic "modes." This is the first successful type of
optical fiber that was developed. Since then, more advanced types of optical
fibers, such as graded-index and single-mode fibers have been produced.