Information Technology Reference
In-Depth Information
Fiber-Optic Transmission
The phenomenon of the transmission of light through fiber-optic media comes from
the optical principles of refraction and reflection. As you know, a beam of light
bends when it passes from one medium into another. The most familiar example of
this is the air-water transition.
Fill a pan with water and place a coin at the midpoint of the pan's bottom.
Now, stand back from the pan and place a straw (or pencil) into the water, along
your line of vision to the coin. The straw appears to bend upward from the straight-
line path at the surface of the water. Place the straw into the water from the right or
left side, and you will see the same upward bend. The straw has obviously not bent,
but the reflected light from the part of the straw below the water's surface has bent
at the air-water interface, making the straw appear to bend.
This bending of light as it passes from material to material is called
refraction
.
Materials that transmit light have a property called the
index of refraction
1
that is pro-
portional to the amount of bending that would occur between a given material and a vac-
uum reference. Both air and water have respective indexes that indicate this behavior.
While the air-water refraction is trivial (unless you are spear fishing), the same
refraction principle is used in forming lenses, such as those used for vision correc-
tion, and can be used to collimate light, as well. As illustrated in Fig. 11.1, the path
of a ray of light bends at the interface between two transmissive materials. The
amount of the bend is determined by the index of refraction. However, beyond a
Less-dense
material
Reflected ray
Critical angle
Denser material
Refracted ray
Un-refracted path
FIGURE 11.1
Reflection and refraction of light at an interface.
1
The index of refraction is defined as the ratio of the speed of light in a vacuum to the speed of light
in the medium under consideration, thus
c / v
m
.
