05-03-2013, 04:09 PM
QCC TechASCEND
Fiber Optics Experiments.pdf (Size: 1.38 MB / Downloads: 107)
Introduction
to Fiber Optics Cables—Stripping and Cleaving
Optical fiber has changed telecommunications all over the world. Because a single optical
fiber can carry really huge numbers of telephone conversations, long distance calls that used
to be very expensive have become cheap enough for many people to make them often. How
optical fibers carry multiple conversations is fascinating.
How does the longdistance
digital telephone system work?
When you talk into a telephone transmitter, the sound waves from your voice move a
diaphragm on the transmitter. This causes electricity to flow. The electric signal varies with
time in a way that imitates the way the air pressure varied in the sound wave from your
voice. In the modern telephone system, the (changing) strength of this electric signal is
measured several thousand times each second. The measured strength is then made into a
code. If you knew how to interpret this code, you could get the strength of the electric
signal as time went forward during the conversation that caused the signal. This code is
what is sent through the telephone system. In the telephone system, the person at the other
end of the conversation does not need to know the code. Instead, the machinery at the
receiving end of the system reads the code and converts it back into an electric signal that
reproduces the original electric signal from the transmitter. This electric signal goes to a
speaker, where it causes the air to vibrate just the way it did when the person on the other
end talked into the transmitter. The listener then hears a faithful reproduction of the original
conversation. Amazingly, all of this takes place so fast that the listener thinks that s/he is
hearing the original talking as it is happening.
Where does fiber optics enter the longdistance
telephone system?
In a fiber optic system, the code that is sent through the system is not electrical. Instead, the
electrical code is changed into a code based on laser light. It is this light that travels through
a fiber optic cable nearly to its final destination. Only when the signal gets quite near its
destination is it changed back into an electric code and from that into the electric signal that
operates the speaker in the receiving phone.
Optical fiber
For much of modern telecommunication, the path over which the signals travel is optical
fiber. Optical fiber for most purposes is made of a very special kind of glass that is drawn
into a very thin, long fiber. In some ways, this is similar to the fiberglass that is used for
insulation in homes. Unlike fiberglass, however, optical fiber is made of a much different
kind of glass and comes in lengths that may be many kilometers long.
Standard optical fiber is shaped like a very long thin cylinder. In the center of the cylinder
there is a core, and surrounding the core is a layer called the cladding. Both core and
cladding are glass; they are slightly different types, however. A cross section of the fiber is
Fiber cross section
In Figure 1, the diameter of the core is half the diameter of the cladding. This is typical of
one type of fiber. In a slightly different type of fiber, the core diameter is 0.4 time that of the
cladding. Both of these are called multimode fiber. A third type of fiber is used for very
long distance telecommunication. Its core diameter is about 1/10 the diameter of its
cladding. This type of fiber is called single mode fiber. There is one thing about Figure 1
that is very misleading—its diameter. The outside diameter of all standard real optical fiber
is 125 microns. A micron is 1/1,000,000 of a meter, or 1/1000 of a millimeter. This means
that the outside diameter of standard optical fiber is only 1/8 of a millimeter. That is really
small. In fact, it is about the same as the diameter of a single hair of a typical human being.
Furthermore, the light used in the telecommunication system travels in the core. The
cladding is necessary to keep the light in the core (and to make the fiber stronger and easier
to handle). It is quite amazing that the diameter of the lightcarrying
part of single mode
fiber is about 1/10 the diameter of a human hair.
We will examine two samples of fiber, one multimode and one singlemode, using a
microscope. Notice the difference between the two types of fiber.
Stripping and cleaving optical fiber
We will also learn some skills necessary for working with optical fiber. Fiber comes with a
thin plastic coating (called the buffer) to protect it. Before we can join two fibers together,
we must first remove (or strip) the coating. Although you can actually strip fiber with an
Xacto knife, it is much easier to do it with a fiber stripping tool. To strip with a knife takes
practice. Use a very sharp knife, such as an Xacto knofe. The plane of the blade should
make a small angle with the fiber, not over about 20 degrees.