Optical fiber has changed telecommunications all over the world. Because a single optical fiber can carry really huge numbers of teleph one 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 long distance 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 long distance 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.
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 justify 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 shown in Figure 1.
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 light carrying 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 single mode, 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.
You have to press hard enough to remove the plastic coating (buffer), but not hard enough to break the fiber. Since fiber is very thin, this takes lots of practice. Ask you instructor to demonstrate the technique.
Each group of participants will have a commercially available fiber optic stripper. As you will discover, this makes the task of stripping much easier. You should strip between 1/4 inch and 1/2 inch of the plastic coating each time you use the tool. Before you strip another 1/4 1/2 inch, you need to brush off the blade with the brush provided. If you do not do this, the coating will accumulate on the blade of the tool, and it will stop stripping the fiber. For most purposes, you will need to strip between 1 and 2 inches (2.55cm) of the buffer.
It is important to keep the work area clean and orderly. This is not just for appearance. A clean, orderly work area improves safety. Optical fiber is made of glass. Since it is very thin, little pieces of it act like rather nasty splinters. If they get into your skin or eyes, they can be painful and dangerous.
In many cases, the next step in preparing a fiber for several processes is to cleave the fiber. The purpose of cleaving is to prepare the end of the fiber so that it makes a very nearly perfect right angle with the cylindrical body of the fiber and that this end face is nearly perfectly smooth. You might think that the only way to achieve this would be to polish the fiber end while holding the fiber at a right angle to the polishing surface. Although this polishing method works (and is sometime necessary), for most applications a much simpler procedure is equally good. This procedure is called controlled fracture. To cleave a fiber using controlled fracture, we put a little stress on the fiber and make a very light scratch on it. When this is done correctly, the fiber will split, leaving ends that are very smooth and are perpendicular to the length of the fiber. Unfortunately, if you don’t do this correctly, you will crush the fiber, and the end will be jagged. To use a hand cleaver, bend the optical fiber around your index finger. (This puts the needed stress on the fiber.) Scratch the fiber lightly by gently touching it with a diamond, sapphire, or tungsten carbide tipped hand cleaver. Your instructor will demonstrate the technique. Please be sure to dispose of the small pieces of fiber that are left on the cleaver in the red covered fiber disposal units that should be on your desk or on the tray of equipment you will receive. You should practice as many times as you can, examining the results with the microscopes that will be available for your use. Your instructor may also show you how to use the semi-automatic cleavers that are in the laboratory. These devices produce an excellent cleave more than 95% of the time. Unfortunately, they are quite expensive. (They cost nearly $1500 each, as of 2001.)