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AN INTRODUCTION

Optical fiber is a solid strand of glass (or, in some cases, plastic conducts light in much the same way that copper wire conducts electricity or pipes conduct water. Light travels through the fiber by re from the fiber's inner surfaces. Because of the construction of t light that passes into the glass does not pass back out, but reflects stays in the fiber. The fiber is very thin and flexible, and, therefore it can be routed around corners and through small opening passing through the fiber can be used for illumination, for sensing changes in temperature or other information, or can be sensing, welding, or cutting (for example, the high-intensity laser beams). However, its most common and most useful function is for communications.

Communication is the process of conveying information from one place to another. In electronic communications, this is accompli imposing the information (signal) onto some form of energy through modulation. The modulated carrier passes through a (medium) and arrives at its destination where the information moved through demodulation. A simple example is a telephone in which a person's voice is the signal used to modulate the (electricity). The carrier passes through the medium of copper wire and then is demodulated and transmitted through a speaker of another telephone.

In today's information age, the need for fast, accurate, and high volume communication is paramount. Electronics and computers have permeated society so that they touch every aspect of our daily lives in some way. Bar code scanners read the prices of our grocery items, optical disks play our music, computers dispense money from automatic tellers, and robots controlled by computers assemble our manufactured items. We can use the personal computer for banking, real estate, accessing news and entertainment, and a hundred and one other functions that are an integral part of our life. This increase in electronic methods for manipulating, interpreting, and communicating information has revealed the limits of traditional communication technologies such as copper wire and radio waves. Optical fiber has been developed to overcome these disadvantages.

Optical fiber is a riverbed along which the huge stream of data genei-ated by modern technology flows. The bed is wide, deep, and capable of handling all the data we can pour into it without overflowing its banks. Originally, fiber was used only for long-distance telephone lines where the need for its greater capacity outweighed its expense and difficulties. As the technology developed and its cost dropped, the fiber became viable for ship and airplane systems, medium- and short-haul telephone lines, local area networks, and even cable TV. This widespread use of optical fiber has occurred because of its advantages over electrical signals in copper wire.

Optical fiber has several advantages over traditional transmission methods (see Table 1.1). Light has a much higher frequency than radio waves or modulated signals in copper wire. Because of this high frequency, light has a very high information-carrying capacity. Optical fiber can transmit this high-frequency carrier with a small amount of distortion or loss of power. Therefore, optical fiber is capable of carrying hundreds and even thousands of times as much information as copper wire. Many more normal telephone conversations can be routed through a single fiber (systems capable of transmitting over 300,000 telephone calls through a single fiber have been demonstrated), as can data that require more capacity such as computer communications, video signals, and even a combination of signal types.

1.2 Advantages and Disadvantages of Optical Fiber

Table 1.1 Advantages of Optical Fiber

Optical fiber does not emit any noise that might interfere with signals or might be picked up by eavesdroppers. Signals sent through copper wire radiate electromagnetic energy that can be picked up by powerful antennas or other devices. The only way to eavesdrop on a signal as it passes through the optical fiber is to actually splice in fiber, and even this is not nearly as simple as splicing into copper Although fiber optic's advantages outweigh its disadvantage most cases, the disadvantages should be considered. Perhaps the most significant of these is that optical fiber requires a new set of ski installation and maintenance by technicians. Technicians who are experienced in splicing, soldering, and installing copper wire will find fiber requires additional knowledge and training. Splicing and connecting optical fiber is a delicate skill that must be carefully learned to produce quality results (splices and connectors are discussed fully in Chapter 9).

Using optical fiber also means acquiring some specialized equipment. Volt-ohm-ammeters, oscilloscopes, and other devices used in testing and measuring electronic signals are replaced by optical power meters and optical time domain reflectometers. The methods and techniques for performing the tests and measurements are also changed (see Chapter 10).

These disadvantages mean that companies that are actively working in optical fiber must invest in equipment and training (or hire another company who already has). However, these challenges also offer opportunities for students studying to be technicians. Technicians trained in the methods of using, testing, and installing optical fiber will find many employment possibilities.

The use of fiber optics in communications has a fairly short history beginning in the late 1970s, but the principles of light in communications have been around much longer. The idea of using light to communicate evolved from simple signal fires and lamps (such as Paul Revere's famous "One if by land, two if by sea") to using mirrors to reflect the sun and transmit messages. The first modern attempt could be attributed to Claude Chappe and his optical telegraph built in France in the 1790s and to Alexander Graham Bell and his photophone patented in the 1880s (shown in Figure 1.1). Both devices relied on direct, line-of-sight transmissions of light. Bell's invention also incorporated the principles of modulating and demodulating the light.

Figure 1.1 Alexander Graham Bell's Photophone

Optical fiber used in communications is a solid glass strand (called the core) which is covered with another layer of glass (called the cladding). The fiber may have a distinct boundary between the core and cladding, or there may be a gradual change in materials between the two (more on this in Chapter 5). The dimensions of the fiber (diameter of core and cladding) vary depending on the application of the fiber, but these dimensions must be kept within tight specifications for a particular type of fiber. The type of glass used will also vary with application, but the purity (lack of any foreign materials as well as lack of imperfections such as cracks or bubbles) must also be kept very high. To produce quality fiber with these criteria in mind, manufacturers begin by making a preform.

A preform is a rod of glass containing the materials of the fiber and constructed the same way as the fiber but with much larger dimensions. The preform may be constructed by using the Modified Chemical Vapor Deposition (MCVD) process which uses a hollow tube and a stream of heated materials (known as the soot) passing through its center. The soot is used to form the inner part (or core) of the fiber, and it sticks to the walls of the tube. A torch follows along the outside of the tube and slightly behind the flow of soot and sinters the soot to the tube. Several passes may be run depending on the intended result. After the layers of soot are deposited, the torch moves along the tube at a slower rate and causes the tube to melt and collapse so that it forms a solid rod. The process is illustrated in Figure 1.2.

Figure 1.2 Making the Preform

Figure 1.3 The Drawing Process

to the preform, and then the molten end is pulled thro neck, producing the fiber. The size of the fiber is monitored pulled through the neck, and adjustments are made to the tem of the preform and the speed of the drawing so that the size of t is constant.