Fiber Optic Reference Guide

David Goff

• Publisher: Routledge
• Year: 2002
• ISBN:
  9781136028175
• Edition: 3
• Topic: Languages & Linguistics
• Subtopic: Communication Studies
• Index: Languages & Linguistics

The Fiber Optic Reference Guide offers readers a solid understanding of the principles of fiber optic technology, especially as it relates to telecommunications, from its early days to developing future trends. Using a minimum of jargon and a wealth of illustrations, this book provides the underlying principles of fiber optics as well as essential practical applications. The third edition is updated to include expanded sections on light emitters, semiconductor optical amplifiers, Bragg gratings, and more systems design considerations. Fiber optics plays a key role in communications, as well as in broadcast and cable systems. Engineers working with fiber optics as well as newcomers to the industry will find the third edition of this reference guide invaluable. It will help the reader develop a solid understanding of the underlying principles of this rapidly changing technology as well as its essential practical applications. The text is thoroughly indexed and illustrated.

1 A HISTORY OF FIBER OPTIC TECHNOLOGY

THE NINETEENTH CENTURY

In 1870, the British Royal Society in London, England witnessed a thought-provoking demonstration given by natural philosopher, John Tyndall. Tyndall, using a jet of water that flowed from one container to another and a beam of light, demonstrated that light used internal reflection to follow a specific path. As water poured out through the spout of the first container, Tyndall directed a beam of sunlight at the path of the water. The light, as seen by the audience, followed a zigzag path inside the curved path of the water. This simple experiment, illustrated in Figure 1.1, marked the first research into the guided transmission of light.

Figure 1.1: John Tyndall's Experiment

William Wheeling expanded upon Tyndall's experiment when, in 1880, he patented a method of light transfer he called “piping light.” Wheeling believed that by using mirrored pipes branching off from a single source of illumination, i.e. a bright electric arc, he could send the light to many different rooms in the same way that water, through plumbing, is carried throughout buildings today. Due to the ineffectiveness of Wheeling's idea and to the concurrent introduction of Edison's highly successful incandescent light bulb, the concept of piping light never took off until late in the 20th century when several commercial systems were introduced. Wheeling's ideas were about a century ahead of the technology required to make “piping light” feasible.

That same year, Alexander Graham Bell developed an optical voice transmission system he called a photophone. Like Wheeling's method of light transfer, the photophone preceded by decades the technology required to make it a commercially viable idea. The photo-phone used free-space light to carry the human voice 200 meters. Specially placed mirrors reflected sunlight onto a diaphragm attached within the mouthpiece of the photophone. At the other end, mounted within a parabolic reflector, was a light-sensitive selenium resistor. This resistor was connected to a battery that was, in turn, wired to a telephone receiver. As one spoke into the photophone, the illuminated diaphragm vibrated, casting various intensities of light onto the selenium resistor. The changing intensity of light altered the current that passed through the telephone receiver which then converted the light back into speech. The technology to support this invention would not be available for many years, but Bell believed this invention was superior to the telephone because it did not need wires to connect the transmitter and receiver. It was, in fact, the world's first optical amplitude modulation (AM) audio link. Today, “free-space” optical links, similar in concept to Edison's photophone, find extensive use in metropolitan applications.

THE TWENTIETH CENTURY

Fiber optic technology experienced a phenomenal rate of progress in the second half of the twentieth century. Early success came during the 1950's with the development of the fiber-scope. This image-transmitting device, which used the first practical all-glass fiber, was concurrently devised by Brian O'Brien at the American Optical Company and Narinder Kapany and colleagues at the Imperial College of Science and Technology in London. (In fact, Narinder Kapany first coined the term “fiber optics” in 1956.) Early all-glass fibers experienced excessive optical loss, the loss of the light signal as it traveled the fiber, limiting transmission distances.

This motivated scientists to develop glass fibers that included a separate glass coating. The innermost region of the fiber, or core, was used to transmit the light, while the glass coating, or cladding, prevented the light from leaking out of the core by reflecting the light within the boundaries of the core. This concept is explained by Snell's Law which states that the angle at which light is reflected is dependent on the refractive indices of the two materials — in this case, the core and the cladding. The lower refractive index of the cladding (with respect to the core) causes the light to be angled back into the core as illustrated in Figure 1.2.

Figure 1.2: Optical Fiber with Cladding

The fiberscope quickly found application inspecting welds inside reactor vessels and combustion chambers of jet aircraft engines as well as in the medical field. Fiberscope technology has evolved over the years to make laparoscopic surgery one of the great medical advances of the twentieth century.

The development of laser technology was the next important step in the establishment of the industry of fiber optics. Only the laser diode (LD) or its lower-power cousin, the light-emitting diode (LED), had the potential to generate large amounts of light in a spot tiny enough to be useful for fiber optics. In 1957, Gordon Gould popularized the idea of using lasers when, as a graduate student at Columbia University, he described the laser as an intense light source. Shortly after, Charles Townes and Arthur Schawlow at Bell Laboratories supported the laser in scientific circles. Lasers went through several generations including the development of the ruby laser and the helium-neon laser in 1960. Semiconductor lasers were first realized in 1962; these lasers are the type most widely used in fiber optics today.

Because of their higher modulation frequency capability, the importance of lasers as a means of carrying information did not go unnoticed by communications engineers. Light has an information-carrying capacity 10,000 times that of the highest radio frequencies being used. However, the laser is unsuited for open-air transmission because it is adversely affected by environmental conditions such as rain, snow, hail, and smog. Faced with the challenge of finding a transmission medium other than air, Charles Kao and Charles Hockham, working at the Standard Telecommunication Laboratory in England in 1966, published a landmark paper proposing that optical fiber might be a suitable transmission medium if its attenuation (loss of signal strength) could be kept under 20 decibels per kilometer (dB/km). At the time of this proposal, optical fibers exhibited losses of 1,000 dB/ km or more. At a loss of only 20 dB/km, 99% of the light would be lost over only 3,300 feet. In other words, only 1/100th of the optical power that was transmitted reached the receiver. Intuitively, researchers postulated that the current, higher optical losses were the result of impurities in the glass and not the glass itself. An optical loss of 20 dB/km was within the capability of the electronics and opto-electronic components of the day.

Intrigued by Kao and Hockham's proposal, glass researchers began to work on the problem of purifying glass. In 1970, Drs. Robert Maurer, Donald Keck, and Peter Schultz of Corning succeeded in developing a glass fiber that exhibited attenuation at less than 20 dB/km, the threshold for making fiber optics a viable technology. It was the purest glass ever made. Concurrent advances in laser technology, semiconductor chips, optical detectors, and optical connectors, combined with the optical fiber, ushered in the true beginnings of the fiber optic communications industry.

The early work on fiber optic light sources and detectors was slow and often had to borrow technology developed for other reasons. For example, the first fiber optic light sources were derived from visible indicator LED's. As demand grew, light sources were developed for fiber optics that offered higher switching speed, more appropriate wavelengths, and higher output power.

Fiber optics developed over the years in a series of generations that can be closely tied to wavelength. Figure 1.3 shows three curves. The top, dashed, curve corresponds to early 1980's fiber, the middle, dotted, curve corresponds to late 1980's fiber, and the bottom, solid, curve corresponds to modern optical fiber. The earliest fiber optic systems were developed at an operating wavelength of about 850 nm. This wavelength corresponds to the so-called “first window” in a silica-based optical fiber. This window refers to a wavelength region that offers low optical loss. It sits between several large absorption peaks caused primarily by moisture in the fiber and Rayleigh scattering, the scattering of light particles due to impurities in the glass, at shorter wavelengths.

Figure 1.3: Four Wavelength Regions of Optical Fiber

The 850 nm region was initially attractive because the technology for light emitters at this wavelength had already been perfected in visible indicator LED's. Low-cost silicon detectors could also be used at the 850 nm wavelength. As technology progressed, the first window became less attractive because of its relatively high 3 dB/km loss limit.

Most companies jumped to the “second window” at 1310 nm, but a few companies, notably IT&T, spent effort developing the wavelength region near 1060 nm. The 1060 nm region allowed low-cost silicon detectors to be used; however, the light emitter technology was more complex than 850 nm. This region did offer lower attenuation, about 1.7 dB/km. Ultimately however, the second window at 1310 nm won out with lower attenuation of about 0.5 dB/km. In late 1977, Nippon Telegraph and Telephone (NTT) developed the “third window” at 1550 nm. It offered the theoretical minimum optical loss for silica-based fibers, about 0.2 dB/km.

Today, 850 nm, 1310 nm, and 1550 nm systems are all manufactured and deployed along with very low-end, short distance, systems using visible wavelengths near 660 nm. Each wavelength has its advantage. Longer wavelengths offer higher performance, but always come with higher cost. The shortest link lengths can be handled with wavelengths of 660 nm or 850 nm. The longest link lengths require 1550 nm wavelength systems. A “fourth window,” near 1625 nm, is being rapidly developed. While it is not lower loss than the 1550 nm window, the loss is comparable, and it might simplify some of the complexities of long-length, multiple-wavelength communications systems.

Most optical fiber is based on silicon, like most of today's electronics. There have been numerous attempts over the last few decades to develop alternate materials to either reduce cost or improve performance. To date, none have shown any real promise in dethroning silicon-based fiber. Much effort has gone into developing plastic fiber, but its impact on the marketplace has been minimal. It is suited for very short distances, typically around tens of meters only. As the cost of glass fiber has plunged over the last decade, the advantage of plastic fiber has faded.

The other big pu...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Foreword
• Chapter 1 A History of Fiber Optic Technology
• Chapter 2 Fiber Optic Fundamentals
• Chapter 3 Optical Fiber
• Chapter 4 Fiber Optic Cables
• Chapter 5 Light Emitters
• Chapter 6 Light Detectors
• Chapter 7 Other Active Devices
• Chapter 8 Interconnection Devices
• Chapter 9 Other Passive Devices
• Chapter 10 System Design Considerations
• Chapter 11 Fiber Optic Applications
• Chapter 12 Video Over Fiber
• Chapter 13 Data Over Fiber
• Chapter 14 Pushing Fiber to the Limits
• Chapter 15 Testing ABCDE Measurement Techniques
• Chapter 16 Future Trends
• Appendix A General Reference Material
• Appendix B Glossary of Terms
• Appendix C Fiber Optic Symbols
• Appendix D Industrial, Telcordia ABCDE ITU Standards
• Appendix E Societies, Conference Sponsors ABCDE Trade Magazines
• Index