Low Voltage — Fiber Optics

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Article by Adam Corbin [ acorbin@corbinelectric.com ]

The History of Light Theories

Fiber optics is a concept that amazes many people. To fully understand the concept, the composition of light needs to be examined. Light has been characterized by six major theories over the past 3,000 years. The six theories are known as:

1. The tactile theory

2. The emission theory

3. The corpuscular theory

4. The wave theory

5. The electromagnetic theory

6. The quantum theory

The tactile theory was based on the ability to touch. The theory stated that the eye sent out invisible probes to "feel" objects. The emissions theory, however, was the opposite of the tactile theory. The emissions theory stated that bright objects sent out beams or particles that would ricochet off objects and enter the eye. The emission theory became generally accepted over the tactile theory by the eleventh century.

The next two theories that manifested were the corpuscular theory of Sir Isaac Newton and the wave theory of Christian Huygens. These theories contradicted each other. Newton corpuscular theory stated that light traveled in straight lines. He described light as a system of corpuscles or particles that were emitted in all directions from a source in straight lines. Newton's laws of physics were able to explain light reflecting off objects.

Huygens wave theory disagreed with Newton. Huygens argued that if light were made of particles, then two crossed beams would cancel each other. He compared this point to two streams of water interrupting each other. Light, however, did not exhibit this phenomenon. Huygens proposed that light was a wave. He described light and its actions completely based on a wave theory. His description, on the other hand, presented Newton's original objection that if light were truly a wave, light would bend around corners. Modern research, however, has since shown that light can bend around corners, but its wavelength is so short that it is difficult to detect under ordinary circumstances. The wave theory held through the 1800s and the particle theory was relatively ignored until about 1900.

At the end of the nineteenth century, James Clerk Maxwell combined electricity, magnetism and light into one theory. He called his theory the electromagnetic theory. According to Maxwell, light was an electromagnetic wave. Because light was an electromagnetic wave, light carried the same properties as the other electromagnetic waves. Maxwell was able to predict the speed of light by using electrical and magnetic constants. His calculations proved to be very close to the accepted value. Maxwell's theory, however, was unable to explain the photoelectric effect.

In 1887, Heinrich Hertz discovered that electrons could be emitted from metal if a certain light was allowed to shine on it. In 1900, Max Planck described another theory about light. He suggested that light was transmitted and absorbed in small bundles of energy. He called the bundles of energy "quanta." The amount of energy in the quanta, he stated, was proportional to the frequency of the light. Albert Einstein agreed with Planck's theory and stated that light not only was transmitted and absorbed as quanta, but also traveled as quanta. Einstein termed the units of quanta as photons. In 1905, Einstein was able to explain the photoelectric effect using the quantum theory. The quantum theory combines the two major theories of light, which are the particle theory and the wave theory. This is necessary because light does not always behave as a particle and light does not always behave as a wave. Light is a form of energy. The unit of light energy is a photon, which is a bundle of energy (quanta). Photons exist as a photon only when they are in motion. The speed of light is 3 x 108 meters per second in a vacuum.

One of the most important properties of light relating to fiber optics is reflection. Newton's laws govern how light is reflected. Newton's laws states "the angle of incidence will equal the angle of reflection." Another very important property light exhibit is that of refraction. Refraction results from light traveling through different media. Light will speed up, or slow down depending on the medium it travels in. For example, light travels faster in air than in glass. Due to light having to slow down, light will bend towards the normal, which is the point at which the light would intersect the medium at 90, as it passes through the glass. The exact opposite would occur for a light wave traveling through a slower medium to a faster one.

A third property of light that is important to fiber optics is recognizing the critical angle. The critical angle is the minimum angle at which total internal reflection takes place. This recognized by Snell's Law. Snell's Law gives n1 sin(q 1) = n2 sin(q 2) where n is the index of refraction of that material and theta is the angle of the ray from the normal. The critical angle then occurs when q 2 = 90. The critical angle is the crux of fiber optic technology.

Fiber optic technology is a result of light technology evolving throughout the past few centuries. Light communication has been around for a long time. Since early history, fire was used as a signaling device. As society advanced, more sophisticated means of signaling came about. Major advancements in fiber optic technology have taken place relatively recently, although light experimentation began centuries ago.

Back in 1621, Willebrord Snell formulated his law on the behavior of light. He realized light would refract as it entered one medium from another. Alexander Graham Bell, in 1860, demonstrated voice transmission using mirrors that were vibrated by the sound waves of the voice. The light reflected off the mirrors was modulated by the sound. He was able to focus the modulations onto a selenium plate, which caused resistance changes. These changes occurred as the intensity of the light changed. From these changes, a device similar to a speaker was activated. In 1870, John Tyndall experimented with light in a stream of water. He concluded that light could propagate through a medium. When in a medium, he realized that light could follow a curved path as well as a straight line. In 1897, John William Strutt presented some basic laws regarding light propagation. At the turn of the twentieth century, Max Planck developed the quantum theory. He developed Planck's constant, which relates electron and photon energy.

In 1905, Albert Einstein, using the quantum theory, was able to explain the photoelectric effect. In 1930, Willis Lamb, Jr. started some of the earliest experiments with guiding light in a glass fiber.

The Beginning of Fiber

In 1951, some American researchers were able to demonstrate the transmission of an image through a bundle of glass fibers. With experiments with light propagation in glass fibers well under way, Narinder Singh Kapany in 1953 developed fibers with cladding. The cladded fibers greatly improved transmission characteristics. The cladding significantly reduced the amount of dispersion of the light. The laser came about in 1960 being developed by Theodore Maiman. Two years later, Maiman invented the semiconductor laser. The laser was very important to advancing fiber optic technology because now it was possible to have a coherent light source. A coherent light source consists of a light beam of a single wavelength. In 1966, a collaboration effort between Charles Kao and Charles Hockman proposed the use of fiber optics for long-distance communication.

In 1970, Corning Glass Company produced the first set of low-loss fibers. This began to revolutionize the fiber optic industry. AT&T, in 1980, began the first major fiber optic communication link between Boston, Massachusetts, and Richmond, Virginia. The next year, Corning Glass Company modified their low loss fibers and came up with single-mode fibers with high bandwidth and low loss capabilities to increase data transmission rates. Starting in the mid-eighties, the major communication companies began installing long-distance fiber optic communication links using single-mode fibers.

In 1988, the first transatlantic fiber optic cable was installed. Starting during the end of the eighties and working into the nineties, cable providers began implementing widespread usage of fiber optic networks. During the past few years cable companies began replacing their coaxial cable lines with fiber optic cable.

Fiber Advantages

Fiber optic systems have many advantages over their conventional coaxial and radio transmission counterparts. Fiber optic cable can transmit at a higher capacity than conventional means. Current technology allows fiber that can carry data at more than a gigabit-per-second. Laboratory testing is being done on fibers allowing an increase of 10 times that amount. The increased amount of data that can be transmitted means better service for telecommunications, high-speed data communications, video, High Definition Television, and other communication needs. This technology also offers new services that require large bandwidths a low cost means of transmission. Fiber optic technology offers higher reliability over other methods. Fiber optic systems have very low bit error ratios on the magnitude of less than 1 x 10-11. With fiber optics, there is no electromagnetic interference because the transmissions all occur through light waves.

The transmission is also unaffected by rain, temperature and humidity. Fiber optic cable is usually buried three to four feet underground. This provides substantial protection from the elements. Fiber technology survived hurricane Hugo in 1989. Telephone service was not completely disrupted because the cables were buried underground and survived substantial flooding.

Fiber optic systems are more cost effective than conventional means. Commercially available fiber optic systems can transmit data for miles without requiring a repeater. A copper wire, however, requires repeater spacing about every mile. On the horizon, repeater spacing is expected to increase which will reduce the overall cost of the line and increase reception quality.

Fiber optic systems also offer higher security. Fiber optic transmissions neither induce nor emit any external energy. Because of its security, fiber optic communication is virtually untappable if it is monitored carefully. A signal loss can be detected almost immediately provided the system is monitored. The military and banking applications are unlimited.

Fiber optics also provides a great size and weight advantage over conventional means. For example, a shipboard advanced radar system requires about 750 feet of coaxial cable, with a weight of seven tons and a diameter of eighteen inches. These cables can be replaced by an optical fiber weighing forty pounds and measuring one inch in diameter. This reduces the weight of the ship above the water line.

Fiber optic systems also possess an unlimited growth potential. The capacity of a fiber optic system is not limited by the fiber, but rather by the equipment used to terminate the fiber. This enables the system to expand as technology advances.

Fiber optic systems also possess a greater financial benefit. The initial and maintenance costs of traditional copper systems are higher than fiber systems. Copper is becoming a scarcer natural resource, and its reserves are expected to be depleted within sixty years, whereas the primary ingredient of fiber optics is silica, which is readily available. Fiber optic systems can also transmit more data and at a faster rate with smaller wires and fewer repeaters than a copper system. Fiber, because it is not metal, is relatively unaffected by its environment. The fiber will not corrode in the atmosphere. Using fiber optic cable also prevents any type of wear from an electrical connection. The system is electrically separated from both ends. A beam of light is the only bridge between the sender and the receiver. With its superior quality and cost advantages, fiber optic systems will only be implemented into more applications.

Fiber Optic System

A fiber optic system consists of five parts. The three main parts of a system are transmit circuitry and light source, light detector and receive circuitry, and fiber.

1. Transmit Circuitry and Light Source

The transmitter circuitry's main job is to convert the electronic signal to modulate a light source. The transmitter circuitry codes the information it receives and gives it to a light source for transmission.

There are generally two types of light sources. The light source could either be a semiconductor diode laser or a light emitting diode (LED). The application at hand will determine which type of light source to use. LED's are relatively inexpensive transmitters with low output power. This type of system is more appropriate for applications requiring only moderate data rates (200 million bits per second), within a relatively short communication distance. A semiconductor laser system is usually more expensive. This system has a higher output power, which enables it to transmit high-speed data (1-2 billion bits per second), over a considerable distance. These two parts, the transmitter circuitry, and the light source are usually packaged together and commonly referred to as the transmitter. The transmitter provides an essential part of the system because without it, the modulated light signal would not occur.

2. Light detector and receiver circuitry

At the other end of the system are two other very important components. Although they are similar to the components at the beginning of the system, they have their own special characteristics. The light transmitted by the light source needs to be received by something that can interpret the signal. The equipment that can interpret the light signal is a photoelectric light detector. This detector takes the modulated light signal and converts it into an electronic form that the receiver circuitry can utilize. An example of a simple detector is a solar cell. A solar cell can detect light and transform it into an electric voltage. This type of detector is not used in sensitive applications because it is slow and generally insensitive. The faster and more sensitive detectors are electrically reverse-biased. This type of detector can be used to handle faster applications like voice and video communication.

The other part of the system is referred to as the receiver circuitry. The receiver circuitry takes the information from the photoelectric detector and converts it back into the information that was inputted into the system. This circuitry can also amplify the signal received. Often, a receiver is normally coupled with an amplifier. The detector and the circuitry are also usually packaged together to form the unit called the receiver.

3. Fiber

Tying together the two ends of the system, transmitters and receivers, are the fibers. A fiber is made up of generally three parts, core, cladding, and buffer coating.

The light transmitted is guided down the fiber by reflecting off the outside of the core. The core's index of refraction is slightly higher than that of the surrounding cladding to insure internal refraction. The core is surrounded by optical material called the cladding. The cladding causes the light to remain in the core. The core and the cladding are usually made of ultra-pure glass, although some fibers are plastic. The materials need to be ultra-pure because impurities in the material can lead to a reduction of power output. Impurities can add to absorption and scattering, which would reduce the effectiveness of the fiber. The buffer coating covers the core and the cladding. The buffer coating is generally made of plastic, which protects the fiber from moisture and other damage.

There are two types of optical fibers in common use, multimode and singlemode fibers. Multimode fibers have a larger core, about 62.5 microns. This type of fiber is used with LED sources at wavelengths of 850-1300 nm, and is commonly used for local area networks (LANs). The singlemode fiber has a much smaller core, about 9 microns. This type of fiber is used with laser sources at wavelengths of 1300-1550 nm, and is usually used for telephone and cable television applications. Due to the higher wavelength, this cable is able to handle high data transmission speeds.

Handle With Care

Optical fibers are fragile and care must be taken when they are installed. For this reason, optical fibers are mated with a cabling structure to increase the mechanical characteristics of the fiber. The cables need to protect the fibers from tensile, torsional, and bending stresses. These stresses can occur at any time during the installation process or during normal use. If a fiber is stretched too far, it can break. The glass fibers are also very susceptible to twisting. Twisting a fiber could easily compromise its integrity. The bending of fibers must also be watched. If a fiber is bent too far, the optical characteristics of the fiber can be changed. Understanding the causes of stress on the fiber enables the engineering of cables that can handle the application. Depending on the application, the cable could consist of a Kevlar coating or a simple thickened rubberized material.

The worst problems come from contractors with backhoes and other earth-moving equipment, who dig up buried cables, applying sharp forces and snapping the cables as well as from falling objects on aerial cables. Although cable designs cannot prevent out of the ordinary mishaps from occurring, with proper design considerations a cable strong enough for the application intended can easily be manufactured.

Fiber optic cable is an ideal medium for transmitting information. Fiber optics can provide faster data transmission at better quality and at a lower cost that their conventional counterparts. They are also considerably smaller and lighter than traditional means enabling usage in locations which otherwise could not accept traditional wiring. Fiber optics is playing an increasing role in the medical profession. Doctors are now able to look inside a person's body through a small incision, whereas a few years ago this procedure required a large operation.

Future of Fiber

Fiber technology is playing an increasing role in cable television systems. It will soon be possible for interactive voice and video communication using fiber optics. The speeds of data transmission required for such applications can only be achieved with fiber. The computer applications of fiber optic networks have only first begun to be utilized. Fiber linked networks will enable faster communication which will make downloading outdated. The Internet will be able to provide instant two-way communication. Fiber optics is being used in almost every industry from the military to banking to the automated industries. The potential applications of fiber optics are limited only by the imagination of the designer.