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Fiberoptics (Click to select text)
Description Fiber Optics "A friend of mine once said that the trouble with being an architect is that everyone is an architect; everyone knows what's good and what's bad. I seem to remember the same story told by an economist. In physics we don't have that problem" (Sobel ix). Fiber optic technology has been inevitable since the first mention or speculation of light by Pythagoras in the 6th century B.C. (Sobel 1) Because of its flexibility and comparatively low cost (to copper cable), fiber optic-systems are now being used in almost every situation in everyday life. Fiber optic technology is fairly new though, considering the first uses of fiber optics were mainly for illumination of signs and for toys. Recently though, technology has started to catch up and the uses of fiber optics are numerous. Fortunately for the high-tech society of today, some scientists and technicians researched the avenues of fiber optics. Fiber optics can be seen in telephone systems, in the military, in cars, in medicine, and in many other fields of study. For all industries and professions, "Fiber optics allow huge amounts of voice, data and video imagery to be transmitted instantaneously, allowing for photographic-quality images, interactive video conferencing and other business multimedia uses" (Inman News Features 1). It's becoming possible to transmit audio, video and data signals over great distances using these fibers of pure glass or plastic. Today, fiber optic technology has almost become a necessity in everyday business. Because fiber optics can simultaneously transmit two audio signals across a single fiber optics cable without interfering with each other (Nelson, Garat, Wong 1), many companies have changed, or are in the process of changing their connections to fiber optic cables rather than copper cables. Fiber optic cables are lighter, take up less space and are less expensive than the cumbersome copper cable that has been used previously by telephone companies and the military. Telephone companies have created international connections using optical fibers. A fiber optics communication link provides a virtually noise free medium for transporting complex signals without distortion and interference. (Nelson, Garat, Wong 1). The military is buying fiber for portable battlefield communications systems not only because of its superior performance and light weight, but also because fiber-optic systems have not been proven to be buggable or tappable. (Myring 32) These reasons explain the extensive use of fiber optics for the transmission of confidential and critical data and commands between the cockpit and all parts and systems of high-tech fighter aircraft. Fiber optic technology can be a matter of life and death. In the medical field, recent advances involve using fiber optics to explore the body from the mouth down through the stomach and into the intestines. In the same way, it has become possible to visually examine veins and arteries for signs of blockage and lasers can be used to unclog arteries. (Myring 20). As fiber optic technology advances, fiber optics will be useful in everyday life situations, such as transporting information over the rapidly-growing Internet, high-definition television, and even the possibility of teaching classes in the home using monitors and signals from the teacher. Information can be spread quicker and at a lower cost than printing newspapers or magazines.. Developments of the fiber optic-system started in France in 1794. A system of relaying messages through "optical telegraphy" was used to transmit an important message over a relay of towers 230 kilometers long (about 144 miles). (Industrial Fiber Optics 3). After optical telegraphy came the electrical telegraph. It was more cost-efficient and was, unlike the optical telegraph, effective regardless of weather and the time of day. After this advance, the idea of optical data transmission was forgotten, but not for long. From this time until 1880, no word of optical data transmission was heard. Alexander Graham Bell changed this when he demonstrated a "Photophone". It wasn't practical, but it illustrated means of using voice (audio) to modulate a beam of light for transmission to a remote point, known today as a laser. (IFO 3). Along with Bell's invention of the Photophone, Charles Vernon Boys was developing a way to make long, thin fibers of glass for use in communications systems during World War I. He did not devise a practical way to put his fibers into effect. In 1934, Norman R. French, of AT&T, patented an "optical telephone system" that could carry voice signals on beams of light through a network of "light cables." (IFO 3). His system also proved to be impractical, but the idea of fiber optics had been reborn. In the 1940's, Kell and Sziklai, of RCA, patented a system very similar to French's, but working with video signals rather than voice. In the late 40's, the MASER (Microwave Amplification by the Stimulated Emission of Radiation) was developed and later led to the development of the LASER (Light Amplification by the Stimulated Emission of Radiation) in 1960 by Theodore H. Maiman, of the Hughes Research Laboratories in Malibu, California. (IFO 4). With the invention of the laser, technology began to progress very rapidly. The importance of the laser is that it emits a single beam of light, narrow enough to fit in small spaces, and uniform so that it can be concentrated for high power. The thousands of individual rays of light making up the beam are all the same frequency/wavelength, (IFO 4) exactly in phase, or synchronized, and they are all exactly parallel with each other. Because of the predictability and accuracy of the path of the laser beam, it has taken many steps ahead of microwave, light and radio beams in systems and businesses alike. It wasn't until the 1950's, though, that optical fibers became practical. They were first used simply for transmitting light, then later for transmitting complex images through bundles of fibers. (IFO 5). They became available in the 1960's when communications scientists working for telephone companies on both sides of the Atlantic began realizing that lasers, when combined with the steadily improving optical fibers, could revolutionize the telecommunications industry. Intensive research began by many companies and they rapidly improved the quality, purity and light-carrying capacities of optical fibers. These developments led to breakthroughs into the very-low-loss fibers (needed for longer-range transmission of optical signals) that arrived in the early 70's. (IFO 5). The availability of fibers led to the design of various light sources to send the signals through the optical fibers. Such sources included laser diodes and light-emitting diodes, or LEDs. The original lasers were bulky, delicate and expensive, so more developments for the semiconductor diode laser began. It was small, but had short life expectancy and wasn't very reliable. Those limitations were "designed out" In the meantime, though, the less expensive and longer-lasting incoherent light-emitting diodes were suitable for most of the less demanding applications. Between the laser diode and the LED (the non-coherent Light Emitting Diode), practical light sources had finally been developed. (IFO 5). Virtually all of the early work had been done in the wavelength range of the laser diodes and LEDs then readily available, 800-900 nanometers. By 1980, scientists had realized that many other improvements could be made in long-range communications if longer-wavelength components were made practical. As a result, special-purpose systems demanding lower-loss operation are becoming available at 1300 and even at 1550 nanometers. Special fibers exist that are capable of simultaneously carrying many different wavelengths of information. There is little interference between two electromagnetic signals if their frequencies are sufficiently far apart. Two laser beams can be transmitted down the same fiber optics cables simultaneously. If their frequencies are sufficiently far apart, they will exit carrying the same information as when they started, with little change due to interference. (Nelson, Garat, Wong 1) Fiber optic technology is normally used as a replacement of other cables in performing some function. More recently though, problems have been addressed with fiber optics that had never been before recognized or addressed. These technologies require fiber optics to function, thus showing that fiber optics technology has created its own market. One of the prime examples of this is the ability for fiber optics to bring light to places that seemed to be forever a mystery. Because of its ability to direct light to a single spot, fiber optics is becoming growingly useful in many areas including lighting for microscopes, illumination during surgical procedures, improved contrast for automated-machine vision, borescopes for gunsmiths, sophisticated maps for use in low-light environments, and illumination in the presence of explosive fumes and gasses. (Bohn, MacDonald 392). The possibilities and space provided for work on aircraft is very limited for electronic systems. Because of rapid changes in temperature and high temperatures for long periods of time, extremely low temperatures in certain areas of the aircraft, frequent severe vibration, and high g-forces in several directions, finding a suitable solution for the massive cable wiring within the aircraft is not easy. Despite all of this, in 1976, the U.S. Air Force, in an experimental program called ALOFT (Airborne Light Optical Fiber Technology), removed the 40 kg, 1260-meter long, 302-cable wiring harness of an A-7 test-bed aircraft and replaced it with fiber cable. Not only was this done using only 1.7 kg, 76-meter, 12-fiber cable, but the cable also performed all of the functions of the heavier cable. The aircraft had improved performance and no drawbacks were seen. Through these experiments, ALOFT demonstrated that fiber optics could equal and exceed the performance of electrical wiring, especially in harsh environments. (IFO 7). Both the B-1 bomber and the MX missile use fiber optics for data transmission. One study showed that, if all the wire cables in a B-1 could be replaced by fiber optics, the bomber's weight would be reduced by up to 2,000 pounds. (IFO 7). This could be used effectively to increase the amount of weaponry that can be carried, to save money on fuel, and to generally decrease the cost of operating the aircraft. The replacement of copper wire by fiber optics in the B-2 Stealth bomber has greatly reduced the chance of the aircraft being detected on radar. They also, unlike wiring harnesses, do not radiate electronic signals, therefore outperforming highly sensitive enemy radar and tracking devices. Another great use for fiber optics is in the automotive environment. The car's wiring is almost as fragile as the wiring in aircraft in some aspects. The conditions surrounding the wiring can be very harsh, including vibration, high temperature and low temperatures, compression and constant exposure to sunlight, water and oil. Although high-speed data transmission isn't required for the average car, each year, the use of fiber increases. The non-communications uses, however, are important and include dash lights, burned-out bulb indicators, ashtray and glove compartment lights, headlight high-beam indicators, and power windows and door locks. (Fales, Kuetemeyer, Brusic 405). General Electric studies are trying to find a solution that would replace all lighting in a car with a routing of light throughout the vehicle from a single light source using fiber optics. Fiber optics may be used to link electronic operations in engines. However, the fibers currently available can't withstand the heat generated by today's engines. Perhaps by redesigning engines, and with the invention of new materials, fiber optics will play an important role in engines as well. (Fales, Kuetemeyer, Brusic 405) The most widely used use of fiber optics in medicine is in the endoscope. It is used for direct visual examination of internal surfaces of organs like the lungs, arteries, veins, the heart itself, and the alimentary canal from one end to the other. (Myring 20). The performance of laser surgery to burn away blockages within arteries is only recently being used to clear blockages in the coronary arteries. When the technique can be used, it replaces the higher-risk, longer-recovery method of surgically by-passing the blockages, the method most commonly used today. Laser surgery is already common in less life-threatening surgery, such as arthroscopic repair of damaged knee ligaments and cartilage. It is widely used in sports medicine, largely because arthroscopic surgery heals much faster than traditional surgery and allows the players to return to their sport in a shorter period of time. The uses of fiber optics are endless. Fiber optics can only work if they have all parts of the system. They must have a light source or transmitters, sensors or receivers, and of course the fibers on which the signals travel through. Other elements in the system include power sources, connectors and transducers (a device for converting energy from one form to another, such as optical energy to electrical energy.) (LASCOM 24). The information is contained within the modulation, while the light itself is simply the carrier. (IFO 9). The working of an optical fiber depends on the basic principles of optics and the interaction of light with matter. Many of light's properties can be explained by thinking of light as an electromagnetic wave. "Light" is a small part of the total electromagnetic spectrum. Light is higher in frequency and shorter in wavelength than the more common radio waves. Visible light's wavelength is from 380 nanometers (far deep violet in color) to 750 nanometers (far deep red). Infrared radiation has longer waves than visible light. Most fiber optic systems use infrared light between 750 and 1500 nm. The higher the frequency, the shorter the wavelength. 8 = c/f, where 8 is the wavelength, f is the frequency and c is the speed of light. (IFO 9). Light also exhibits some particle-like properties. A light particle is called a photon and the amount of energy contained in a photon depends on its frequency. The higher the frequency, the greater the energy. The energy (E) that is contained in a photon, measured in joules, is equal to the frequency (f), measured in Hertz, multiplied by 6.63 X 10-34, called Planck's constant, after the man who first determined it. E = h X f where h = Planck's constant. (Sobel 92). The most important optical measurement for any transparent material is its refractive index (n). The refractive index of any light-conductive medium is the ratio of the speed of light in a vacuum to the speed of light in the medium. (LASCOM 11). The speed of light in any material is always slower than in a vacuum, so the refractive index is always greater than one. The refractive index is measured by comparing the speed of light in the material to that in air, rather than in a vacuum. This simplifies the measurements and does not make any practical difference, since the refractive index of air is very close to that of a vacuum, 1.00029. (IFO 11). Light travels in straight lines through most optical material, but when light hits a boundary shared with another medium, light is bent as it passes through a surface in which the refractive index changes. The amount of bending depends on the refractive indices of the two materials and the angle of the incident ray striking the transition surface. (IFO 11). The angles of incidence and transmission are measured from a line perpendicular to the surface. Snell's Law expresses the relationship between the incident rays and the transmitted rays. Snell's Law states "n'sin 1' = n" sin 1" where n' and n" are the refractive indices of the initial and secondary media, respectively, while 1' and 1" are the angles of incidence and transmission, respectively." (Sobel 5). Snell's Law applies to light passage through a boundary between two optically conductive media, or materials. More important to the transmission of light along the length of an optical fiber are the cases in which the light strikes the boundary surface at an angle that prevents the light from passing through the boundary and, instead, reflects the light back into the material from which it was trying to escape. That happens when the light strikes the boundary surface at a "glancing" angle, i.e., an incident angle greater than the critical angle. (IFO 12). Snell's Law indicates that refraction cannot occur when the angle of incidence becomes too large when light is traveling from a high index to a low index. (Sobel 5). If light traveling through one material hits a boundary with another material that has a lower refractive index, and if it strikes that boundary at a low enough, or "glancing," angle, it cannot get out into that other material and all of the light is reflected back into the higher-index material through which it had already been traveling. This is called "total internal reflection" and this is what keeps the light signals going through optical fiber systems. The simplest fiber optic cable consists of two coaxial layers of transparent materials contained within a protective layer or "cladding." The inner portion, the core, carries the light. The next layer is the cladding. The cladding must have a lower refractive index than the core. The core is made of pure glass or plastic fibers. Light transmission through an optical fiber is not 100 percent efficient. The loss of light in transmission is called attenuation. Absorption by materials within the fiber, scattering of light out of fiber core, and leakage of light out of the core caused by environmental factors are the three most significant contributors to attenuation. Attenuation is directly influenced by the wavelength of the light being transmitted. (LASCOM 1). Attenuation is measured by comparing output power, and the results are usually measured in decibels. (dB). The decibel is the unit of measure for the ratio of output power to input power. (LASCOM 7). All optical fibers have a characteristic attenuation of decibels per unit length, normally expressed in decibels per attenuation times the length of the fiber in kilometers. (IFO 15). Optical fibers can carry light around corners because light doesn't always travel in a straight line. Instead, it travels in a straight line until it meets a surface whose properties either allow the light to pass through (usually at some angle other than the incident angle) or that cause the light to reflect off the surface at an angle equal to that at which it met the surface. Materials used in the construction of optical fibers cause the fibers to present a reflective type of surface to light rays entering into the core of the fiber approximately along the core's axis at the point of entrance. (IFO 15). Each ray of light will then multiply along the core in a straight line until it hits the surface forming the boundary between the core and its cladding, at which time that ray will glance off the boundary and continue along its way until it meets fiber, adjusting its path slightly, through reflection, as many times as necessary to pass through and travel along the bends or curves in the fiber. When it reaches a properly connected end of the fiber, having bounced off the walls of the core possibly thousands of times, the ray will exit the fiber alongside all of the other light rays transmitted through the fiber, giving the appearance of having traveled along a single curved path identical to the fiber and its core. (IFO 15). The two types of fiber optic light sources that provide greater than 95 percent of the communications market with fiber optic communication are light-emitting diodes (LEDs) and laser diodes. The LED is the simplest and is most widely used in fiber optic systems because they are sturdy, inexpensive, require low input power, and have very long life expectancy. They are made from a variety of materials and the color or emission wavelength depends upon the material. Such materials include gallium phosphide, gallium arsenic phosphide, gallium aluminum arsenide, gallium arsenide and indium gallium arsenic phosphide, producing green, yellow-red, and near-infrared colors of emission wavelengths. Simple LEDs emit light in every direction and are designed to optimize light coming from a particular surface, but the light rays being emitted are random in direction, phase and precise frequency with respect to each other. (IFO 16). The differences between a laser and an LED are that the opposing ends of the laser chip are highly reflective and perfectly parallel surfaces, forming an optical cavity within the chip. The laser diodes' high efficiency, small size, reliability, and low cost make them ideal for communication devices. (Nelson, Garat. Wong 1) At low electrical drive currents, the laser acts like an LED, but as the drive current increases, it crosses a threshold, above which lasing occurs. A laser diode relies on a very high current density to stimulate lasing. At high current densities, many electrons are in the excited state. Just as in LEDs, holes and electrons combine, creating photons which, initially, are confined by the reflecting ends of the optical cavity. Traveling between those ends, the photons collide with other electrons, producing more photons that are identical to the first photons. The first light photon amplified itself by stimulating an electron to emit another photon, which stimulated another, etc., etc. (IFO 17). At least one end of the optical cavity is less than 100 percent reflective, or there could be no optical output from the diode. Some of the optical power escapes to be used in the fiber optic system and the laser diode then becomes a light source. While the light source is essential to the transmitter, it is not sufficient by itself. A housing is required to mount and protect the light source and to communicate with the electronic signal source and the transmitting optical fiber. Internal components may be necessary to optimize the coupling to the fiber. Drive circuitry may be needed and output monitoring may be critical for sophisticated laser diodes. (IFO 17). Practical boundaries between source and transmitter may be vague, since simple LED sources can be mounted in a case having just optical and electronic connections, with little or no drive circuitry required. However, a high-performance laser may require a transmitter that also contains an output monitor and a thermoelectric cooler. (Burkig 103). The basic elements commonly found in a transmitter consist of housing, electronic interface, electronic preprocessing, drive circuits, optical interface, light sources, temperature sensing and control and the optical monitor. (IFO 17). The receiver of an optical fiber system functions to accept the information-containing modulated light from the fiber and convert it into usable output, or an electrical signal. The basic parts of any receiver include a light detector, a preamplifier, a main amplifier, a signal processor and a housing, electrical interfaces and optical interfaces. Besides these parts of the receiver, there is also a detector, which is its most critical component. Most fiber optic detectors are made from semiconductor materials similar to those found in LEDs and lasers. After receiving an incident optical signal, it responds by providing an electrical output signal. The current produced depends on the amount of light actually received and on the type of device used. (LASCOM 7). There are four basic types of photoconductive detectors. The two seldom used are the phototransistor and the photodarlington, both of which are too slow for most modern fiber optic applications. Most used are the photodiode and the avalanche photodiode, called the APD. (IFO 21). Many types of photodiodes are on the market, but the most useful in fiber optics is the PIN photodiode, named that because of the types and sequence of layers used in making the chip - Positive, Intrinsic and Negative. (LASCOM 18). The PIN photodiode has high efficiency and a faster rise time than other photodiodes. In PIN photodiodes, one photon creates one hole/electron pair. (IFO 21). The avalanche photodiode is similar to the laser diode. In a laser, a few primary carriers produce many emitted photons. The avalanche photodiode is the opposite of that, with a few photons producing many carriers. When the avalanche photodetector absorbs a photon, it creates a hole/electron pair in the Intrinsic region. The APD is reverse biased, causing the holes and electrons to move in the electric field. In an avalanche photodiode, this electric field is much stronger than in a PIN diode due to the higher bias voltage (typically 100 - 400 volts). The hole/electron pairs accelerate while traveling in this strong electric field. These pairs collide with electron/hole pairs, generating more sets of carriers - which is called avalanching. The avalanche process increases the number of carriers produced from a single photon, producing typical magnifications of from 10 to 100. (IFO 22). Avalanche photodiodes are used in fiber optic systems because the system noise level is limited. Disadvantages of the APDs include gain variation with temperature, high voltage power supply required, power dissipation, and a higher price than PIN photodiodes. (IFO 22). The preamplifier sets the two most important performance levels in most fiber optic systems - minimal detectable signal and electrical bandwidth. At the preamplifier, the signal is the weakest and is at a point at which noise can enter the system to interfere with the desired signal because the noise entering here will be amplified along with the signal. The main amplifier further amplifies the preamplifier signals to higher levels and sends the signal on its way. The signal processor separates the amplified signal from the usable signal and the useful form of data is then received. (IFO 24). The fiber optic-system is a very complex system of transmitted and received light signals. Because of its complexity, it is hard to comprehend the procedure of transmitting light through the fiber optic cable. Though this process is very complex, it is being used more and more because of the terrific advantages it has over other cable systems and also the advantage of being the only known resource for lighting confined areas. The fiber optic-systems of today will continue to be developed and by the end of the century, great advances and inventions that never seemed possible will be available.
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