Optical : Optics is a major field of study in itself and far beyond the scope of this book. Most courses in physics introduce the basic principles of light and optics. We review briefly those principles that relate directly to optical communication systems and components.
Light, radio waves, and microwaves are all forms of electromagnetic radiation. Light frequencies fall between those of microwaves and X-rays, as shown in Fig. 19-1(a). Radio frequencies range from approximately 10 kHz to 300 GHz. Microwaves extend from 1 to 300 GHz. The range of about 30 to 300 GHz is generally defined as millimeter waves.
Further up the scale is the optical spectrum, made up of infrared, visible, and ultraviolet light. The frequency of the optical spectrum is in the range of 3 x 1011 to 3 x 1016 Hz. This includes both the infrared and the ultraviolet bands as well as the visible parts of the spectrum. The visible spectrum is from 4.3 x 1014 to 7.5 x1014 Hz.
We rarely refer to the “frequency of light.” Light is expressed in terms of wavelength. Recall that wavelength is a distance measured in meters between peaks of a wave. It is calculated with the familiar expression
where λ (lambda) is the wavelength in meters, 300,000,000 is the speed of light in meters per second, and f is the frequency in hertz. The details of the optical spectrum are shown in Fig. 19-1(b).
Light waves are very short and usually expressed in nanometers (nm, one-billionth of a meter) or micrometers (μm, one-millionth of a meter). An older term for micrometer is micron. Visible light is in the 400- to the 700-nm range or 0.4 to 0.7 μm depending upon the color of the light. Short-wavelength light is violet (400 nm) and red (700 nm) is long-wavelength light.
Another unit of measure for light wavelength is the angstrom. One angstrom (Å) is equal to 10-10 m or 10-4 μm. To say it the other way, 1 μm equals 10,000 Å. Right below visible light is a region known as infrared. Its spectrum is from 0.7 to 1000 μm. Sometimes you will hear infrared referred to as near or far-infrared. Near-infrared is those frequencies near the optical spectrum and far infrared is lower in frequency near the upper microwave region [see Fig. 19-1(b)].
Right above the visible spectrum is the ultraviolet range. The ultraviolet range is above violet visible light, which has a wavelength of 400 nm or 0.4 μm up to about 10-8 m or 0.01 μm or 10 nm. The higher the frequency of the light, the shorter the wavelength. The primary source of ultraviolet light is the sun. Infrared and ultraviolet are included in what we call the optical spectrum.
Speed of Light
Light waves travel in a straight line as microwaves do. Light rays emitted by a candle, lightbulb, or other light source move out in a straight line in all directions. Light waves are assumed to have a spherical wavefront as do radio waves. The speed of light is approximately 300,000,000 m/s, or about 186,000 mi/s, in free space. These are the values normally used in the calculation, but for a more accurate outcome, the actual values are closer to 2.998 x 108 m/s, or 186,280 mi/s.
The speed of light depends upon the medium through which the light passes. The figures given above are correct for light traveling in free space, i.e., for light traveling in air or a vacuum. When light passes through another material such as glass, water, or plastic, its speed is slower.
Physical optics refers to the ways that light can be processed. Light can be processed or manipulated in many ways. For example, lenses are widely used to focus, enlarge, or decrease the size of light waves from some source.
The simplest way of manipulating light is to reflect it. When light rays strike a reflective surface, such as a mirror, the light waves are thrown back or reflected. By using mirrors, the direction of a light beam can be changed.
The reflection of light from a mirror follows a simple physical law. That is, the direction of the reflected light wave can be easily predicted if the angle of the light beam striking the mirror is known (refer to Fig. 19-2). Assume an imaginary line that is perpendicular to the flat mirror surface. A perpendicular line, of course, makes a right angle with the surface, as shown. This imaginary perpendicular line is referred to as the normal. The normal is usually drawn at the point where the mirror reflects the light beam. If the light beam follows the normal, the reflection will simply go back along the same path. The reflected light ray will exactly coincide with the original light ray.
If the light ray strikes the mirror at some angle A from the normal, the reflected light ray will leave the mirror at the same angle B to the normal. This principle is known as the law of reflection. It is usually expressed in the following form: The angle of incidence is equal to the angle of reflection.
The light ray from the light source is usually called the incident ray. It makes an angle A with the normal at the reflecting surface, called the angle of incidence. The reflected ray is the light wave that leaves the mirror surface. Its direction is determined by the angle of reflection B, which is exactly equal to the angle of incidence.
The direction of the light ray can also be changed by refraction, which is the bending of a light ray that occurs when the light rays pass from one medium to another. In reflection, the light ray bounces away from the reflecting surface rather than being absorbed by or passing through the mirror. Refraction occurs only when light passes through a transparent material such as air, water, and glass. Refraction takes place at the point where two different substances come together. For example, where air and water come together, refraction will occur. The dividing line between the two different substances or media is known as the boundary or interface.
To visualize refraction, place a spoon or straw into a glass of water, as shown in Fig. 19-3(a). If you observe the glass of water from the side, it will look as if the spoon or straw is bent or offset at the surface of the water.
Another phenomenon caused by refraction occurs whenever you observe an object underwater. You may be standing in a clear stream and observing a stone at the bottom. The rock is in a different position from where it appears to be from your observation [see Fig. 19-3(b)]. The refraction occurs because light travels at different speeds in different materials.
The speed of light in free space is typically much higher than the speed of light in water, glass, or other materials. The amount of refraction of the light of a material is usually expressed in terms of the index of refraction n. This is the ratio of the speed of light in air to the speed of light in the substance. It is also a function of the light wavelength.
Naturally, the index of refraction of air is 1, simply because 1 divided by itself is 1. The refractive index of water is approximately 1.3, and that of glass is 1.5.
To get a better understanding of this idea, consider a piece of glass with a refractive index of 1.5. This means that light will travel through 1.5 ft of air, but during that same time, the light will travel only 1 ft through the glass. The glass slows down the light wave considerably. The index of the refraction is important because it tells exactly how much a light wave will be bent in various substances.
When a light ray passes from one medium to another, the light wave is bent according to the index of refraction. In Fig. 19-2, the incident ray strikes the surface at angle A to the normal but is refracted at an angle C. The relationship between the angles and indices of refractions are
n1 sin A = n2 sin C
Now refer to Fig. 19-4. A light ray is passing through the air. It makes an angle A with the normal. At the interface between the air and the glass, the direction of the light ray is changed. The speed of light is slower; therefore, the angle that the light beam makes to the normal is different from the incident angle. If the index of refraction is known, the exact angle can be determined with the formula given earlier.
If the light ray passes from the glass back into the air, it will again change direction, as Fig. 19-4 shows. The important point to note is that the angle of the refracted ray B is not equal to the angle of incidence A.
If the angle of incidence is increased, at some point the angle of refraction will equal 90° to the normal, as shown in Fig. 19-5(a). When this happens, the refracted light ray in red travels along with the interface between the air and glass. In this case, the angle of incidence A is said to be the critical angle. The critical angle value depends upon the index of refraction of the glass.
If you make the angle of incidence greater than the critical angle, the light ray will be reflected from the interface [see Fig. 19-5(b)]. When the light ray strikes the interface at an angle greater than the critical angle, the light ray does not pass through the interface into the glass. The effect is as if a mirror existed at the interface. When this occurs, the angle of reflection B is equal to the angle of incidence A as if a real mirror were used. This action is known as total internal reflection, which occurs only in materials in which the velocity of light is slower than that in air. This is the basic principle that allows a fiber-optic cable to work.
Optical Communication Systems
Optical communication systems use light as the carrier of the information to be transmitted. As indicated earlier, the medium may be free space as with radio waves or a special light “pipe” or waveguide known as fiber-optic cable. Both media are used, although the fiber-optic cable is far more practical and more widely used. This article focuses on fiber-optic cable.
Rationale for Light Wave Communication
The main limitation of communication systems is their restricted information-carrying capabilities. In more specific terms, this means that the communication medium can carry just so many messages. This information-handling ability is directly proportional to the bandwidth of the communication channel. Using light as the transmission medium provides vastly increased bandwidths. Instead of using an electric signal traveling over a cable or electromagnetic waves traveling through space, the information is put on a light beam and transmitted through space or through a special fiberoptic waveguide.
Light Wave Communication in Free Space
Fig. 19-6 shows the elements of an optical communication system using free space. It consists of a light source modulated by the signal to be transmitted, a photodetector to pick up the light and convert it back into an electric signal, an amplifier, and a demodulator to recover the original information signal.
A transmitter is a light source. Other common light sources are light-emitting diodes (LEDs) and lasers. These sources can follow electric signal changes as fast as 100 GHz or more.
Lasers generate monochromatically, or single-frequency, the light that is fully coherent; i.e., all the light waves are lined up in sync with one another and as a result, produce a very narrow and intense light beam.
A modulator is used to vary the intensity of the light beam in accordance with the modulating baseband signal. Amplitude modulation, also referred to as intensity modulation, is used where the information or intelligence signal controls the brightness of the light. Analog signals vary the brightness continuously over a specified range. This technique is used in some cable TV systems. Digital signals simply turn the light beam off and on at the data rate. Digital modulation is usually NRZ-formatted binary data that turns a laser off or on to produce off-on keying (OOK) or amplitude-shift keying (ASK).
A modulator for analog signals can be a power transistor in series with the light source and its dc power supply (see Fig. 19-7). The voice, video, or other information signal is applied to an amplifier that drives the class A modulator transistor. As the analog signal goes positive, the base drive on the transistor increases, turning the transistor on harder and decreasing its collector-to-emitter base voltage. This applies more of the supply voltage to the light source, making it brighter. A negative-going or decreasing signal amplitude drives the transistor toward cutoff, thereby reducing its collector current and increasing the voltage drop across the transistor. This decreases the voltage to the light source.
Frequency modulation is not used in light communication. There is no practical way to vary the frequency of the light source, even a monochromatic source such as an LED or a laser. Amplitude modulation is used with analog signals, but otherwise, most lightwave communication is accomplished by pulse modulation.
Pulse modulation refers to turning the light source off and on in accordance with some serial binary signal. The most common type of pulse modulation is pulse-code modulation (PCM), which is serial binary data organized into bytes or longer words. NRZ, RZ, and Manchester formats are common.
The modulated light wave is picked up by a photodetector. This is usually a photodiode or transistor whose conduction is varied by the light. The small-signal is amplified and then demodulated to recover the originally transmitted signal. Digital processing may be necessary. For example, if the original signal is voice that was digitized by an A/D converter before being transmitted as a PCM signal, then a D/A converter will be needed at the receiver to recover the voice signal.
Communication by the light beam in free space is impractical over very long distances because of the great attenuation of the light due to atmospheric effects. Fog, haze, smog, rain, snow, and other conditions absorb, reflect, refract, and disperse the light, greatly attenuating it and thereby limiting the transmission distance. Artifi cial light beams used to carry information are obliterated during daylight hours by the sun. And they can be interfered with by any other light source that points in the direction of the receiver. Distances are normally limited to several hundred feet with low-power LEDs and lasers. If high-powered lasers are used, a distance of several miles may be possible.
Light beam communication has become far more practical with the invention of the laser, a special high-intensity, single-frequency light source. It produces a very narrow beam of brilliant light of a specific wavelength (color). Because of its great intensity, a laser beam can penetrate atmospheric obstacles better than other types of light can, thereby making light beam communication more reliable over long distances. When lasers are used, the light beam is so narrow that the transmitter and receiver must be perfectly aligned with each other for communication to occur.
Although this causes initial installation alignment problems, it also helps to eliminate external interfering light sources.
Fiber-Optic Communication System
Instead of free space, some type of light-carrying cable can be used. Today, fiberoptic cables have been highly refined. Cables many miles long can be constructed and then interconnected for the purpose of transmitting the information. Thanks to these fiber-optic cables, a new transmission medium is now available. Its great advantage is its immense information-carrying capacity (wide bandwidth). Whereas hundreds of telephone conversations may be transmitted simultaneously at microwave frequencies, many thousands of signals can be carried on a light beam through a fiber-optic cable. When multiplexing techniques similar to those in telephone and radio systems are used, fiber-optic communication systems have an almost limitless capacity for information transfer.
The components of a typical fiber-optic communication system are shown in Fig. 19-8. The information signal to be transmitted may be the voice, video, or computer data. The first step is to convert the information to a form compatible with the communication medium, usually by converting continuous analog signals such as voice and video (TV) signals to a series of digital pulses. An A/D converter is used for this purpose.
These digital pulses are then used to flash a powerful light source off and on very rapidly. In simple low-cost systems that transmit over short distances, the light source is usually a light-emitting diode that emits a low-intensity infrared light beam. Infrared beams such as those used in TV remote controls are also used in transmission.
The light beam pulses are then fed into a fiber-optic cable, which can transmit them over long distances. At the receiving end, a light-sensitive device is known as a photocell, or light detector is used to detect the light pulses. It converts the light pulses to an electric signal. The electrical pulses
are amplified and reshaped back into digital form. They are fed to a decoder, such as a D/A converter, where the original voice or video is recovered.
In very long transmission systems, repeater units must be used along the light is greatly attenuated when it travels over long runs of cable, at some point, it may be too weak to be received reliably. To overcome this problem, special relay stations are used to pick up the light beam, convert it to electrical pulses that are amplified, and then retransmit on another light beam.
Applications of Fiber Optics
Fiber-optic communication systems are being used more and more each day. Their primary use is in long-distance telephone systems and cable TV systems. Fiber-optic networks also form the core or backbone of the Internet. Fiber-optic cables are no more expensive or complex to install than standard electrical cables, yet their information-carrying capacity is many times greater.
Fiber-optic communication systems are being used in other applications. For example, they are being used to interconnect computers in networks within a large building, to carry control signals in airplanes and in ships, and in TV systems because of the wide bandwidth. In all cases, the fiber-optic cables replace conventional coaxial or twisted-pair cables. Fig. 19-9 lists some of the applications in which fi ber-optic cables are being used.
Benefits of Fiber Optics
The main benefi t of fi ber-optic cables is their enormous information-carrying capability. This capacity is dependent upon the bandwidth of the cable. Bandwidth refers to the range of frequencies that a cable will carry. Electric cables such as coaxial have a wide bandwidth (up to about 750 MHz), but the bandwidth of fi ber-optic cable is much greater. Data rates in excess of 100 GHz have been achieved, and even higher rates have been achieved with multiplexing. Fiber-optic cable has many other benefi ts, summarized in Fig. 19-10.
There are some disadvantages to fi ber-optic cable. High cost is the greatest disadvantage. Otherwise, its small size and brittleness make it diffi cult to work with. Special, expensive tools and test instruments are required.
A fiber-optic cable is a thin glass or plastic cable that acts as a light “pipe.” It is not really a hollow tube carrying light, but a long, thin strand of glass or plastic fiber. Fiber cables have a circular cross-section with a diameter of only a fraction of an inch. Some fiberoptic cables are the size of a human hair. A light source is placed at the end of the fi ber, and light passes through it and exits at the other end of the cable. How the light propagates through the fi ber depends upon the laws of optics.
Principles of Fiber-Optic Cable
Fiber-optic cables operate on the optical principles of total internal reflection as described earlier in this chapter. Fig. 19-11(a) shows a thin fiber-optic cable. A beam of light is focused on the end of the cable. It can be positioned in a number of different ways so that the light enters the fiber at different angles. For example, light ray A enters the cable perpendicular to the end surface. Therefore, the light beam travels straight down the fiber and exits at the other end. This is the most desirable condition.
The angle of light beam B is such that its angle of incidence is less than the critical angle, and therefore refraction takes place. The light wave passes through the fiber and exits the edge into the air at a different angle. The angle of incidence of light beams C and D is greater than the critical angle. Therefore, total internal reflection takes place, and the light beams are reflected off the surface of the fiber cable. The light beam bounces back and forth between the surfaces until it exits at the other end of the cable.
When the light beam reflects off the inner surface, the angle of incidence is equal to the angle of reflection. Because of this principle, light rays entering at different angles will take different paths through the cable, and some paths will be longer than others. Therefore, if multiple light rays enter the end of the cable, they will take different paths, and so some will exit sooner and some later than others.
In practice, the light source is placed so that the angle is such that the light beam passes directly down the center axis of the cable so that the refl ection angles are great. This prevents the light from being lost because of refraction at the interface. Because of total internal refl ection, the light beam will continue to propagate through the fi ber even though it is bent. With long slow bends, the light will stay within the cable.
The core of the fiber has a higher index of refraction than the cladding surrounding the core as shown in Fig. 19-11(b). The light enters the core at an infi nite number of angles but only those rays entering the core at an angle less than the critical angle actually pass down the core. These light rays are refl ected off the interface between core and cladding as they pass down the cable.
Fig. 19-11(b) shows the critical angle of the cable θc, sometimes referred to as the acceptance angle θA. This angle is formed between the center axis line of the core and the line that defines the maximum point where light entering the cable will undergo total internal reflection. If the light beam entering the end of the cable has an angle less than the critical angle, it will be internally reflected and propagated down the cable.
Refer to the angles in Fig. 19-11(b). External to the end of the cable is what is called a cone of acceptance; it is defi ned by the critical angle. Any light beam outside the cone will not be internally refl ected and transmitted down the cable.
the cone of acceptance defines the numerical aperture (NA) of the cable. This is a number less than 1 that gives some indication of the range of angles over which a particular cable will work. The NA can be calculated with the expression
NA = sin θc
For example, if the critical angle is 20°, the NA is
NA = sin 20° = 0.342
The NA can also be determined from the indices of refraction of the core and
The numerical aperture of a fi ber-optic cable is 0.29. What is the critical angle?
NA = sin θc
0.29= sin θc
θc = sin21 0.29= arcsin 0.29
θc = 16.86°
Fiber-Optic Cable Construction
Fiber-optic cables come in a variety of sizes, shapes, and types. The simplest cable contains a single strand of fi ber; a complex cable is made up of multiple fi bers with different layers and other elements.
The portion of a fiber-optic cable that carries the light is made from either glass or plastic. Another name for glass is silica. The optical characteristics of glass are superior to those of plastic. However, glass is far more expensive and more fragile than plastic. Although plastic is less expensive and more flexible, its attenuation of light is greater. For a given intensity, the light will travel a farther distance in glass than in plastic.
The construction of a fiber-optic cable is shown in Fig. 19-11(b). The glass or plastic optical fiber is contained within an outer cladding. The index of refraction of the outer cladding N2 is slightly less than the index of refraction N1 of the core. Typical values for N1 and N2 are 1.5 and 1.4, respectively. Over the cladding is a plastic jacket similar to the outer insulation on an electrical cable.
The fi ber, which is called the core, is usually surrounded by a protective cladding (see Fig. 19-12). The cladding is also made of glass or plastic but has a lower index of refraction. This ensures that the proper interface is achieved so that the light waves remain within the core. In addition to protecting the fi ber core from nicks and scratches, the cladding gives strength. Some fi ber-optic cable has a glass core with a glass cladding.
Other cables have a plastic core with plastic cladding. Another arrangement, plastic-clad silica (PCS) cable, is a glass core with plastic cladding.
Typically the division between the core and the cladding cannot be seen; they are usually made of the same types of material. Over the cladding is usually a plastic jacket similar to the outer insulation on an electrical cable.
In addition to the core, it’s cladding, and a jacket, fiber-optic cable usually contains one or more elements to form a complete cable. The simplest cable is the core with its cladding surrounded by a protective jacket. As in electrical cables, this outer jacket, or insulation, is made of some type of plastic, typically polyethylene, polyurethane, or polyvinyl chloride (PVC). The main purpose of this outer jacket is to protect the core and cladding from damage. Usually, fiber-optic cables are buried underground or strung between supports. Therefore, the fibers must be protected from moisture, dirt, temperature variations, and other conditions. The outer jacket also helps minimize physical damage, such as cuts, nicks, and crushing.
The more complex cables may contain two or more fiber-optic elements. Typical cables are available with 2, 6, 12, 18, and 24 optical fiber cores.
There are many different types of cable configurations. Many have several layers of protective jackets. Some cables incorporate a flexible strength or tension element that helps minimize damage to the fiber-optic elements when the cable is being pulled or must support its own weight. Typically, this strength member is made up of stranded steel or a special yarn known as Kevlar, which is strong and preferred over steel because it is an insulator. In some cables, Kevlar forms a protective sleeve or jacket over the cladding. Most claddings are covered with a clear protective coating for added strength and resistance to moisture and damage (see Fig. 19-13).
Fiber-optic cables are also available in a flat ribbon form (see Fig. 19-14). Flat ribbon cable works well with multiple fibers. Handling and identification of individual fi bers are easy. The flat cable is also more space-efficient for some applications.
Types of Fiber-Optic Cables
There are two basic ways of classifying fiber-optic cables: The first method is by the index of refraction, which varies across the cross section of the cable. The second method of classification is by mode, which refers to the various paths the light rays can take in passing through the fiber. Usually these two methods of classification are combined to define the types of cable.
Step Index Cable
The two ways to define the index of refraction variation across a cable are the step-index and the graded-index. Step index refers to the fact that there is a sharply defined step in the index of refraction where the fiber core and the cladding interface. It means that the core has one constant index of refraction N1 and the cladding has another constant index of refraction N2. When the two come together, there is a distinct step (see Fig. 19-15). If you were to plot a curve showing the index of refraction as it varies vertically from left to right across the cross-section of the cable, you would see a sharp increase in the index of refraction as the core is encountered and then a sharp decline in the index of refraction as the cladding is encountered.
Graded Index Cable
The other type of cable has a graded-index. Here, the index of refraction of the core is not constant. Instead, it varies smoothly and continuously over the diameter of the core (see Fig. 19-16). As you get closer to the center of the core, the index of refraction gradually increases, reaching a peak at the center and then declining as the other outer edge of the core is reached. The index of refraction of the cladding is constant.
Mode refers to the number of paths for the light rays in the cable. There are two classifications: single-mode and multimode. In a single-mode, the light follows a single path through the core; in a multimode, the light takes many paths.
Each type of fiber-optic cable uses one of these methods of rating the index or mode. In practice, there are three commonly used types of fiber-optic cable: multimode step-index, single-mode step-index, and multimode graded-index.
Multimode Step Index Cable
The multimode step-index fiber cable is probably the most common and widely used type. It is also the easiest to make and therefore the least expensive. It is widely used for short to medium distances at relatively low pulse frequencies.
The main advantage of a multimode stepped index fiber is its large size. Typical core diameters are in the 50- to the 1000-μm range. Such large-diameter cores are excel at gathering light and transmitting it efficiently. This means that an inexpensive light source such as an LED can be used to produce the light pulses. The light takes many hundreds or even thousands of paths through the core before exiting (refer to Fig. 19-11). Because of the different lengths of these paths, some of the light rays take longer to reach the other end of the cable than do others. The problem with this is that it stretches the light pulses. This is called dispersion.
Dispersion is the distortion of the optical signal due to the characteristics of the cable. For example, in Fig. 19-17, a short light pulse is applied to the end of the cable by the source. Light rays from the source travel in multiple paths. At the end of the cable, the rays that travel the shortest distance reach the end first. Other rays begin to reach the end of the cable later, until the light ray with the longest path finally reaches the end, concluding the pulse. In Fig. 19-17, ray A reaches the end first, then B, and then C. The result is a pulse at the other end of the cable that is lower in amplitude because of the attenuation of the light in the cable and increased in duration because of the different arrival times of the various light rays. This stretching of the pulse is referred to as modal dispersion.
Because the pulse has been stretched, pulses at the input cannot occur at a rate faster than the output pulse duration permits. Otherwise, the pulses will essentially merge (see Fig. 19-18). At the output, one long pulse will occur and will be indistinguishable from the three separate pulses originally transmitted. This means that incorrect information will be received. The only cure for this problem is to reduce the pulse repetition rate of the frequency of the pulses. When this is done, the proper operation occurs. But with pulses at a lower frequency, less information can be handled. The key point here is that the type of cable selected must have a modal dispersion sufficiently low to handle the desired upper frequency of operation.
Another type of dispersion is chromatic dispersion, which also causes pulse stretching. Chromatic dispersion occurs when multiple wavelengths of light are used, as in dense wavelength-division multiplexing (DWDM) systems. Since higher light frequencies travel faster than lower light frequencies, pulse stretching occurs. This type of dispersion is the most troubling at data rates above 10 Gbps. Special fiber sections or filters are used to correct it.
One more type of pulse stretching is caused by polarization mode dispersion (PMD). This is a phenomenon that occurs in single-mode fiber (SMF). SMF essentially supports two orthogonal (at a 90° angle) polarizations along the cable. Since the cable is not a perfect cylinder and because of cable distortions caused by bending, twisting, and other stresses, pulses with different polarization orientations can travel at different velocities in different parts of the cable. The orientations of connectors and splices also introduce this effect and distort the signal. The most common effect
is two pulses produced for each transmitted pulse. PMD is generally measured in picoseconds, so it does not materially affect pulses at rates under about 5 Gbps. At 10 Gbps and above, PMD becomes noticeable, especially in long-haul WANs and MANs. As with chromatic dispersion, special cable and physical compensators are available to correct it.
One of the newer and better ways to deal with dispersion is to use electronic dispersion compensation (EDC). EDC uses equalization techniques to adjust the received waveform to compensate for any type of dispersion. Equalizers can be made to correct the frequency response of a circuit, but also can be designed to correct for phase and amplitude differences. Most equalizers are fixed circuits and so only correct for problems of one type. However, adaptive equalizers adjust the correction for different speeds and distances. Adaptive equalizers adjust themselves to the fiber link into which they are connected after a short “training” period provides them with information about the type and degree of distortion produced. The equalizer then adds or subtracts energy to or from the signal in the preceding two or more pulse intervals to
correct for the dispersion.
An electronic dispersion compensation device is usually deployed at the serial output of the receiver. The equalizer is essentially a finite impulse response (FIR) DSP filter. The equalizer uses feedforward and feedback taps whose coefficients can be automatically adjusted. The received signal is sampled by a very fast A/D converter, and the samples are processed in a DSP chip. The DSP chip computes coefficients that are actively applied to the equalizer. The equalizer in effect performs the inverse function of the dispersive cable, thereby correcting the problem. Most modern 10-Gbps optical transceivers employ EDC. It essentially doubles the range that a given optical transceiver can cover.
Single-Mode Step Index Cable
A single-mode or mono mode step-index fiber cable essentially eliminates modal dispersion by making the core so small that the total number of modes or paths through the core is minimized (see Fig. 19-19). Typical core sizes are 2 to 15 μm. The only path through the core is down the center. With minimum refraction, little pulse stretching occurs. The output pulse has essentially the same duration as the input pulse.
Single-mode step-index fibers are by far the best because the pulse repetition rate can be high and the maximum amount of information can be carried. For very long-distance transmission and maximum information content, single-mode step-index fiber cables should be used.
The main problem with this type of cable is that it is extremely small, difficult to make, and therefore very expensive. It is also more difficult to handle. Splicing and making interconnections are more difficult. Finally, for proper operation, and expensive, superintense light source such as a laser must be used. For long distances, however, this is the type of cable preferred.
Multimode Graded Index Cable
Multimode graded-index fiber cables have several modes, or paths, of transmission through the cable, but they are much more orderly and predictable. Fig. 19-20 shows the typical paths of the light beams. Because of the continuously varying index of refraction across the core, the light rays are bent smoothly and repeatedly converge at points along the cable. The light rays near the edge of the core take a longer path but travel faster because the index of refraction is lower. All the modes or lightpaths tend to arrive at one point simultaneously. The result is less modal dispersion. As a result, this cable can be used at very high pulse rates, and therefore a considerable amount of information can be carried. This type of cable is also much wider in diameter, with core sizes in the 50- to 100- μm range. Therefore, it is easier to splice and interconnect, and cheaper, less intense light sources can be used.
The most popular kind of graded-index multimode fiber is designated OM1 through OM4. OM means optical multimode. These types are standardized by the TIA/EIA and are used in applications with shorter ranges up to about several thousand feet, such as a small campus or a multifl oor building. OM cables can handle the most common data rates from 1 Gbps and 10 Gbps up to 100 Gbps. Both LED and vertical cavity surface emitting laser (VCSEL) sources are common at wavelengths of 850 nm and 1300 nm.
The OM fibers are limited in their data rate per unit of length because of dispersion. The measure of this characteristic is the effective modal bandwidth (EMB). The bandwidth of an OM fiber is the frequency point where the optical power level drops to −3 dB relative to the zero frequency power. EMB is expressed as the bandwidth per unit of length, usually 1 kilometer. An example is 500 MHz-km. One MHz-km approximately translates to 0.7 to 0.8 Mbps of data rate. Therefore, an EMB of 500 MHz-km would allow a maximum data rate of 350 to 400 Mbps.
OM1 is 62.5/125 fiber with an EMB of 200 MHz-km. It is used with 100-Mbps Ethernet 100BASE-FX to 2000 meters or 1-Gbps Ethernet 1000BASE-SX to 275 meters.
It can be used with 10-Gbps Ethernet 10GBASE-SR to 33 meters. OM2 is 50/125 fiber with an EMB or 500 MHz-km. It is used with 100-Mbps Ethernet to 2000 meters, 1-Gbps Ethernet to 550 meters, and 10-Gbps Ethernet to 82 meters.
OM3 is 50/125 fiber with an EMB in the 1500 to 2000 MHz-km. It also works with 100-MHz Ethernet to 2000 meters, 1-Gbps Ethernet to 550 meters, and 10-Gbps Ethernet to 300 meters.
OM4 is also 50/125 fiber with an EMB in the 3500 to 4700 MHz-km. It also works with 100-MHz Ethernet to 2000 meters, 1-Gbps Ethernet to 1000 meters, and 10-Gbps Ethernet to 550 meters.
Both OM3 and OM4 can also be used with 40-Gbps or 100-Gbps Ethernet, up to 100 meters with OM3 or up to 150 meters with OM4.
Fiber-Optic Cable Specifications
The most important specifications of a fiber-optic cable are size, attenuation, and bandwidth. The NA is also sometimes given, although this specification is needed only when connectors are being designed—a rare occurrence because standard connectors are available.
The fiber-optic cable comes in a variety of sizes and configurations as previously indicated. Size is normally specified as the diameter of the core, and cladding is given in micrometers (μm), where 1 micrometer is one-millionth of a meter. For example, a common size for multimode fiber is 62.5/125, where 62.5 is the diameter of the core and 125 is the diameter of the cladding, both in micrometers (see Fig. 19-21). A PVC or polyurethane plastic jacket over the outer cladding gives a total outside diameter of about 3 mm or 0.118 in. Other common cable sizes for multimode fiber are 50/125 and 100/140, although the 62.5/125 is by far the most widely used. Common sizes for
single-mode fibers are 9/125 or 8.3/125 μm.
Cables come in two common varieties, simplex and duplex. Simplex cable, as the name implies, is just a single-fiber core cable, as shown in Fig. 19-21. In a common duplex cable, as shown in Fig. 19-22, two cables are combined within a single outer cladding. Cables are available with 4, 10, and 12 parallel fibers.
The most important specification of a fiber-optic cable is its attenuation. Attenuation refers to the loss of light energy as the light pulse travels from one end of the cable to the other. The light pulse of a specific amplitude or brilliance is applied to one end of the cable, but the light pulse output at the other end of the cable will be much lower in amplitude. The intensity of the light at the output is lower because of various losses in the cable. The main reason for the loss in light intensity over the length of the cable is light absorption, scattering, and dispersion.
Absorption refers to how light energy is converted to heat in the core material because of the impurity of the glass or plastic. This phenomenon is similar to electrical resistance. Scattering refers to the light lost due to light waves entering at the wrong angle and being lost in the cladding because of refraction. Dispersion, as mentioned, refers to the pulse stretching caused by the many different paths through the cable.
Although no light is lost as such in dispersion, the output is still lower in amplitude than the input, although the length of the light pulse has increased in duration.
The amount of attenuation varies with the type of cable and its size. Glass has less attenuation than plastic. Wider cores have less attenuation than narrower cores of the same material. Wider cores have less absorption and much more dispersion. Wider cores are also usually plastic, which has a greater absorption capacity than glass.
More important, attenuation is directly proportional to the length of the cable. It is obvious that the longer the distance the light has to travel, the greater the loss due to absorption, scattering, and dispersion.
The attenuation of a fiber-optic cable is expressed in decibels (dB) per unit of length. The standard is decibels per kilometer. The standard decibel formula is used
dB = 10 log Pout/Pin
where Pout is the power out and Pin is the power in. Because light intensity is a type of electromagnetic radiation, it is normally expressed and measured in power units, watts, or some fraction thereof. Fig. 19-23 shows the percentage of output power for various decibel losses.
For example, 3 dB represents half power. In other words, a 3-dB loss means that only 50 percent of the input appears at the output. The other 50 percent of the power is lost in the cable. The higher the decibel figure, the greater the attenuation and loss. A 30-dB loss means that only one-thousandth of the input power appears at the end. The standard specification for fiber-optic cable is attenuation expressed in terms of decibels per kilometer (dB/km).
The attenuation ratings of fiber-optic cables vary over a considerable range. The finest single-mode step-index cables have an attenuation of only 1 dB/km. However, very large core plastic fiber cables can have an attenuation of several thousand decibels per kilometer. A typical 62.5/125 cable has a loss in the 3- to 5-dB/km range. Typically, fibers with an attenuation of less than 10 dB/km are called low-loss fibers, and those with an attenuation between 10 and 100 dB/km are medium-loss fibers. High-loss fi bers have over 100-dB/km ratings. Naturally, the smaller the decibel number, the less the attenuation and the better the cable.
The total attenuation for a particular cable can be determined from the attenuation rating of the cable. For example, if a cable has an attenuation of 3.75 dB/km, a 5-km long cable has a total attenuation of 3.75 X 15 or 56.25 dB. If two cables are spliced together and one has an attenuation of 17 dB and the other 24 dB, the total attenuation is simply the sum, or 17 + 24, or 41 dB.
The bandwidth of a fiber-optic cable determines the maximum speed of the data pulses the cable can handle. The bandwidth is normally stated in terms of megahertz-kilometers (MHz·km). A common 62.5/125-μm cable has a bandwidth in the 100- to 300-MHz·km range. Cables with 500 and 600 MHz·km are also common. Even higher-bandwidth cables up to 5000 MHz-km are available to carry gigahertz-range signals.
As the length of the cable is increased, the bandwidth decreases in proportion. If a 160-MHz·km cable length is doubled from 1 to 2 km, its bandwidth is halved to 80 MHz·km.
A fi ber-optic cable has a bandwidth rating of 600 MHz·km. What is the bandwidth of
a 500-ft segment of cable?
Most fiber-optic cable operates over a relatively wide light frequency range, although it is normally optimized for a narrow range of light frequencies. The most commonly used light frequencies are 850, 1310, and 1550 nm (or 0.85, 1.31, and 1.55 μm). The cable has minimum attenuation to these frequencies.
Fig. 19-24 shows attenuation versus wavelength for a typical cable. Note that there is a peak in the attenuation at approximately 1.4 μm. This is caused by hydroxyl ions or undesired hydrogen-oxygen ions produced during the manufacturing process. These so-called water losses are avoided by carefully selecting the frequency of light transmission. Minimal loss occurs at 1.3 and 1.55 μm. Most LED or laser light sources operate at or near one of these wavelengths.
Connectors and Splicing
When long fi ber-optic cables are needed, two or more cables can be spliced together. The ends of the cable are perfectly aligned and then fused together with heat. A variety of connectors are available that provide a convenient way to splice cables and attach them to transmitters, receivers, and repeaters.
Connectors are special mechanical assemblies that allow fiber-optic cables to be connected to one another. Fiber-optic connectors are the optical equivalent of electrical plugs and sockets. They are mechanical assemblies that hold the ends of a cable and cause them to be accurately aligned with the ends of another cable. Most fiber-optic connectors either snap or twist together or have threads that allow the two pieces to be screwed together.
Connectors ensure precise alignment of the cables. The ends of the cables must be aligned with precision so that maximum light from one cable is transferred to another. A poor splice or connection will introduce excessive attenuation as the light exits one cable and enters the other. Fig. 19-25 shows several ways that cores can be misaligned. A connector can correct these problems.
A typical fiber-optic connector is shown in Fig. 19-26(a). One end of the connector, called the ferrule, holds the fiber securely in place. A matching fitting holds the other fiber securely in place. When the two are screwed together, the ends of the fibers touch, thereby establishing a low-loss coupling. Fig. 19-26(b) shows in greater detail how the connector aligns the fibers.
Dozens of different kinds of connectors are available for different applications. They are used in the parts of the system where it may be desirable to occasionally disconnect the fiber-optic cable for making tests or repairs. Connectors are normally used at the end of the cable applied to the light source or the end of the cable connected to the photodetector.
Connectors are also used at the repeater units where the light is picked up, converted to an electrical pulse, amplified and reshaped, and then used to create a new pulse to continue the transmission over a long line. Connectors are used on the back of interface adapters that plug into computers.
The two most common connector designations are ST and SMA. The ST connectors also referred to as bayonet connectors, use a half-twist cam-type arrangement like that used in BNC coaxial connectors. They are convenient for quick connect and disconnect. The SMA connectors are about the same size but have threaded connections. Two other popular connector types are the LC and SC. Both are snap-in type connectors with the LC connector being smaller.
Splicing fiber-optic cable means permanently attaching the end of one cable to another. This is usually done without a connector. The first step is to cut the cable, called cleaving the cable so that it is perfectly square on the end. Cleaving is so important to minimizing light loss that special tools have been developed to ensure perfect cuts. The two cables to be spliced are then permanently bonded together by heating them instantaneously to high temperatures so that they fuse or melt together. Special tools and splicing machines must be used to ensure perfect alignment.
Installing a connector begins with cleaving the fiber so that it is perfectly square. Polishing usually follows. Again, the special cleaving and polishing machines devised for this purpose must always be used. Poorly spliced cable or poorly installed connectors create an enormous loss.
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