Fiber-Optic Transmission Systems
JIM JACHETTAMultiDyne Video & Fiber-optic Systems Locust Valley, New York
An excerpt from the
National Association of Broadcasters ENGINEERING HANDBOOK 10th Edition
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C H A P T E R
6.10Fiber-Optic Transmission SystemsJIM JACHETTAMultiDyne Video & Fiber-optic Systems Locust Valley, New York
INTRODUCTION TO FIBER OPTICS Fiber-Optic MediumFiber optics is a method of carrying information using optical fibers. An optical fiber is a thin strand of glass or plastic that serves as the transmission medium over which information is sent. It thus fills the same basic function as a copper cable carrying a telephone conversation, computer data, or video. Unlike the copper cable, however, the optical fiber carries light instead of electrons. In so doing, it offers many distinct advantages that make it the transmission medium of choice for applications ranging from telephone calls, television, and machine control. The basic fiber-optic system is a link connecting two electronic circuits. Figure 6.10-1 shows a simple fiber-optic link. There are three basic parts to a fiber-optic system: Transmitter: The transmitter unit converts an electrical signal to an optical signal. The light source is typically a light-emitting diode, LED, or a laser diode. The light source performs the actual conversion from an electrical signal to an optical signal. The driving circuit for the light source changes the electrical signal into the driving current. Fiber-optic cable: The fiber-optic cable is the transmission medium for carrying the light. The cable includes the optical fibers in their protective jacket. Receiver: The receiver accepts the light or photons and converts them back into an electrical signal. In most cases, the resulting electrical signal is identical to the original signal fed into the transmitter. There are two basic sections of a receiver. First is the
detector that converts the optical signal back into an electrical signal. The second section is the output circuit, which reshapes and rebuilds the original signal before passing it to the output. Depending on the application, the transmitter and receiver circuitry can be very simple or quite complex. Other components that make up a fiber-optic transmission system, such as couplers, multiplexers, optical amplifiers, and optical switches, provide the means for building more complex links and communications networks. The transmitter, fiber, and receiver, however, are the basic elements in every fiber-optic system. Beyond the simple link, the fiber-optic medium is the fundamental building block for optical communications. Most electrical signals can be transported optically. Many optical components have been invented to permit signals to be processed optically without electrical conversion. Indeed, one goal of optical communications is to be able to operate entirely in the optical domain from system end to end.
Basic building blocks of a fiber-optic
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SECTION 6: TELEVISION TRANSMISSION another depends on the refractive indices of the two materials. In the case of fiber optics, this is the refractive index between the core and the cladding. Figure 6.10-2 illustrates the equations for Snells Law. In this figure, the upper region of the frame, n1, indicates a higher refractive index than the lower region n2. The refractive index of the upper region is designated as n1 while the lower region refractive index is n2. The figure on the top shows the case with the angle of the indices less than the critical angle. Note that the angle of the light changes at the interface between the higher refractive index, in region 1, and the lower refractive index, in region 2. In the center figure, the angle of indices has increased to the critical angle. At this point all the refracted light rays travel parallel to the interface region. In the figure on the bottom, the angle of indices has increased to a value greater than the critical angle. In this case 100% of the light refracts at the interface region. Advancements in laser technology next elevated the fiber-optics industry. Only the light-emitting diode or its higher powered counterpart, the laser diode, had the potential to generate large amounts of light in a focused beam small enough to be useful for fiber-optic transport. Communications engineers quickly noticed the importance of lasers and their higher modulation frequency capabilities. Light has the capacity to carry 10,000 times more information than radio frequencies. Because environmental conditions, such as rain, snow, and fog, disrupt laser light, a transmission scheme other than free space was needed. In 1966, Charles Kao and Charles Hockham, working at the Standard Telecommunications Laboratory, presented optical fibers as an ideal transmission medium, assuming fiber-optic attenuation could be kept under 20 dB per kilometer. Optical fibers of the day exhibited losses of 1,000 dB/km or more. At a loss of 20 dB/km, 99% of the light would be lost over only 1000 meters (3300 ft). Scientists theorized that the high levels of loss were due to impurities in the glass and not the glass itself. At the time in 1970, an optical loss of 20 dB/km was within the capabilities of electronics and optoelectronic components for short distances (less than 1 km) but not for longer distances (greater than 1 km). Dr. Robert Maurer, Donald Keck, and Peter Schultz of Corning succeeded in developing a glass fiber that exhibited attenuation at less than 20 dB/km, the limit for making fiber optics a usable technology. Other advances of the day, such as semiconductor chips, optical detectors, and optical connectors, initiated the true beginnings of the fiber-optic communications industry.
refraction index of the core, n1, is always less than that of the cladding, n2. Light incident on the boundary at less than the critical angle, 1, propagates through the boundary, but is refracted away from the normal to the boundary (a) at the critical angle, C, along the boundary (b). Light incident on the boundary at angles 1 above the critical angle is totally internally reflected (c). (Adapted from Force, Inc., illustration used with permission.)
FIGURE 6.10-2 Light wave refraction principles. The
Snells LawEarly fiber optics exhibited high loss that limited transmission distances. To correct this, glass fibers were developed that included a separate glass coating. The innermost region of the fiber, the core, carried the light, while the glass coating or cladding prevented the light from leaking out of the core by refracting the light back into the inner boundaries of the core. Snells Law explained this concept. It states that the angle at which a light reflects as it passes from one material to
Optical Windows and SpectrumWavelength remains a significant factor in fiber-optic developments. Figure 6.10-3 illustrates the wavelength windows. Table 6.10-1 shows the wavelength of each optical window and the typical application for multimode (MM) or single-mode (SM) operation.
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS TABLE 6.10-1 De Facto Standard Light WavelengthsFiber Types Nominal spectrum (nm) 850 30 (short wavelength) 1300 30 (long wavelength) Multimode Window (MM) I II III X X X X Single Mode (SM)
Fiber attenuation versus light wavelength characteristics.FIGURE 6.10-3
1550 30 (extra-long wavelength)
The earliest fiber-optic systems were developed at an operating wavelength of about 850 nm. This wavelength corresponded to the so called first window in a silica-based optical fiber, as shown in Figure 6.10-3. This window refers to the wavelength region that will offer a low optical loss that sits between several large absorption peaks. The absorption peaks are caused primarily by moisture in the fiber and Rayleigh scattering, which is the scattering of light due to random variations in the index of refraction caused by irregularities in the structure of the glass. The attraction to the 850 nm region came from its ability to use low-cost infrared LEDs and low-cost silicon detectors. As technology progressed, the first window lost its appeal due to its relatively high 3 dB/km losses. Most companies began to exploit the second window at 1310 nm with a lower attenuation of about 0.5 dB/km. In late 1977, Nippon Telegraph and Telephone developed the third window at 1550 nm. The third window offers an optical loss of about 0.2 dB/km. The three optical windows850 nm, 1310 nm, and 1550 nmare used in many fiber-optic installations today. The visible wavelength near 660 nm is used in low-end, short-distance systems. Each wavelength has its advantages. Longer wavelengths offer higher performance, but always come with higher cost.
Table 6.10-2 provides the typical optic attenuation for each of the common wavelengths versus the fiberoptic cable diameter. A narrower core fiber has less optical attenuation. The International Telecommunication Union (ITU), an international organization that promotes worldwide telecommunications standards, has specified six transmission bands for fiber-optic transmission. The first is the O band (original band), which is from 12601310 nm. The second band is the E band (extended band), which is 13601460 nm. The third band is the S band (short band), which is 14601530 nm. The fourth band in the spectrum is the C band (conventional band), which is 15301565 nm. The fifth band is the L band (longer band), which is 15601625 nm. The sixth band is the U band (ultra band), which is 16251675 nm. There is a seventh band that has not been defined by the ITU that is in the 850 nm region. It is mostly used in private networks. The seventh band is widely used in high-speed computer networking, video distribution, and corporate applications. Researchers have attempted to develop new fiber optics that could reduce costs or improve performance. Some alternative fiber materials have found specialized usage. Plastic fiber is ideal for short transmission distances that are ideal for home theater installations. Lower cost glass fiber reduces the need
TABLE 6.10-2 Typical Optical Fiber LossFiber Size () 9/125 50/125 62.5/125 100/140 110/125 200/230 MM Type SM 780 nm 3.0 3.57.0 4.08.0 4.58.0 Optical Loss (dB/km) 850 nm 2.5 2.56.0 3.07.0 3.57.0 15 12 1300 nm 0.50.8 0.74.0 1.04.0 1.55.0 1550 nm 0.20.4 0.63.5 1.04.0 1.55.0
SECTION 6: TELEVISION TRANSMISSION to develop longer distance plastic fiber and the higher cost of copper wire has expanded glass fiber-optic cable applications. fiber without passing through the axis. The path of the skew ray is typically helical, wrapping around and around the center axis. To simply analyze, skewer rays are ignored in most fiber-optics analysis. A cone known as the acceptance cone, shown in Figure 6.10-5, defines which light will be accepted and propagated by a total internal reflection. Light that enters the core from within this acceptance cone refracts down the fiber. Light outside the cone will not strike the core-to-cladding interface at the proper angle that allows total internal reflection. This light will not propagate. The specific characteristics of light propagation through fiber depend on many factors. The factors include the size and composition of the fiber as well as the light source injected into the fiber. An understanding of the interplay between these properties will clarify many aspects of fiber optics. Fiber itself has a very small diameter. Table 6.10-3 provides the core and cladding diameters of four commonly used fibers. To realize how small these fibers are, note that human hair has a diameter of about 100 . Fiber sizes are usually expressed by first giving the core size, followed by the cladding size. Thus, 50/125 means a core diameter of 50 microns (m) and a cladding diameter of 125 microns (m). Optical fibers are classified in two ways. One way is by the material makeup: Glass fiber: Glass fibers have a glass core and glass cladding. They are the most widely used type of fiber. The glass used in an optical fiber is an ultra pure and transparent silicon dioxide or fused quartz. If ocean water was as clear as fiber, one could see to the bottom of the Marianas Trench in the Pacific Ocean, a depth of 36,000 feet. Impurities are purposely added to the pure class to achieve the desired index of refraction. The elements germanium and phosphorus are added to increase the refractive index of the glass. Boron or fluorine is used to decrease the index. There are other impurities that are not removed when the class is purified. These additional impurities also affect the fiber properties by increasing attenuation from scattering or by the absorbing light.
Types of Fiber-optic MaterialThere are two distinct parts of a fiber optic cablethe optical fiber that carries the signal and the protective covering that keeps the fiber safe from environmental and mechanical damage. This section deals specifically with the optical fiber. An optical fiber has two concentric layers called core and cladding. The core (inner part) is the light-carrying part. The surrounding cladding provides the difference in refractive index that allows total internal reflection of light through the core. The index of refraction of the cladding is less than 1% lower than that of the core. Typical values, for example, are a core index of 1.47 and cladding index of 1.46. Fiber manufacturers must carefully control this difference to obtain the desired fiber characteristics. Fibers have an additional coating around the cladding. This coating, which is usually one or more layers of polymer, protects the core and cladding from shocks that might affect their optical or physical properties. The coating has no optical properties affecting the propagation of light within the fiber. This coating is just a shock absorber. Figure 6.10-4 shows the light traveling through a fiber. Light injected into the fiber and striking the coreto-cladding interface at a critical angle reflects back into the core. Since the angles of incident and reflection are equal, the light will again be reflected. The light will continue as expected down the length of the fiber. Light, however, striking the interface at less than the critical angle passes into the cladding, where it is lost over distance. The cladding is usually inefficient as a light carrier, and light in the cladding becomes attenuated fairly rapidly. The propagation of light is governed by the indices of the core and cladding and by Snells Law. Such total internal reflection forms the basis of light propagation through a simple optical fiber. This analysis considers only meridional rays, the rays that pass through the fiber center axis each time they are reflected. Other rays, called skew rays, travel down the
FIGURE 6.10-4 Total internal reflection in an optical fiber. Rays of light incident on the core/cladding boundary at greater than the critical angle, determined by the quotient n1/n2, propagate down the fibers core at a velocity determined by that fibers value. One ray is shown to keep the diagram simple. (From AMP, Inc., copyright illustration, used with permission.)
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS
Light ray acceptance cone geometry. The acceptance cone is an imaginary right angle cone extending outward coaxially from the fibers core. It is a measure of the light-gathering capability of a fiber. Its ray acceptance angle, called the numerical aperture (NA) of the fiber, is uniquely determined by the refractive indices of that fibers core and cladding. (From AMP, Inc., copyright illustration, used with permission.)
FIGURE 6.10-6 Optical fiber types. The core diameter
Plastic-clad silica (PCS): PCS fibers have a glass core and plastic cladding. The performance of PCS fiber is limited compared to a fiber made of all glass. Plastic: Plastic fibers have a plastic core and plastic cladding. Plastic fibers are limited by high optical loss and low bandwidth. The very low cost and ease of use make them attractive for applications where low bandwidth or high losses are acceptable. Plastic and PCS fibers do not have the buffer coating surrounding the cladding. The second way to classify fibers is by the refractive index of the core and the modes that the fiber propagates. Fiber can be categorized into three general types; Figure 6.10-6 shows the three general fiber types and their basic characteristics. Figure 6.10-6 shows the difference between the input pulse injected into a fiber and the output pulses exiting the fiber. The decrease in the height of the pulse shows the loss of optical signal power. The broadening of the pulse shows the bandwidth limiting effects of the fibers. It also shows the different paths of rays of light traveling down the fiber. And, it shows the relative index of refraction of the core and cladding for each type of fiber. TABLE 6.10-3 Core and Cladding Diameters of Four Commonly Used FibersCore () 8 50 62.5 100 Cladding () 125 125 125 140
and its refractive index characteristics determine the light propagation path or paths within the fibers core. (From AMP, Inc., copyright illustration, used with permission.)
ModesMode is a mathematical or physical concept describing the propagation of an electromagnetic wave through any media. In its mathematical form, mode theory derives from Maxwells equations. James Maxwell first developed mathematical expressions to the relationship between electric and magnetic energy. He proved that they were both a single form of electromagnetic energy, not two different forms as was then commonly believed. His equations also showed that the propagation of electromagnetic energy follows strict rules. Maxwells equations form the basis of electromagnetic theory. A mode is a solution to Maxwells equations. For purposes of this chapter, a mode is simply a path that a ray of light travels down a fiber. The number of modes that a given fiber will support ranges from 1 to over 100,000 individual rays of light. This depends on the physical properties of the fiber and fiber diameter.
Refractive Index ProfileThe refractive index profile describes the relationship between the indices of the core and cladding. Two main relationships exist: step index and graded index. The step index fiber has a core with a uniform index throughout. The profile shows a sharp step at the junction of the core and cladding. In contrast, graded index has a nonuniform core. The index is highest at the center of the core and gradually decreases until it matches that of the cladding. Therefore, there is no sharp transition between the core and the cladding. By this classification, there are three types of fibers: Multimode step index fiber, commonly called step index fiber.
SECTION 6: TELEVISION TRANSMISSION Single-mode step index fiber, called single-mode fiber. Multimode degraded index fiber, called graded index fiber. The characteristics of each type have an important bearing on its suitability for particular applications. Step Index Multimode Fiber The multimode step index fiber is the simplest type. It has a core diameter from 100970 microns. This fiber type includes glass, PCS, and plastic fibers. The step index fiber is the most widely used fiber type. This is despite relatively low bandwidth and high losses. Since light reflects at different angles for different paths, the different rays of light take a shorter or longer time to propagate down the fiber. The ray of light that travels straight down the center of the core arrives at the other end first. Other rays of light arrive later, since they refract back and forth in a zigzag path. Therefore, rays of light that enter the fiber at the same time exit the fiber at different times. The effect is that the light has spread out in time. This spreading of an optical pulse is called modal dispersion. A pulse of light that began as a tight and precisely defined shape has dispersed or spread over time. Dispersion describes the spreading of light by various mechanisms. Modal dispersion is that type of dispersion that results from the varying path lengths of each mode of light as it propagates through the fiber. The typical modal dispersion for a stepped index fiber ranges from 1530 ns per kilometer. This means that when rays of light enter a 1 km long fiber at the same time, the ray of light that takes the longest path will arrive 1530 ns after the ray of light that took the shortest path. The modal dispersion of 1530 billionths of a second does not seem to be very much, but dispersion is a fibers main limiting factor to bandwidth. Pulse spreading results in the overlapping of adjacent pulses, as shown in Figure 6.10-7. Eventually the pulses will merge so that one pulse cannot be distinguished from another. This results in the loss of information. Reducing the modal dispersion in a fiber will increase a fibers bandwidth. Graded Index Multimode Fiber One way to reduce modal dispersion is to use graded index fiber. Here the core has numerous concentric layers of glass, somewhat like the annular rings of a tree. Each successive layer outward from the central axis of the core has a lower index of refraction. Light travels faster in a lower index of refraction, so the further the light is from the center axis, the greater the speed. Each layer of the core refracts the light. Instead of being sharply refracted as it is in a step index fiber, the light is now bent or continually refracted in almost a sinusoidal pattern. Those rays that follow the longest path by traveling in the outside of the core have a faster average velocity. The light traveling near the center of the core has the slowest average velocity. As a result, all rays tend to reach the end of the fiber at the same time. The graded index reduces modal dispersion to 1 ns per kilometer or less. Popular graded index fibers have a core diameter of 50 or 62.5 microns and a cladding diameter of 125 microns. The fiber is popular in applications requiring high bandwidth, especially telecommunications, local area networks, computers, and video applications. Single-Mode Fiber Another way to reduce modal dispersion is to reduce the cores diameter until the fiber propagates only one mode efficiently. The single-mode fiber has a very small core diameter of only 512 microns. The standard cladding diameter is 125 microns. The cladding diameter was chosen for three reasons: The cladding must be about 10 times thicker than the core in a single-mode fiber. For a fiber with an 8 or 9 m core, the cladding should be at least 80 m. It is the same size as a graded index fiber that promotes size standardization. It promotes easy handling because it makes the fiber less fragile and because the diameter is reasonably large so that it can be handled by technicians.
Pulse spreading due to modal dispersion.
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS Since the single-mode fiber only carries one mode, modal dispersion does not exist. Single-mode fibers have a potential bandwidth of 50100 GHz-kilometers. Present fiber has a bandwidth of several GHz and allows transmissions of tens of kilometers. Dispersion-Shifted Single-Mode Fibers There are three types of single-mode optical fibers usually found in typical applications for telecommunications and data networks. Beyond standard singlemode fibers, there are also dispersion-shifted (DS) fibers and nonzero-dispersion-shifted (NZ-DS) fibers. The purpose of these fibers is to reduce dispersion in the transmission window having the lowest attenuation. Normally, attenuation is lowest in the 1550 nm window and dispersion is lowest in the 1310 nm window. Dispersion shifting creates a fiber that shifts the lowest dispersion to the 1550 nm region. This shifting of dispersion results in a fiber suited for highest data rates and longest transmission distances. In a standard single-mode fiber, the points of lowest loss and highest bandwidth do not coincide. Dispersion shifting brings them closer together. ing the 1980 winter Olympics the first use of fiber optics for a live television production in history. The telecommunications industries took advantage of this new technology. In 1977, both AT&T and GTE created fiber-optic telephone systems in Chicago and Boston, respectively. Soon after, fiber-optic telephone networks increased in number and reach. Network designers originally specified multimode-grated index fiber, but by the early 1980s, single-mode fiber operating in the 1310 nm and later in the 1550 nm wavelength windows became the standard. In 1983, British Telecoms entire phone system used single-mode fiber exclusively. Computer and information networks slowly moved to fiber. Today fiber is favored over copper due to lighter-weight cables, lightning strike immunity, and the increased bandwidth over longer distances. In the mid-1980s, the U.S. government deregulated telephone service, allowing small telephone companies to compete with the giant, AT&T. MCI and Sprint led the way by installing regional fiber-optic telecommunications networks throughout the world. Existing natural rights of way, such as railroad lines and gas pipes, allowed these and other companies to install thousands of miles of fiber-optic cable. With this boom, the fiber manufacturers output capacity struggled to keep up with the demand of the optical fiber needed to increase bandwidth over greater distances. In 1990, Bell Labs sent a 2.5 gigabit-per-second signal over 7,500 km without regeneration. With the use of soliton lasers and an erbium-doped fiber amplifier (EDFA), the light pulses maintained their shape and intensity. In 1998, Bell Labs researchers transmitted 100 simultaneous optical signals. Each optical signal was at a data rate of 10 Gbps and was transported for a distance of nearly 250 miles. The bandwidth on one fiber was increased to 1 terabit per second. This was achieved using dense wavelength-division multiplexing (DWDM) technology, which allows multiple wavelengths to be combined into one optical signal. Figure 6.10-8 illustrates a basic DWDM system. DWDM technologies continue to develop as the need for bandwidth increases. The potential bandwidth of fiber is 50 terahertz or better. DWDM technology has decreased greatly in cost and power consumption over the years. The DWDM laser technology requires strict temperature control and compensation. This makes the device draw high amounts of power and adds to the system costs. Today it is still a rather expensive form of optical multiplexing.
EARLY APPLICATIONS FOR FIBER OPTICSThe U.S. armed services immediately took advantage of fiber optics to improve its communications and tactical systems. In the early 1970s, the U.S. navy installed a fiber-optic telephone system aboard the USS Little Rock. In 1976, the air force followed suit by developing its Airborne Light Optical Fiber Technology (ALOFT) program. The early successes of these applications spawned a number of military research and development programs to create stronger fibers, tactical cables, and ruggedized, high-performance components for applications ranging from aircraft to undersea. Soon after, commercial applications began to appear. The broadcast television industry was always interested in systems that offered superior video transmission quality. In 1980, broadcasters of the winter Olympics, in Lake Placid, New York, requested a fiberoptic video transmission system for backup video feeds. The fiber-optic feed, because of its quality and reliability, soon became the primary video feed, mak-
FIGURE 6.10-8 Dense wave-division multiplexing.
SECTION 6: TELEVISION TRANSMISSION More commonly used in the broadcast television industry is coarse wave-division multiplexing (CWDM). CWDM technology gives the ability for up to 18 simultaneous optical signals on one fiber. This gives a usable bandwidth of more than 70 Gbps. CWDM optics are relatively common that operate at 4 Gbps. The Federal Communications Commission (FCC) mandated that all broadcasters switch from analog to digital transmission, which provides the capacity for high definition (HDTV). This presented researchers with the challenge to provide high bandwidth fiberoptic transport for HDTV. Beyond broadcast television, however, consumers are requesting to have broadband services, including data, audio, and video, delivered to the home.
ADVANTAGES OF FIBER-OPTIC TRANSMISSIONIn addition to fiber optics technical advantages, the cost of materials for Fiber optics is becoming more attractive because the cost of copper wire has risen substantialllyin revent years.
Longer DistancesA significant benefit of fiber-optic transmission is the capability to transport signals long distances. Basic systems are capable of sending signals up to 5 km over multimode fiber and up to 80 km over single mode without repeaters. Most modern fiber-optic systems transport information digitally. A digital fiber-optic system can be repeated or regenerated virtually indefinitely. An electro-optical repeater or an erbium doped fiber amplifier (EDFA) can be used to regenerate or amplify the optical signal.
INFORMATION TRANSMISSION OVER FIBER OPTICSA fiber-optic cable provides a pipeline that can carry large amounts of information. Copper wires or copper coaxial cable carry modulated electrical signals but only a limited amount of information, due to the inherent characteristics of copper cable. Free-space transmission, such as radio and TV signals, provides information transmission to many people, but this transmissions scheme cannot offer private channels. Also, the free-space spectrum is becoming a costly commodity with access governed by the FCC. Fiber-optic transmission offers high bandwidth and data rates, but it does not add to the crowded freespace spectrum.
Multiple SignalsAs discussed in previous sections, fiber has a bandwidth of more than 70 GHz using typical off-the-shelf fiber-optic transport equipment. Theoretically, hundreds, even thousands, of video and audio signals can be transported over a single fiber. This is achieved by using a combination of time-division multiplexing (TDM) and optical multiplexing. Fiber-optic transport equipment is readily available to transport more than 8 video and 32 audio channels per wavelength. Offthe-shelf coarse wave-division multiplexing CWDM equipment easily provides up to 18 wavelengths. This combination of equipment provides up to 144 video and 576 audio channels, as shown in Figure 6.10-9.
Information Modulation SchemesThe modulation scheme is the manner in which the information to be transported is encoded. Encoding information can improve the integrity of the transmission, allow more information to be sent per unit time, and in some cases, take advantage of some strength of the communication medium or overcome some weakness. Three basic techniques exist for transmitting information such as video signals over fiber optics: Amplitude modulation (AM) includes baseband AM, radio frequency (RF) carrier AM, and vestigial sideband AM. Frequency modulation (FM) includes sine wave FM, square wave FM, pulse FM, and FM-encoded vestigial sideband. Digital modulation of the optical light source with the ones and zeros of a digital data stream. A simplified explanation is that the light or laser source is off for a digital zero and on for a digital one. In actual practice, the light source never completely shuts off. The light source modulates darker and lighter for digital zero and one information.
SizeFiber-optic cable is very small in diameter and size when compared to copper. A single strand of fiberoptic cable is about 3 mm. A video coaxial cable is typically much larger. Fiber cable facilitates higher capacity in building conduits. There is often limited space in existing building conduits for infrastructure expansion. In mobile and field productions for sports and news events, fiber is often the cable of choice due to space limitations in a mobile and electronic newsgathering vehicle.
WeightA fiber-optic cable is substantially lighter in weight than copper cable. A single core PVC-jacketed fiber weighs about 25 pounds per kilometer; RG-6 copper coaxial cable may be three to four times as much.
Noise ImmunityA signal traveling on a copper cable is susceptible to electromagnetic interference. In many applications it
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS
Time-division and optical multiplexing equipment offers substantial capacity for carrying video and audio signals. (Courtesy of Multidyne Video & Fiber Optic Systems.)
is unavoidable to have to route cabling near power substations; heating, ventilating, and air-conditioning (HVAC) equipment; and other industrial sources of interference. A signal traveling as photons in an optical fiber is immune to such interference. The photons traveling down a fiber cable are immune to the effects of electromagnetic interference. In military applications, fiber systems are immune to an electromagnetic pulse (EMP) generated by a nuclear explosion in the Earths atmosphere. Fiber-optic equipment is used in command and control bunkers to isolate facilities and systems from EMP interference. A fiber-optic signal does not radiate any interference or noise.
cable. The connectors are already prepolished. No polishing equipment is needed.
Connector TypesOver the years as fiber-optic communications have grown and changed, there have been many different types of connectors. Today there are four common connector types that are used in most fiber-optic applications (Figure 6.10-10). The first is the ST connector (Figure 6.10-10(a)). It is a bayonet-style connector similar to a coaxial BNC connector, and is available for single-mode and multimode applications. The next style is the FC connector (Figure 6.10-10(b)). This connector has a threaded screwtype receptacle. It is similar to an RF-type connector, and is only used for single-mode applications. The telecommunications industry standardized on the SC connector (Figure 6.10-10(c)). It is a square snap-intype connector and has gained popularity in the video and computer networking industries. Telecommunications and networking applications typically require two fibers: one for transmitted data and one for received data traffic. Since SC-type connectors were popular in these types of applications,
Ease of InstallationOne of the myths regarding fiber is that it is difficult to install and maintain. This may have been true in the early days, but now it is as simple to terminate an optical fiber with a connector as it is to install a BNC connector on coax. Fiber-optic termination kits are now available that require no epoxy and special polishing. Simple cable stripping tools are used, similar to those used for copper coax, to prepare the fiber for termination. Epoxy-free connectors are available to terminate both multimode and single-mode fiber-optic
SECTION 6: TELEVISION TRANSMISSION two SC connectors were required. As the size of fiber
Radiation and SecurityFiber-optic transport is a secure means of communications. Since a fiber-optic cable emits or radiates no RF energy, it is impossible to passively listen or to tap into a fiber-optic circuit. The only way to tap into a fiberoptic cable is to physically cut the cable. An eavesdropper would have to cut the fiber and install a splitter to tap into the fiber-optic link. The cut in the fiber and the inserted splitter can be detected by fiber-optic test equipment.
Environmental ConditionsFiber-optic cable is immune to most environmental conditions. Fiber-optic cable is capable of tolerating temperature extremes. Unlike copper cable, fiber is immune to moisture. Fiber is available with jacketing that is resistant to nuclear radiation. Many fiber-optic systems are used for the inspection of nuclear reactors. Many military applications require fiber-optic equipment and cable to have resistance to radiation.
END-TO-END SYSTEM DESIGNFIGURE 6.10-10
Fiber-optic communications connec-
equipment reduced and the density of fiber-optic input/outputs (I/Os) increased, a small alternative to the SC connector was required. This led to the LC connector as shown in Figure 6.10-10(d). An LC is approximately half the size of an SC connector. It is rectangular in shape and has a locking clip.
A common misconception is that it is difficult to design a fiber-optic system. There are simple calculations to be made using information from the fiberoptic product datasheet. When designing a fiber-optic system it is necessary to know the number and type of signals to be sent through the fiber as well as the transmission distance or required optical budget. We also need to know the transmission distance or required optical budget.
Transmitter Launch PowerThe datasheet of any fiber-optic transport system will provide the transmitter units output optical power. There may be different models with varying levels of output power. A more powerful transmitter can be chosen to reach a further transmission distance. A typical fiber-optic transmitter has an output optical power of 8 dBm or 0.158 mW.
Ease of SplicingAnother myth is the repair or maintenance of a broken or cut fiber. The cost of fusion splicing equipment has come down significantly. The fusion splicer is a small portable device that is easily carried in the field. A fusion splice is easy to perform. First, the fiber is stripped and prepared using simple tools. The fiber is then placed in the fusion-splicing machine. An LCD screen shows the device automatically aligning the fibers. With the press of a button a fusion arc is generated to splice the fibers together. The fusion splicer even tests the connection when complete. There is now an even simpler way to splice a fiber in the fieldmechanical splicing. A mechanical splice consists of a small device that is used to splice a fiber. It is about 2 inches long by 1/2 inch wide. The process involves first stripping the fiber-optic cable and then inserting the ends into the splicing unit with mating gel. A key is used to close and clamp the unit shut. The mechanical splice gives fiber installers the ability to splice and repair with inexpensive equipment in areas where no electrical power is available.
Receiver SensitivityThe receiver sensitivity is another parameter found on any fiber-optic equipment datasheet. The receiver sensitivity is the minimum optical signal or power required for the receiver unit to operate properly. Many systems have a minimum receiver sensitivity of 28 dBm or 0.00158 mW. The 28 dBm value represents an optical power that is 28 decibels below the 0 dBm or 1 mW reference point.
Optical Power BudgetThe optical budget of a fiber-optic transport system takes into account the optical power of the transmitter, loss in the fiber for a given distance, receiver sensitiv-
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS ity, and signal-to-noise required. Optical power, like electrical power, is measured in watts or milliwatts. Fiber-optic systems are typically designed using decibels referenced to 1 milliwatt or 0 dBm. The following formula shows the conversion from watts to decibels: dBm = 10 log(laser power in mW). The output power of an optical laser may be 1 milliwatt. The equivalent power in dBm would be 10 * log (1mW) = 0 dBm. For 0.5 mW laser, output power would be 10 * log(0.5 mW) = 3 dBm. The optical attenuation of a multimode fiber at the 850 nm wavelength is about 3 dB/km. The attenuation on single-mode fiber at 1310 and 1550 nm is 0.5 and 0.2 dB/km, respectively. Using these numbers we can calculate how much optical power is required to reach a certain transmission distance. For example, a 10 km run over single-mode fiber at 1310 nm would incur a loss of 5 dB (10 km 0.5 dB/km). The optical budget that a fiber-optic system provides is the difference between the fiber-optic transmitter optical output power and the receiver sensitivity. For example, if the transmitter power is 8 dBm and the minimum receiver level is 28 dBm, then the maximum loss the system can withstand is 20 dBm. In many cases it may seem that a multimode or single-mode fiber run has optical power to reach 4060 km. When transmissions exceed about 5 km in multimode systems and about 15 km in single-mode systems, other factors due to dispersion come into play and limit the transmission distance. of the laser light source, the more dispersion. The analysis of dispersion of a single-mode fiber is very complex. An approximate calculation can be made with the following formula: BW = 0.187/(disp SW L), where: disp is the dispersion of the fiber at the operating wavelength with units seconds per nanometer per kilometer. SW is the spectral width (rms) of the light source in nanometers. L is the length of fiber cable in kilometers. For example, with a dispersion equal to 4 ps/nm/ km, spectral width of 3 nm, and a transmission length of 20 km, then: BW = 0.187/(4 1012 s/nm/km) (3 nm) (20 km) BW = 779,166,667 Hz or about 800 MHz. If the spectral width of the laser light source is doubled to 6 nm the bandwidth will drop to about 390 MHz. This shows how significant the spectral width of the laser source is on the usable bandwidth of a fiber. If a laser light source with a narrow optical spectral width is used, or a fiber with a lower dispersion figure, the bandwidth and transmission distance will increase. In single-mode fiber communications, there are two basic types of laser light sources. The first type is the less expensive laser that uses Fabre-Perot laser diode (FP-LD) technology. The FP-LD is an inexpensive choice for digital fiber-optic communication. With a spectral width of typically 4 nm or more, it is primarily used for lower bandwidth or short-distance applications. The second is the distributed feedback laser diode (DFB-LD) technology. These light sources are more expensive and are widely used for longdistance fiber-optic communications. The typical spectral width for a DFB laser is about 1 nm. When a DBF laser is used in combination with a low dispersion fiber, the transmission bandwidth and distance can be significantly higher. See Table 6.10-2, which shows the typical fiber-optic cable losses, and Table 6.10-4, which shows the bandwidth for different types of fiber cable.
BandwidthThe optical losses and usable bandwidth of a fiberoptic system have to be taken into account. As mentioned previously, multimode fibers have greater losses and less bandwidth compared to single mode. Single mode has lower losses and very high bandwidth than does multimode. Most manufacturers of multimode fiber-optic cable do not specify dispersion. They will provide a figure of merit known as the bandwidth-length product or just bandwidth with units of MHz-kilometer. For example, 500 MHz-km translates to a 500 MHz signal that can be transported 1 km. The product of the required bandwidth and transmission distance cannot exceed 500: BW L 500 A lower bandwidth signal can be sent a longer distance. A 100 MHz signal can be sent L = BW product/BW = 500 MHz-km/100 MHz = 5 km Single-mode fiber typically has a dispersion specification provided by the manufacturer. The dispersion is specified in picoseconds per kilometer per nanometer of light source spectral width or ps/km/nm. This loosely translates to the wider the spectral bandwidth
Optical LossesOptical loss or attenuation can vary from 300 to 0.2 dBm/km for plastic or single-mode fibers, respectively. Optical fiber has different loss characteristics at different wavelengths. The optical windows, as mentioned earlier, are regions within the optical fiber spectrum with low loss. The earliest fiber-optic systems operated in the first optical window in the 850 nm range. The second window is the 1310 nm range, which has zero dispersion. The third window is the 1550 nm window. A multimode fiber has an attenuation of about 4 dB/km at 850 nm and about 2.5 dB/km at 1310 nm. The multimode
SECTION 6: TELEVISION TRANSMISSION TABLE 6.10-4 Typical Fiber-Optic BandwidthFiber Size (m) 9/125 50/125 62.5/125 Type SM MM Bandwidth-Distance Product (MHz km) 850 nm 2000 200800 100400 1300 nm 20,000+ 4001500 2001000 1550 nm 400020,000+ 3001500 150500
fiber spectrum attenuation curve is shown in Figure 6.10-3. Note the high loss regions at 700, 1250, and 1380 nm. The single-mode fiber attenuation curve is shown in Figure 6.10-11. There are high-loss regions at 800, 1100, and 1490 nm regions. The high-loss region at about 1100 nm is called the mode transition region. This is where the fiber changes from multimode to singlemode characteristics. In order to make use of the low-loss properties of a given region in the fiber, the optic light source must generate light at that wavelength. For multimode fiber, light sources are used in the 850 and 1310 nm wavelengths. In single-mode fiber, light sources are typically at 1310 and 1550 nm. CWDM lasers are in the 14701610 nm range. The curve in Figure 6.10-11 shows that the fiber has low loss and a flat spectrum at these wavelengths. Corning introduced a CWDM metro fiber that eliminated the high water peak or the high-loss region centered at about 1380 nm. Most single-mode fibers, on new installation, use this flatspectrum fiber with a usable spectrum from about 12701610 nm. The new fiber gives the ability to have up to 18 CWDM wavelengths on one single-mode fiber. Most video fiber-optic systems take advantage of the 18 usable wavelengths. CWDM is far less expensive than its 42 wavelength counterpart, DWDM. With the fiber-optic systems available with up to 8 channels of video per wavelength, when combined with the
capabilities of CWDM optical multiplexing, more than 144 channels of video can be transported over one fiber. Plastic fiber is used over short distances due to high attenuation. The visible light region at around 650 nm is used over plastic fiber. Optical attenuation is constant at all bit rates and modulation frequencies. The attenuation in copper cable increases at higher bit rates and modulation frequencies. In a copper cable, a 100 MHz signal will be attenuated more per foot than a 50 MHz signal. This results in distances and bandwidth limitation. In a fiber cable, the 100 Mhz and 50 MHz signals are attenuated the same.
SYSTEM TESTING, TROUBLESHOOTING, AND MAINTENANCEThere are simple procedures to test, troubleshoot, and maintain a fiber-optic system. For basic procedures only simple inexpensive equipment is required. More sophisticated equipment can be used for advanced analysis.
Measuring Optical PowerThe optical power output from a fiber-optic transmitter or fiber cable can be measured with a simple and inexpensive light meter. The light meter is calibrated for each of the three optical windows850, 1310, and 1550 nm. The meters are available with interchangeable connectors so that systems with any fiber connector type can be tested. The meter gives a reading in milliwatts or dBm. When troubleshooting a fiber-optic system, the first step is to see if the transmitter unit is sending any optical power. The technician will attach the meter to the transmitter with a fiber patch cord. The output optical power of the transmitter can then be confirmed against the manufacturers datasheet. If the transmitter is within specification, the next step is the see if light is making it through the fiber to the receiver side. If the light level output of the transmitter does not meet specifications, this indicates the source of a possible failure. After reconnecting the transmitter back to the fiber, the optical meter is connected to the receiver side of the fiber. Measure the output fiber from the end of the fiber. The theoretical attenuation for the fiber length can be calculated. Using the theoretical attenua-
FIGURE 6.10-11 Single-mode fiber attenuation curve.
(Courtesy of Corning Glass Works.)
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS tion of the fiber and subtracting it from the transmitter optical output power, the power level that should be present at the fiber end near the receiver can be calculated. As long as the optical power level at the received side is higher than the receiver optical sensitivity, the fiber link should operate. If there is a low or no optical signal at the end of the fiber on the receiver side, the fiber may be damaged or have faulty or dirty connectors. Lint-free optical wipes, isopropyl alcohol, and a can of compressed air can be used to clean all optical connectors. A test of a fiber-optic cable can be performed prior to the purchase of fiber equipment. If a calibrated optical source is not available, handheld calibrated light sources are available as a companion device to the optical power meter. The calibrated light source can be attached to one side of the fiber and the optical meter to the other end.
FIBER-OPTIC TRANSMISSION SYSTEMS Digital ModulationThe digital bit is the basic unit of digital information. This unit has two values: one or zero. The bit represents the electronic equivalent of the circuit being on or off where a zero equals off and a one equals on. One bit of information is limited to these two values. The digital information is transmitted through the fiber serially one bit at a time. A digital pulse train represents the ones and zeros of digital information. The pulse train can also depict high and low electrical voltage levels or the presence and absence of a voltage.
Digital in the Television and Video IndustriesA digital signal can mean different things to video and cable TV system engineers, causing much confusion. The most common types of digital video and audio are: Uncompressed digital video and audio Lossless compression of digital video and audio Lossy compression of digital video and audio Complex digital modulations schemes such as 64 QAM, 256 QAM, 16 VSB, 64 QPSK, etc. SONET, ATM, or other telecom base standards Serial digital interface (SDI) High-definition serial digital interface (HD-SDI) Digital audio or AES/EBU
Optical Time Domain ReflectometerMore extensive tests can be performed with an optical time domain reflectometer (OTDR). An OTDR is a sophisticated device that sends a calibrated light source at a specific wavelength down one end of a fiber. The unit is extremely sensitive and measures the extremely low levels of light reflected back through the fiber. OTDR works very much like sonar. In sonar an audio tone is bounced off objects. The size of the reflection and the delay determine the size and distance of the objects. As the downstream light beam from the OTDR hits connectors, splices, and other defects in the fiber, it reflects small amounts of light back to the OTDR. Based on the size of the reflection and the time it takes for the reflection to return to the OTDR, the system will provide a calibrated representation of the attenuation and flaws in an optical fiber. The OTDR analysis can be performed at different wavelengths with various modulation schemes. An OTDR analysis is typically only necessary on very long fiber runs with many optical connectors, patch panels, and splices. It is easier to predict optical losses and bandwidth on the majority of fiber-optic cable runs since there will be a minimal number of connectors and splices. Most applications will not require an OTDR.
Cleaning and Maintaining Optical ConnectorsFiber-optic connectors should be cleaned with lint-free optical wipes, and 100% pure isopropyl alcohol should be used with the wipes. Compressed air is also useful to clean any dirt and debris from connectors or receptacles. There are cassette-type cleaning devices that have an advanceable cleaning ribbon. The tips of a male fiber-optic connector are typically ceramic. A protective cap should always be applied to the connector on the fiber cable as well as to the connector on the fiber equipment. This prevents damage and dirt build up.
The process of digitizing a standard NTSC video signal is straightforward. The typical bandwidth of a video signal is 4.5 MHz. Typically a sample rate of four times the video bandwidth is used or about 18 megasamples per second. The analog-to-digital (A/D) converter typically has a sampling resolution of 8, 10, or 12 bits. This process generates a serial digital data stream of about 144270 Mbps. The video signal is typically encoded in a digital format at the video source or in the video camera. Depending on the digital video format, the analog video will be encoded in one of several standard formats such as 4:2:2, 4:1:1, or 4:2:0. While these encoding schemes are not referred to as compression, they omit or remove certain information to reduce the systems bandwidth requirement. In the encoding schemes above, the three digits refer to the three common components of video. The first component is luminance (Y) or the light intensity of the video signal. The second is the color signal of red minus luminance (RY). The third component is the color signal of blue minus luminance (BY). These three components are referred to as YUV. The numbers 4:2:2 have to do with the fact that twice the bandwidth is used for the Y channel than the two color channels. This technique is a form of compression that will be addressed later in the chapter. HDTV or high-definition video requires a data rate of 1.485 Gbps for one uncompressed signal.
SECTION 6: TELEVISION TRANSMISSION The most efficient means of analog video transport utilizes analog to digital conversion. Once video and audio signals are converted to digital information, many channels can be combined into one high-speed data stream using TDM. The high-speed serial digital data stream is then converted to light via a laser or LED. The receiver unit performs the reverse function. The light or optical signal is received by a PIN photo detector. The optical signal is converted back into a serial data stream. The data stream is demultiplexed using TDM. The digital data is then converted back to video and audio via digital-to-analog (D/A) converters. Digital video transmission has many advantages over analog transmission. An analog fiber-optic system requires high-linearity optical components that are expensive and require fine tuning and complex calibration procedures. Once a video or audio signal has been digitized, it can be transported via fiber using readily available digital telecom optical components for both multimode and single-mode applications. A digital system has a higher immunity to noise and superior performance characteristics compared to an analog system. A digital signal can be regenerated and repeated virtually indefinitely without signal or performance degradation.
16-QAM encoding phase constella-
Compressed Digital VideoWhen compression is introduced into a video transport system, a substantial reduction in bandwidth can be implemented. A digital composite signal requires 144 Mbps and an HD-SDI signal requires 1.485 Gbps. When considering a system that will transport many channels of digital video, an enormous amount of bandwidth is required. A compression system removes redundant or repetitive information from the digital data stream. A compression or transmission encoding scheme will take advantage of limitations in the human eye. The human eye has lower sensitivity or resolution to color detail. Many compression or encoding schemes take this into account and compress or omit certain color details. There are two basic types of compression systems: lossless and lossy. A lossless compression system does not degrade the video or audio quality. The receiver unit recovers the original uncompressed information. A lossless compression system strictly removes repetitive information from the data stream. Most video content has repetitive information from one video frame to the next. For example, the background image may not change from frame to frame. Therefore, there is no need to send this information repetitive times. Unfortunately, a lossless compression scheme does not offer significant bandwidth savings. A compression rate of three to four times can be expected. A lossy compression scheme can achieve very high levels of compression but at the cost of image or signal quality. A lossy compression algorithm removes detail from the original image. Once the information has been removed, it cannot be reconstructed. There are many compression and encoding schemes used in
video transport. The 4:2:2, 4:1:1, and 4:1:0 encoding schemes mentioned earlier are a technique used to reduce bandwidth. Since the human eye has less sensitivity or less resolution for color, these encoding schemes have less bandwidth for the color information. The human eye has higher resolution horizontally than vertically. When taking this into account, most video formats have a higher horizontal resolution than vertical resolution.
QAM Digital EncodingQuadrature amplitude modulation (QAM) is a widely used modulation technique for video transport applications, particularly in digital cable TV systems. In a serial digital modulation scheme there are only two informational states: 1 and 0, or on and off. With 256QAM there are 256 states. The information is encoded by a varying 360 degree quadrature phase and amplitude. This modulation scheme can provide an enormous amount of data throughput in a limited amount of bandwidth, but a higher signal-to-noise band ratio is required. Figure 6.10-12 is the phase constellation for a 16-QAM signal.
MULTIPLEXINGIn communication, there are many techniques to transport multiple signals over one transmission medium. These techniques apply to fiber-optic transport.
Time-division MultiplexingTime-division multiplexing (TDM) is an encoding technique that combines many data streams into one
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS high-speed serial digital stream by combining each data stream in turn on a time basis and converting it into a single data stream. As a result, the single data stream is a sum of the total data from the streams to be multiplexed, plus some overhead bits to organize the data stream. To produce a serial data stream from a parallel data source, a parallel to serial converter or serializer may be employed that takes, for example, an 8-bit parallel data word and converts it into a 1-bit serial digital signal. If a system with eight 150 Mbps serializers is to be multiplexed together in a TDM system, the output serial data stream will be 8 times 150, or 1200 Mbps. The typical DWDM channel spacing is 0.8 nm. The tight spectrum of DWDM permits optical amplification using EDFA. DWDM technology is expensive and is seldom used in video applications. It is rare that an application will require the use of 80 wavelengths. DWDM technology is typically used for fiber-optic systems with long fiber-optic cables between continents and on the ocean floor. DWDM technology is only available for single-mode fiber.
APPLICATIONS FOR VIDEO FIBER-OPTIC TRANSPORTThere are many applications for fiber-optic communications. Any application that requires high bandwidth or high bit rate communications is ideally suited for fiber-optic transport. The television and video industries are a perfect application for fiber-optic transport. Analog television is a relatively high bandwidth signal of more than 5 MHz. Digital television (in particular HDTV) has bit rates of more than 1.5 Gbps. Highresolution computer graphics can have a bandwidth exceeding 500 MHz. All of these television and video applications are ideal for fiber.
Optical MultiplexingOptical multiplexing techniques can be used in addition to the TDM techniques mentioned above. If both optical multiplexing and TDM are combined, very large bandwidths of information can be transported by one fiber.
Wave-Division MultiplexingWave-division multiplexing (WDM) is the technique of taking two or more wavelengths or colors of light and combining them onto one fiber. On one end of the fiber the two wavelengths are combined, and then on the other end separated. Basic WDM uses two wavelengths. In multimode the 850 and 1310 nm wavelengths are used. In single mode, 1310 and 1550 nm are used. The two wavelengths can travel in the same direction or in opposite directions. Coarse Wave-Division Multiplexing Coarse wave-division multiplexing (CWDM) gives the ability to combine up to 18 wavelengths onto one fiber. The 18 wavelengths are evenly spaced from 12701610 nm in 20 nm increments. Each laser source is precisely tuned to a given wavelength to within 1 nm. What makes CWDM technology possible is extreme temperature stability in laser light sources from 070C without active cooling. CWDM lasers are relatively inexpensive and provide very high and scalable bandwidth. A system could be initially designed and installed using only one wavelength. At any point in the future, up to 17 more wavelengths can be added to increase the system capacity. Components are available to provide both multimode and single-mode CWDM systems. Dense Wave-Division Multiplexing Dense wave-division multiplexing (DWDM) takes optical bandwidth and throughput to a higher level. DWDM permits up to 80 wavelengths to share one fiber. The DWDM spectrum is spaced very tightly over a narrow range. The laser systems are complex in order to provide precise wavelength accuracy and stability over temperature. If the wavelength of a laser drifts, it will interfere with an adjacent channel.
Broadcast Television TransmissionAs mentioned earlier, television production and broadcast engineers have always sought out the best technology for media events such as the Olympics. In the mid-1980s, fiber-optic transport was introduced into the television industry. Fiber optics are used in all aspects of production and distribution of video and audio signals. The state of the art for the transport of analog video is to use 12-bit video digital encoding. The serial digital bit rate can vary from about 144300 Mbps. With the introduction of digital video in the 1990s, fiber-optic transport continued to enjoy growth in the broadcast industry. Digital video was encoded into data rates ranging from 144360 Mbps. These high bit rate video signals could only travel over copper up to about 300 meters. Transport distance beyond 300 meters with a coax required a repeater (which needs power) or a fiber optic system. The transition to 100% DTV/HDTV has created a need to transport signals with a bit rate as high as 1.5 Gbps. HDTV using an SDI interface (HD-SDI), in its native or uncompressed form, is 1.485 Gbps. HD-SDI can better reach about 150 meters over a coax. Once again, fiber is the only choice to reach distances beyond 150 meters. Systems can be designed using many of the technologies described above. Analog and digital signal transport can be mixed. Time-division and optical multiplexing can be combined. A broadcast television station may typically reside in a downtown metropolitan area. The television transmitter and satellite up and down links may be on a distant mountaintop outside the city. This situation is a perfect application for fiber transport. The system
SECTION 6: TELEVISION TRANSMISSION may require both analog video and digital video since the station may be in the midst of its conversion from analog to digital broadcast. Signals in both directions will be required to support downlink satellite video. Another typical application is that of back-haul feeds, where many channels of video and audio are trunked together over one fiber. Such a system can use TDM to combine groups of eight channels of video with audio into single wavelengths. The optical multiplexing or CWDM technology is used to combine the wavelengths with groups of eight videos onto one fiber. The combined technique of TDM and CWDM provides a fiber transport capacity of more than 144 video channels on one fiber. An example may be an airport terminal where arrival and departure information is displayed every 100 ft. This application requires a long daisy-chain of units that can drop and repeat the same signals to each monitor every 100 ft. Systems are available with the drop-and-repeat or daisy-chain feature. As shown in Figure 6.10-14, one transmitter can send the video signal to the first receiver. The first receiver decodes the optical signal and generates an output for the local monitor. The receiver also repeats and regenerates the optical signal to send to the next receiver in the chain. This technique saves on installation and equipment costs. The alternative would be to run a fiber from each monitor back to the control room. Instead, one fiber can feed many monitors.
High-Resolution Graphics and Video TransmissionThe quality and fidelity of an analog signal over long distances are difficult to maintain over copper. As signals increase in bandwidth and bit rate, it becomes more and more difficult for systems to transport these high bandwidth signals even a short distance over copper. This becomes very apparent when working with high-resolution video and graphics. A computer-generating RGB-HV or UXGA signal at 1,600 1,200 pixels requires an analog bandwidth of close to 500 MHz. If the signal is digitized, it requires a data transport bit rate of 34 Gbps. There are many copper-based products that will transport these signals but at a cost in performance and video quality. Figure 6.10-13 shows a typical RGB/UXGA high-resolution video and audio fiber-optic link. Many applications today require the same video or graphical signal to displayed on a series of monitors.
Optical Repeaters and Distribution AmplifiersThere are applications in fiber-optic communications where a signal requires regeneration and replication. The function required is similar to that of a distribution amplifier or digital signal reclocker. A passive splitter can be used to split an optical signal, but each signal is significantly weaker after the split. A device called an optical repeater or distribution amplifier can be used to repeat or regenerate a weak optical signal. This is helpful on long fiber-optic runs where a fiber signal is reaching its limit. The repeater can be used to regenerate the signal for further distribution. The same device can be used to replicate an optical signal. One optical signal can be replicated up to eight times with one device. Unlike the passive split were the optical output is diminished, the output optical signals are regenerated to full optical power. The device can also be used as a mode converter or wavelength remapper. The device can be configured with a single-mode input and multimode outputs. This gives the ability to convert from multimode to single mode or from one wavelength to another wavelength. The devise can convert optical signals to CWDM wavelengths.
Broadband Cable Television TransportBroadband cable television signals are traditionally transported over a hybrid system of fiber and coax. Hundreds of video sources are modulated onto individual carriers and combined into one broadband RF signal. These RF signals can have a frequency bandwidth of 48870 MHz. This is an enormous bandwidth to transport strictly over coax. Cable systems are designed to transport the high bandwidth signals over fiber to each residential community. The last mile of distribution is then accomplished over coax for delivery to each home. Multiple line amplifiers are used to transport the high bandwidth signals over coax the last mile. Another application is in a campus environment such as a corporation, university, or military base, where the buildings are spread over some distance. Many of these facilities have an internal cable televi-
FIGURE 6.10-13 Typical RGB/UXGA high-resolution
video and audio fiber-optic link. (Photo courtesy of Multidyne Video & Fiber Optic Systems.)
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS
FIGURE 6.10-14 Daisy-chain or drop-and-repeat video fiber transport.
sion system. These facilities will receive a commercial cable TV signal from a local provider. They will strip out the channels of interest, such as news and educational programming. They will then combine these channels with their own internal programming, such as human resource and training channels. The channels are then modulated and combined into one broadband RF signal. Many of the buildings on the campus may be several miles away. Broadband fiber-optic links can be used to get the cable TV signal to each building. The cable TV is then distributed in each building via coax the last several 100 ft.
Broadband RF and Satellite Link TransportThere are many broadband RF applications for fiber. One important application is satellite uplinks and downlinks. Commercial satellites, for long wavelength applications, typically use an intermediate frequency (IF) signal as a means of communication. The IF signal is typically 70 or 140 MHz. The signal has a limited transport distance over copper coax. The noise level and sensitivity of a satellite system will suffer with the use of copper coax. The relatively high bandwidth and noise sensitivity issues make IF signal transport ideal for fiber. Consumer and residential satellite service is finding its way into more and more commercial, corporate, and military applications. Consumer satellite equip-
ment is used in many applications as a source of information and news. A large corporation may use consumer satellite instead of cable for news and informational content. In a corporate building or military bunker, the satellite dish may be on the roof many floors above the control or conference room. The Lband signal has a bandwidth from 9502,150 MHz. A coax run will not transport the L-band satellite signal very far. A consumer satellite dish is small due to a shorter wavelength. It typically has a device called a low-noise block downconverter (LNB). The LNB is an active device that receives the satellite signal and translates and amplifies it to be sent down the coax to the receiver. The LNB requires a DC voltage, which is typically generated by the receiver and sent up the coax. If a fiber link is used, the DC voltage needs to be generated on the dish side of the fiber link. When designing an L-band fiber link, the system needs to provide the appropriate LNB DC power.
FIBER-OPTIC ROUTING SWITCHERSJust about every broadcast and audio visual system today has some sort of a video and audio routing switcher. The switcher gives the user the ability to control the source and destination of a given video and audio signal. As more and more video and communications migrate from copper to fiber, it makes sense that the need for an optical routing switcher arose. The optical routing switcher is a new concept for the video
SECTION 6: TELEVISION TRANSMISSION market, but it has been used for many years in the telecommunications industry. It has been used to route and control telephone traffic. As video and broadcast television industries become more and more complex with dozens of different video and encoding formats, optical switching starts to make more sense. In broadcast or video application there may be analog video, component video, SDI, and HD-SDI. An optical switch can switch most signals in the optical domain. There are two basic types of optical switching.
Photonic Fiber-Optic SwitcherThe first is 100% optical switching using 3D microelectromechanical mirror (MEMS) technology. It uses electronically controlled mirrors to route optical signals. This type of switch has an optical input, an optical cross-point, and an optical output. The abbreviation for this technology is OOO. An OOO switch provides only point-to-point switching. One input cannot be multicast to many outputs. The mirrors cannot point to more than one output at a time. The use of mirrors does permit multiple wavelengths and wavelengths in both directions. Switches are available is sizes from 8 8 to 256 256. Pure optical switching is available for multimode and single-mode applications. Optical switching supports both analog and digital optical signals. Pure optical switching is performed using 3D MEMS arrays. Tiny mirrors are fabricated out of silicon. The mirrors are positioned and controlled with electrostatic charges. The core of the optical switch is a 1 inch square cube. The cube has an array of up to 256 input fibers on the left side as shown in Figure 6.10-15. Each fiber has a lens that focuses the optical light onto a MEMS mirror. Each input has its own mirror. On the right side is an array of output fibers. Each output has a MEMS mirror. An optical connection is made when one input mirror aligns with one of the output mirrors. Fiber-optic switching is ideal for video broadcast, production, security, and other video applications requiring transmission, switching, and replication of high-quality optical signals. The fiber-optic switcher revolutionizes how video is distributed and managed. It is based on state-of-the-art field proven photonic switching technology. Laser light is switched in a pure optical format, without electrical conversion, allowing it to support transparent connections compatible with any video or data format including uncompressed HD video at 1.5 Gbps. Also, since the switching is done optically, the switch eliminates video degradation. With a traditional electrical switcher, electrical-to-optical (EO) and optical-to-electrical (OE) conversions are required that cause signal degradation and jitter. An optical switch supports a wide range of formats from 19.4 Mbps ATSC through 1.5 Gbps HDTV as well as NTSC, PAL, SECAM, SMPTE 259M serial digital (SDI) video, broadband analog, L-band, IF, and many more. The optical switcher will also transparently switch CWDM and DWDM signals.
FIGURE 6.10-15 Three dimensional MEMS pure optic switching element. (Photo provided by Calient Networks.)
Optical switcher technology can be used in the field to support applications requiring reliable, high-quality video distribution such as mobile production trucks, sports venues, and professional video facilities; campus video and surveillance networks; remote video monitoring; as well as government and military. Optical layer protection and fault tolerant switching can be configured for mission critical, nonstop applications. Optical switching is cost effective for any applications requiring 32 or more switched optical ports. It eliminates the need for expensive video transceivers to convert signals between electrical and optical formats. Switching the signals in optical format can substantially reduce the cost per port in fiber-optic transport equipment costs.
Electro-Optical SwitchThe second type is the electro-optical switch. The electro-optical switch uses a hybrid approach. The input is optical, the cross-point is electrical, and the output is optical. The abbreviation for this technology is OEO. An OEO switch supports point-to-multipoint or multicast switching. Any input can be switched to every output if necessary. Since the optical signal is converted to electrical, only one wavelength can be switched at a time. Also an electrical cross-point only operates in one direction. Therefore, only one wavelength in one direction is supported.
CHAPTER 6.10: FIBER-OPTIC TRANSMISSION SYSTEMS
THE FUTURE OF VIDEO FIBER-OPTIC TRANSPORTSystems are currently in development for the transport of high-resolution video at bit rates exceeding 10 Gbps. Digital cinema and the proliferation of HDTV television will demand fiber-optic transport systems with high bandwidth capabilities. Fiber transport to the home of video, telephone, and Internet traffic is slowly becoming a reality in many North American communities. This will fuel the demand for highspeed content delivery and distribution throughout the globe.
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