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Page 1: NAB Handbook-Fiber Optic Transmission Systems

Fiber-Optic Transmission Systems

JIM JACHETTA

MultiDyne Video & Fiber-optic Systems Locust Valley, New York

An excerpt from the National Association of Broadcasters

ENGINEERING HANDBOOK 10th Edition

Page 2: NAB Handbook-Fiber Optic Transmission Systems

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Page 3: NAB Handbook-Fiber Optic Transmission Systems

201NAB ENGINEERING HANDBOOKCopyright © 2007 Academic Press.

All rights of reproduction in any form reserved.

C H A P T E R

6.10

Fiber-Optic Transmission SystemsJIM JACHETTA

MultiDyne Video & Fiber-optic SystemsLocust Valley, New York

INTRODUCTION TO FIBER OPTICS

Fiber-Optic Medium

Fiber optics is a method of carrying information usingoptical fibers. An optical fiber is a thin strand of glassor plastic that serves as the transmission medium overwhich information is sent. It thus fills the same basicfunction as a copper cable carrying a telephone con-versation, computer data, or video. Unlike the coppercable, however, the optical fiber carries light instead ofelectrons. In so doing, it offers many distinct advan-tages that make it the transmission medium of choicefor applications ranging from telephone calls, televi-sion, and machine control.

The basic fiber-optic system is a link connectingtwo electronic circuits. Figure 6.10-1 shows a simplefiber-optic link.

There are three basic parts to a fiber-optic system:

• Transmitter: The transmitter unit converts an electri-cal 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 conver-sion 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 trans-mission 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 andreceiver circuitry can be very simple or quite complex.Other components that make up a fiber-optic trans-mission system, such as couplers, multiplexers, opticalamplifiers, and optical switches, provide the meansfor building more complex links and communicationsnetworks. The transmitter, fiber, and receiver, how-ever, are the basic elements in every fiber-optic sys-tem.

Beyond the simple link, the fiber-optic medium isthe fundamental building block for optical communi-cations. Most electrical signals can be transportedoptically. Many optical components have beeninvented to permit signals to be processed opticallywithout electrical conversion. Indeed, one goal of opti-cal communications is to be able to operate entirely inthe optical domain from system end to end.

FIGURE 6.10-1 Basic building blocks of a fiber-opticsystem.

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Snell’s Law

Early fiber optics exhibited high loss that limitedtransmission distances. To correct this, glass fiberswere developed that included a separate glass coating.The innermost region of the fiber, the core, carried thelight, while the glass coating or cladding preventedthe light from leaking out of the core by refracting thelight back into the inner boundaries of the core. Snell’sLaw explained this concept. It states that the angle atwhich a light reflects as it passes from one material to

another depends on the refractive indices of the twomaterials.

In the case of fiber optics, this is the refractive indexbetween the core and the cladding. Figure 6.10-2 illus-trates the equations for Snell’s Law. In this figure, theupper region of the frame, n1, indicates a higher refrac-tive index than the lower region n2. The refractiveindex of the upper region is designated as n1 while thelower region refractive index is n2. The figure on thetop shows the case with the angle of the indices lessthan the critical angle. Note that the angle of the lightchanges at the interface between the higher refractiveindex, in region 1, and the lower refractive index, inregion 2. In the center figure, the angle of indices hasincreased to the critical angle. At this point all therefracted light rays travel parallel to the interfaceregion. In the figure on the bottom, the angle of indi-ces has increased to a value greater than the criticalangle. In this case 100% of the light refracts at theinterface region.

Advancements in laser technology next elevatedthe fiber-optics industry. Only the light-emitting diodeor its higher powered counterpart, the laser diode, hadthe potential to generate large amounts of light in afocused beam small enough to be useful for fiber-optictransport.

Communications engineers quickly noticed theimportance of lasers and their higher modulation fre-quency capabilities. Light has the capacity to carry10,000 times more information than radio frequencies.Because environmental conditions, such as rain, snow,and fog, disrupt laser light, a transmission schemeother than free space was needed. In 1966, CharlesKao and Charles Hockham, working at the StandardTelecommunications Laboratory, presented opticalfibers as an ideal transmission medium, assumingfiber-optic attenuation could be kept under 20 dB perkilometer. Optical fibers of the day exhibited losses of1,000 dB/km or more. At a loss of 20 dB/km, 99% ofthe light would be lost over only 1000 meters (3300 ft).

Scientists theorized that the high levels of loss weredue to impurities in the glass and not the glass itself.At the time in 1970, an optical loss of 20 dB/km waswithin the capabilities of electronics and opto-electronic components for short distances (less than 1km) but not for longer distances (greater than 1 km).Dr. Robert Maurer, Donald Keck, and Peter Schultz ofCorning succeeded in developing a glass fiber thatexhibited attenuation at less than 20 dB/km, the limitfor making fiber optics a usable technology. Otheradvances of the day, such as semiconductor chips,optical detectors, and optical connectors, initiated thetrue beginnings of the fiber-optic communicationsindustry.

Optical Windows and Spectrum

Wavelength remains a significant factor in fiber-opticdevelopments. Figure 6.10-3 illustrates the wave-length “windows.” Table 6.10-1 shows the wavelengthof each optical window and the typical application formultimode (MM) or single-mode (SM) operation.

FIGURE 6.10-2 Light wave refraction principles. Therefraction index of the core, n1, is always less than thatof the cladding, n2. Light incident on the boundary atless than the critical angle, ϕ1, propagates through theboundary, but is refracted away from the normal tothe boundary (a) at the critical angle, ϕC, along theboundary (b). Light incident on the boundary atangles ϕ1 above the critical angle is totally internallyreflected (c). (Adapted from Force, Inc., illustrationused with permission.)

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The earliest fiber-optic systems were developed atan operating wavelength of about 850 nm. This wave-length corresponded to the so called “first window” ina silica-based optical fiber, as shown in Figure 6.10-3.This window refers to the wavelength region that willoffer a low optical loss that sits between several largeabsorption peaks. The absorption peaks are causedprimarily by moisture in the fiber and Rayleigh scat-tering, which is the scattering of light due to randomvariations in the index of refraction caused by irregu-larities in the structure of the glass.

The attraction to the 850 nm region came from itsability to use low-cost infrared LEDs and low-cost sili-con detectors. As technology progressed, the first win-dow lost its appeal due to its relatively high 3 dB/kmlosses. Most companies began to exploit the “secondwindow” at 1310 nm with a lower attenuation ofabout 0.5 dB/km. In late 1977, Nippon Telegraph andTelephone developed the “third window” at 1550 nm.The third window offers an optical loss of about 0.2dB/km.

The three optical windows—850 nm, 1310 nm, and1550 nm—are used in many fiber-optic installationstoday. The visible wavelength near 660 nm is used inlow-end, short-distance systems. Each wavelength hasits advantages. Longer wavelengths offer higher per-formance, but always come with higher cost.

Table 6.10-2 provides the typical optic attenuationfor each of the common wavelengths versus the fiber-optic cable diameter. A narrower core fiber has lessoptical attenuation.

The International Telecommunication Union (ITU),an international organization that promotes world-wide telecommunications standards, has specified sixtransmission bands for fiber-optic transmission. Thefirst is the O band (“original band”), which is from1260–1310 nm. The second band is the E band(“extended band”), which is 1360–1460 nm. The thirdband is the S band (“short band”), which is 1460–1530nm. The fourth band in the spectrum is the C band(“conventional band”), which is 1530–1565 nm. Thefifth band is the L band (“longer band”), which is1560–1625 nm. The sixth band is the U band (“ultraband”), which is 1625–1675 nm. There is a seventhband that has not been defined by the ITU that is inthe 850 nm region. It is mostly used in private net-works. The seventh band is widely used in high-speedcomputer networking, video distribution, and corpo-rate applications.

Researchers have attempted to develop new fiberoptics that could reduce costs or improve perfor-mance. Some alternative fiber materials have foundspecialized usage. Plastic fiber is ideal for short trans-mission distances that are ideal for home theaterinstallations. Lower cost glass fiber reduces the need

FIGURE 6.10-3 Fiber attenuation versus light wave-length characteristics.

TABLE 6.10-1De Facto Standard Light Wavelengths

Nominal spectrum(nm)

Fiber Types

WindowMultimode

(MM)

SingleMode(SM)

850 ± 30(short wavelength)

I X

1300 ± 30(long wavelength)

II X X

1550 ± 30(extra-long wavelength)

III X

TABLE 6.10-2Typical Optical Fiber Loss

Fiber Optical Loss (dB/km)

Size (μ) Type 780 nm 850 nm 1300 nm 1550 nm

9/125 SM 3.0 2.5 0.5–0.8 0.2–0.4

50/125

MM

3.5–7.0 2.5–6.0 0.7–4.0 0.6–3.5

62.5/125 4.0–8.0 3.0–7.0 1.0–4.0 1.0–4.0

100/140 4.5–8.0 3.5–7.0 1.5–5.0 1.5–5.0

110/125 15

200/230 12

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to develop longer distance plastic fiber and the highercost of copper wire has expanded glass fiber-opticcable applications.

Types of Fiber-optic Material

There are two distinct parts of a fiber optic cable—theoptical fiber that carries the signal and the protectivecovering that keeps the fiber safe from environmentaland mechanical damage. This section deals specifi-cally with the optical fiber.

An optical fiber has two concentric layers calledcore and cladding. The core (inner part) is the light-car-rying part. The surrounding cladding provides the dif-ference in refractive index that allows total internalreflection of light through the core. The index ofrefraction of the cladding is less than 1% lower thanthat of the core. Typical values, for example, are a coreindex of 1.47 and cladding index of 1.46. Fiber manu-facturers must carefully control this difference toobtain the desired fiber characteristics.

Fibers have an additional coating around the clad-ding. This coating, which is usually one or more layersof polymer, protects the core and cladding fromshocks that might affect their optical or physical prop-erties. The coating has no optical properties affectingthe propagation of light within the fiber. This coatingis just a shock absorber.

Figure 6.10-4 shows the light traveling through afiber. Light injected into the fiber and striking the core-to-cladding interface at a critical angle reflects backinto the core. Since the angles of incident and reflec-tion are equal, the light will again be reflected. Thelight will continue as expected down the length of thefiber.

Light, however, striking the interface at less thanthe critical angle passes into the cladding, where it islost over distance. The cladding is usually inefficientas a light carrier, and light in the cladding becomesattenuated fairly rapidly. The propagation of light isgoverned by the indices of the core and cladding andby Snell’s Law.

Such total internal reflection forms the basis of lightpropagation through a simple optical fiber. This analy-sis considers only meridional rays, the rays that passthrough the fiber center axis each time they arereflected. Other rays, called skew rays, travel down the

fiber without passing through the axis. The path of theskew ray is typically helical, wrapping around andaround the center axis. To simply analyze, skewer raysare ignored in most fiber-optics analysis.

A cone known as the acceptance cone, shown in Fig-ure 6.10-5, defines which light will be accepted andpropagated by a total internal reflection. Light thatenters the core from within this acceptance conerefracts down the fiber. Light outside the cone will notstrike the core-to-cladding interface at the properangle that allows total internal reflection. This lightwill not propagate.

The specific characteristics of light propagationthrough fiber depend on many factors. The factorsinclude the size and composition of the fiber as well asthe light source injected into the fiber. An understand-ing of the interplay between these properties will clar-ify many aspects of fiber optics.

Fiber itself has a very small diameter. Table 6.10-3provides the core and cladding diameters of four com-monly used fibers.

To realize how small these fibers are, note thathuman hair has a diameter of about 100 μ. Fiber sizesare usually expressed by first giving the core size, fol-lowed by the cladding size. Thus, 50/125 means a corediameter of 50 microns (μm) and a cladding diameterof 125 microns (μm).

Optical fibers are classified in two ways. One way isby 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 germa-nium and phosphorus are added to increase the refractive index of the glass. Boron or fluorine is used to decrease the index. There are other impuri-ties 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.

FIGURE 6.10-4 Total internal reflection in an optical fiber. Rays of light incident on thecore/cladding boundary at greater than the critical angle, determined by the quotientn1/n2, propagate down the fiber’s core at a velocity determined by that fiber’s value.One ray is shown to keep the diagram simple. (From AMP, Inc., copyright illustration,used with permission.)

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• 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 coat-ing surrounding the cladding.

The second way to classify fibers is by the refractiveindex of the core and the modes that the fiber propa-gates. Fiber can be categorized into three generaltypes; Figure 6.10-6 shows the three general fibertypes and their basic characteristics.

Figure 6.10-6 shows the difference between theinput pulse injected into a fiber and the output pulsesexiting the fiber. The decrease in the height of thepulse shows the loss of optical signal power. Thebroadening of the pulse shows the bandwidth limitingeffects of the fibers. It also shows the different paths ofrays of light traveling down the fiber. And, it showsthe relative index of refraction of the core and clad-ding for each type of fiber.

Modes

Mode is a mathematical or physical concept describ-ing the propagation of an electromagnetic wavethrough any media. In its mathematical form, modetheory derives from Maxwell’s equations. James Max-well first developed mathematical expressions to therelationship between electric and magnetic energy. Heproved that they were both a single form of electro-magnetic energy, not two different forms as was thencommonly believed. His equations also showed thatthe propagation of electromagnetic energy followsstrict rules. Maxwell’s equations form the basis of elec-tromagnetic theory.

A mode is a solution to Maxwell’s equations. Forpurposes of this chapter, a mode is simply a path thata ray of light travels down a fiber. The number ofmodes that a given fiber will support ranges from 1 toover 100,000 individual rays of light. This depends onthe physical properties of the fiber and fiber diameter.

Refractive Index Profile

The refractive index profile describes the relationshipbetween the indices of the core and cladding. Twomain relationships exist: step index and graded index.The step index fiber has a core with a uniform indexthroughout. The profile shows a sharp step at the junc-tion of the core and cladding. In contrast, gradedindex has a nonuniform core. The index is highest atthe center of the core and gradually decreases until itmatches that of the cladding. Therefore, there is nosharp transition between the core and the cladding. Bythis classification, there are three types of fibers:

• Multimode step index fiber, commonly called step index fiber.

FIGURE 6.10-5 Light ray acceptance cone geometry.The acceptance cone is an imaginary right angle coneextending outward coaxially from the fiber’s core. Itis a measure of the light-gathering capability of afiber. Its ray acceptance angle, called the numericalaperture (NA) of the fiber, is uniquely determined bythe refractive indices of that fiber’s core and clad-ding. (From AMP, Inc., copyright illustration, usedwith permission.)

TABLE 6.10-3Core and Cladding Diameters of

Four Commonly Used Fibers

Core (μ) Cladding (μ)

8 125

50 125

62.5 125

100 140

FIGURE 6.10-6 Optical fiber types. The core diameterand its refractive index characteristics determine thelight propagation path or paths within the fiber’s core.(From AMP, Inc., copyright illustration, used with per-mission.)

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• Single-mode step index fiber, called single-mode fiber.

• Multimode degraded index fiber, called graded index fiber.

The characteristics of each type have an importantbearing on its suitability for particular applications.

Step Index Multimode Fiber

The multimode step index fiber is the simplest type. Ithas a core diameter from 100–970 microns. This fibertype includes glass, PCS, and plastic fibers. The stepindex fiber is the most widely used fiber type. This isdespite relatively low bandwidth and high losses.

Since light reflects at different angles for differentpaths, the different rays of light take a shorter orlonger time to propagate down the fiber. The ray oflight that travels straight down the center of the corearrives at the other end first. Other rays of light arrivelater, since they refract back and forth in a zigzag path.Therefore, rays of light that enter the fiber at the sametime exit the fiber at different times. The effect is thatthe light has spread out in time.

This spreading of an optical pulse is called modaldispersion. A pulse of light that began as a tight andprecisely defined shape has dispersed or spread overtime. Dispersion describes the spreading of light byvarious mechanisms. Modal dispersion is that type ofdispersion that results from the varying path lengthsof each mode of light as it propagates through thefiber.

The typical modal dispersion for a stepped indexfiber ranges from 15–30 ns per kilometer. This meansthat when rays of light enter a 1 km long fiber at thesame time, the ray of light that takes the longest pathwill arrive 15–30 ns after the ray of light that took theshortest path.

The modal dispersion of 15–30 billionths of a sec-ond does not seem to be very much, but dispersion is afiber’s main limiting factor to bandwidth. Pulsespreading results in the overlapping of adjacentpulses, as shown in Figure 6.10-7. Eventually thepulses will merge so that one pulse cannot be distin-guished from another. This results in the loss of infor-

mation. Reducing the modal dispersion in a fiber willincrease a fiber’s bandwidth.

Graded Index Multimode Fiber

One way to reduce modal dispersion is to use gradedindex fiber. Here the core has numerous concentriclayers of glass, somewhat like the annular rings of atree. Each successive layer outward from the centralaxis of the core has a lower index of refraction.

Light travels faster in a lower index of refraction, sothe further the light is from the center axis, the greaterthe speed. Each layer of the core refracts the light.Instead of being sharply refracted as it is in a stepindex fiber, the light is now bent or continuallyrefracted in almost a sinusoidal pattern. Those raysthat follow the longest path by traveling in the outsideof the core have a faster average velocity. The lighttraveling near the center of the core has the slowestaverage velocity. As a result, all rays tend to reach theend of the fiber at the same time. The graded indexreduces modal dispersion to 1 ns per kilometer or less.

Popular graded index fibers have a core diameter of50 or 62.5 microns and a cladding diameter of 125microns. The fiber is popular in applications requiringhigh bandwidth, especially telecommunications, localarea networks, computers, and video applications.

Single-Mode Fiber

Another way to reduce modal dispersion is to reducethe core’s diameter until the fiber propagates only onemode efficiently. The single-mode fiber has a verysmall core diameter of only 5–12 microns. The stan-dard cladding diameter is 125 microns. The claddingdiameter 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 μm core, the cladding should be at least 80 μm.

• It is the same size as a graded index fiber that pro-motes size standardization.

• It promotes easy handling because it makes the fiber less fragile and because the diameter is reason-ably large so that it can be handled by technicians.

FIGURE 6.10-7 Pulse spreading due to modal dispersion.

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Since the single-mode fiber only carries one mode,modal dispersion does not exist. Single-mode fibershave a potential bandwidth of 50–100 GHz-kilome-ters. Present fiber has a bandwidth of several GHz andallows transmissions of tens of kilometers.

Dispersion-Shifted Single-Mode Fibers

There are three types of single-mode optical fibersusually found in typical applications for telecommuni-cations and data networks. Beyond standard single-mode fibers, there are also dispersion-shifted (DS)fibers and nonzero-dispersion-shifted (NZ-DS) fibers.The purpose of these fibers is to reduce dispersion inthe transmission window having the lowest attenua-tion. Normally, attenuation is lowest in the 1550 nmwindow and dispersion is lowest in the 1310 nm win-dow. Dispersion shifting creates a fiber that shifts thelowest dispersion to the 1550 nm region. This shiftingof dispersion results in a fiber suited for highest datarates and longest transmission distances. In a standardsingle-mode fiber, the points of lowest loss and high-est bandwidth do not coincide. Dispersion shiftingbrings them closer together.

EARLY APPLICATIONS FOR FIBER OPTICS

The U.S. armed services immediately took advantageof fiber optics to improve its communications and tac-tical systems. In the early 1970s, the U.S. navyinstalled a fiber-optic telephone system aboard theUSS Little Rock. In 1976, the air force followed suit bydeveloping its Airborne Light Optical Fiber Technol-ogy (ALOFT) program. The early successes of theseapplications spawned a number of military researchand development programs to create stronger fibers,tactical cables, and ruggedized, high-performancecomponents for applications ranging from aircraft toundersea.

Soon after, commercial applications began toappear. The broadcast television industry was alwaysinterested in systems that offered superior video trans-mission quality. In 1980, broadcasters of the winterOlympics, in Lake Placid, New York, requested a fiber-optic video transmission system for backup videofeeds. The fiber-optic feed, because of its quality andreliability, soon became the primary video feed, mak-

ing the 1980 winter Olympics the first use of fiberoptics for a live television production in history.

The telecommunications industries took advantageof this new technology. In 1977, both AT&T and GTEcreated fiber-optic telephone systems in Chicago andBoston, respectively. Soon after, fiber-optic telephonenetworks increased in number and reach. Networkdesigners originally specified multimode-grated indexfiber, but by the early 1980s, single-mode fiber operat-ing in the 1310 nm and later in the 1550 nm wave-length windows became the standard. In 1983, BritishTelecom’s entire phone system used single-mode fiberexclusively. Computer and information networksslowly moved to fiber. Today fiber is favored over cop-per due to lighter-weight cables, lightning strikeimmunity, and the increased bandwidth over longerdistances.

In the mid-1980s, the U.S. government deregulatedtelephone service, allowing small telephone compa-nies to compete with the giant, AT&T. MCI and Sprintled the way by installing regional fiber-optic telecom-munications networks throughout the world. Existingnatural rights of way, such as railroad lines and gaspipes, allowed these and other companies to installthousands of miles of fiber-optic cable. With thisboom, the fiber manufacturer’s output capacity strug-gled to keep up with the demand of the optical fiberneeded to increase bandwidth over greater distances.

In 1990, Bell Labs sent a 2.5 gigabit-per-second sig-nal over 7,500 km without regeneration. With the useof soliton lasers and an erbium-doped fiber amplifier(EDFA), the light pulses maintained their shape andintensity. In 1998, Bell Labs researchers transmitted100 simultaneous optical signals. Each optical signalwas at a data rate of 10 Gbps and was transported fora distance of nearly 250 miles. The bandwidth on onefiber was increased to 1 terabit per second. This wasachieved using dense wavelength-division multiplex-ing (DWDM) technology, which allows multiplewavelengths to be combined into one optical signal.Figure 6.10-8 illustrates a basic DWDM system.

DWDM technologies continue to develop as theneed for bandwidth increases. The potential band-width of fiber is 50 terahertz or better. DWDM tech-nology has decreased greatly in cost and powerconsumption over the years. The DWDM laser tech-nology requires strict temperature control and com-pensation. This makes the device draw high amountsof power and adds to the system costs. Today it is stilla rather expensive form of optical multiplexing.

FIGURE 6.10-8 Dense wave-division multiplexing.

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More commonly used in the broadcast televisionindustry is coarse wave-division multiplexing (CWDM).CWDM technology gives the ability for up to 18simultaneous optical signals on one fiber. This gives ausable bandwidth of more than 70 Gbps. CWDMoptics are relatively common that operate at 4 Gbps.

The Federal Communications Commission (FCC)mandated that all broadcasters switch from analog todigital transmission, which provides the capacity forhigh definition (HDTV). This presented researcherswith the challenge to provide high bandwidth fiber-optic transport for HDTV. Beyond broadcast televi-sion, however, consumers are requesting to havebroadband services, including data, audio, and video,delivered to the home.

INFORMATION TRANSMISSION OVER FIBER OPTICS

A fiber-optic cable provides a pipeline that can carrylarge amounts of information. Copper wires or coppercoaxial cable carry modulated electrical signals butonly a limited amount of information, due to theinherent characteristics of copper cable.

Free-space transmission, such as radio and TV sig-nals, provides information transmission to many peo-ple, but this transmissions scheme cannot offer privatechannels. Also, the free-space spectrum is becoming acostly commodity with access governed by the FCC.Fiber-optic transmission offers high bandwidth anddata rates, but it does not add to the crowded free-space spectrum.

Information Modulation Schemes

The modulation scheme is the manner in which theinformation to be transported is encoded. Encodinginformation can improve the integrity of the trans-mission, allow more information to be sent per unittime, and in some cases, take advantage of somestrength of the communication medium or overcomesome weakness.

Three basic techniques exist for transmitting infor-mation 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 sim-plified 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.

ADVANTAGES OF FIBER-OPTIC TRANSMISSION

In addition to fiber optics technical advantages, thecost of materials for Fiber optics is becoming moreattractive because the cost of copper wire has risensubstantialllyin revent years.

Longer Distances

A significant benefit of fiber-optic transmission is thecapability to transport signals long distances. Basicsystems are capable of sending signals up to 5 km overmultimode fiber and up to 80 km over single modewithout repeaters. Most modern fiber-optic systemstransport information digitally. A digital fiber-opticsystem can be repeated or regenerated virtually indef-initely. An electro-optical repeater or an erbium dopedfiber amplifier (EDFA) can be used to regenerate oramplify the optical signal.

Multiple Signals

As discussed in previous sections, fiber has a band-width of more than 70 GHz using typical off-the-shelffiber-optic transport equipment. Theoretically, hun-dreds, even thousands, of video and audio signals canbe transported over a single fiber. This is achieved byusing a combination of time-division multiplexing(TDM) and optical multiplexing. Fiber-optic transportequipment is readily available to transport more than8 video and 32 audio channels per wavelength. Off-the-shelf coarse wave-division multiplexing CWDMequipment easily provides up to 18 wavelengths. Thiscombination of equipment provides up to 144 videoand 576 audio channels, as shown in Figure 6.10-9.

Size

Fiber-optic cable is very small in diameter and sizewhen compared to copper. A single strand of fiber-optic cable is about 3 mm. A video coaxial cable is typ-ically much larger. Fiber cable facilitates higher capac-ity in building conduits. There is often limited space inexisting building conduits for infrastructure expan-sion. In mobile and field productions for sports andnews events, fiber is often the cable of choice due tospace limitations in a mobile and electronic news-gathering vehicle.

Weight

A fiber-optic cable is substantially lighter in weightthan copper cable. A single core PVC-jacketed fiberweighs about 25 pounds per kilometer; RG-6 coppercoaxial cable may be three to four times as much.

Noise Immunity

A signal traveling on a copper cable is susceptible toelectromagnetic interference. In many applications it

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is unavoidable to have to route cabling near powersubstations; heating, ventilating, and air-conditioning(HVAC) equipment; and other industrial sources ofinterference. A signal traveling as photons in an opti-cal fiber is immune to such interference. The photonstraveling down a fiber cable are immune to the effectsof electromagnetic interference. In military applica-tions, fiber systems are immune to an electromagneticpulse (EMP) generated by a nuclear explosion in theEarth’s atmosphere. Fiber-optic equipment is used incommand and control bunkers to isolate facilities andsystems from EMP interference. A fiber-optic signaldoes not radiate any interference or noise.

Ease of Installation

One of the myths regarding fiber is that it is difficult toinstall and maintain. This may have been true in theearly days, but now it is as simple to terminate anoptical fiber with a connector as it is to install a BNCconnector on coax. Fiber-optic termination kits arenow available that require no epoxy and special pol-ishing. Simple cable stripping tools are used, similar tothose used for copper coax, to prepare the fiber for ter-mination. Epoxy-free connectors are available to ter-minate both multimode and single-mode fiber-optic

cable. The connectors are already prepolished. No pol-ishing equipment is needed.

Connector Types

Over the years as fiber-optic communications havegrown and changed, there have been many differenttypes of connectors. Today there are four commonconnector types that are used in most fiber-optic appli-cations (Figure 6.10-10).

The first is the ST connector (Figure 6.10-10(a)). It isa bayonet-style connector similar to a coaxial BNCconnector, and is available for single-mode and multi-mode applications.

The next style is the FC connector (Figure 6.10-10(b)).This connector has a threaded screw–type receptacle.It is similar to an RF-type connector, and is only usedfor single-mode applications.

The telecommunications industry standardized onthe SC connector (Figure 6.10-10(c)). It is a squaresnap-in–type connector and has gained popularity inthe video and computer networking industries.

Telecommunications and networking applicationstypically require two fibers: one for transmitted dataand one for received data traffic. Since SC-type con-nectors were popular in these types of applications,

FIGURE 6.10-9 Time-division and optical multiplexing equipment offers substantialcapacity for carrying video and audio signals. (Courtesy of Multidyne Video & FiberOptic Systems.)

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two SC connectors were required. As the size of fiber

equipment reduced and the density of fiber-opticinput/outputs (I/Os) increased, a small alternative tothe SC connector was required. This led to the LC con-nector as shown in Figure 6.10-10(d). An LC is approx-imately half the size of an SC connector. It isrectangular in shape and has a locking clip.

Ease of Splicing

Another myth is the repair or maintenance of a brokenor cut fiber. The cost of fusion splicing equipment hascome down significantly. The fusion splicer is a smallportable device that is easily carried in the field.

A fusion splice is easy to perform. First, the fiber isstripped and prepared using simple tools. The fiber isthen placed in the fusion-splicing machine. An LCDscreen shows the device automatically aligning thefibers. With the press of a button a fusion arc is gener-ated to splice the fibers together. The fusion splicereven tests the connection when complete.

There is now an even simpler way to splice a fiberin the field—mechanical splicing. A mechanical spliceconsists of a small device that is used to splice a fiber.It is about 2 inches long by 1/2 inch wide. The processinvolves first stripping the fiber-optic cable and theninserting the ends into the splicing unit with matinggel. A key is used to close and clamp the unit shut. Themechanical splice gives fiber installers the ability tosplice and repair with inexpensive equipment in areaswhere no electrical power is available.

Radiation and Security

Fiber-optic transport is a secure means of communica-tions. Since a fiber-optic cable emits or radiates no RFenergy, it is impossible to passively listen or to tap intoa fiber-optic circuit. The only way to tap into a fiber-optic cable is to physically cut the cable. An eaves-dropper would have to cut the fiber and install a split-ter to tap into the fiber-optic link. The cut in the fiberand the inserted splitter can be detected by fiber-optictest equipment.

Environmental Conditions

Fiber-optic cable is immune to most environmentalconditions. Fiber-optic cable is capable of toleratingtemperature extremes. Unlike copper cable, fiber isimmune to moisture. Fiber is available with jacketingthat is resistant to nuclear radiation. Many fiber-opticsystems are used for the inspection of nuclear reactors.Many military applications require fiber-optic equip-ment and cable to have resistance to radiation.

END-TO-END SYSTEM DESIGN

A common misconception is that it is difficult todesign a fiber-optic system. There are simple calcula-tions to be made using information from the fiber-optic product datasheet. When designing a fiber-opticsystem it is necessary to know the number and type ofsignals to be sent through the fiber as well as the trans-mission distance or required optical budget. We alsoneed to know the transmission distance or requiredoptical budget.

Transmitter Launch Power

The datasheet of any fiber-optic transport system willprovide the transmitter unit’s output optical power.There may be different models with varying levels ofoutput power. A more powerful transmitter can bechosen to reach a further transmission distance. A typ-ical fiber-optic transmitter has an output opticalpower of –8 dBm or 0.158 mW.

Receiver Sensitivity

The receiver sensitivity is another parameter found onany fiber-optic equipment datasheet. The receiver sen-sitivity is the minimum optical signal or powerrequired 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 repre-sents an optical power that is 28 decibels below the 0dBm or 1 mW reference point.

Optical Power Budget

The optical budget of a fiber-optic transport systemtakes into account the optical power of the transmitter,loss in the fiber for a given distance, receiver sensitiv-

FIGURE 6.10-10 Fiber-optic communications connec-tor types.

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ity, and signal-to-noise required. Optical power, likeelectrical power, is measured in watts or milliwatts.Fiber-optic systems are typically designed using deci-bels referenced to 1 milliwatt or 0 dBm. The followingformula shows the conversion from watts to decibels:

dBm = 10 × log(laser power in mW).

The output power of an optical laser may be 1 milli-watt. The equivalent power in dBm would be 10 * log(1mW) = 0 dBm. For 0.5 mW laser, output powerwould be 10 * log(0.5 mW) = –3 dBm.

The optical attenuation of a multimode fiber at the850 nm wavelength is about 3 dB/km. The attenuationon single-mode fiber at 1310 and 1550 nm is 0.5 and0.2 dB/km, respectively. Using these numbers we cancalculate how much optical power is required to reacha certain transmission distance. For example, a 10 kmrun over single-mode fiber at 1310 nm would incur aloss of 5 dB (10 km × 0.5 dB/km).

The optical budget that a fiber-optic system pro-vides is the difference between the fiber-optic trans-mitter optical output power and the receiversensitivity. For example, if the transmitter power is –8dBm and the minimum receiver level is –28 dBm, thenthe maximum loss the system can withstand is 20dBm.

In many cases it may seem that a multimode or sin-gle-mode fiber run has optical power to reach 40–60km. When transmissions exceed about 5 km in multi-mode systems and about 15 km in single-mode sys-tems, other factors due to dispersion come into playand limit the transmission distance.

Bandwidth

The optical losses and usable bandwidth of a fiber-optic system have to be taken into account. As men-tioned previously, multimode fibers have greaterlosses and less bandwidth compared to single mode.Single mode has lower losses and very high band-width than does multimode.

Most manufacturers of multimode fiber-optic cabledo not specify dispersion. They will provide a figureof merit known as the bandwidth-length product orjust bandwidth with units of MHz-kilometer. Forexample, 500 MHz-km translates to a 500 MHz signalthat can be transported 1 km. The product of therequired bandwidth and transmission distance cannotexceed 500:

BW × L ≤ 500

A lower bandwidth signal can be sent a longer dis-tance. 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 specifi-cation provided by the manufacturer. The dispersionis specified in picoseconds per kilometer per nanome-ter of light source spectral width or ps/km/nm. Thisloosely translates to the wider the spectral bandwidth

of the laser light source, the more dispersion. The anal-ysis of dispersion of a single-mode fiber is very com-plex. An approximate calculation can be made withthe 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 lengthof 20 km, then:

BW = 0.187/(4 × 10–12 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 dou-bled to 6 nm the bandwidth will drop to about 390MHz. This shows how significant the spectral width ofthe laser source is on the usable bandwidth of a fiber.If a laser light source with a narrow optical spectralwidth is used, or a fiber with a lower dispersion fig-ure, the bandwidth and transmission distance willincrease.

In single-mode fiber communications, there are twobasic types of laser light sources. The first type is theless expensive laser that uses Fabre-Perot laser diode(FP-LD) technology. The FP-LD is an inexpensivechoice for digital fiber-optic communication. With aspectral width of typically 4 nm or more, it is pri-marily used for lower bandwidth or short-distanceapplications. The second is the distributed feedbacklaser diode (DFB-LD) technology. These light sourcesare more expensive and are widely used for long-distance fiber-optic communications. The typical spec-tral width for a DFB laser is about 1 nm. When a DBFlaser is used in combination with a low dispersionfiber, the transmission bandwidth and distance can besignificantly higher.

See Table 6.10-2, which shows the typical fiber-opticcable losses, and Table 6.10-4, which shows the band-width for different types of fiber cable.

Optical Losses

Optical loss or attenuation can vary from 300 to 0.2dBm/km for plastic or single-mode fibers, respec-tively. Optical fiber has different loss characteristics atdifferent wavelengths. The optical windows, as men-tioned earlier, are regions within the optical fiber spec-trum with low loss.

The earliest fiber-optic systems operated in the firstoptical window in the 850 nm range. The second win-dow is the 1310 nm range, which has zero dispersion.The third window is the 1550 nm window. A multi-mode fiber has an attenuation of about 4 dB/km at 850nm and about 2.5 dB/km at 1310 nm. The multimode

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fiber spectrum attenuation curve is shown in Figure6.10-3. Note the high loss regions at 700, 1250, and1380 nm. The single-mode fiber attenuation curve isshown in Figure 6.10-11. There are high-loss regions at800, 1100, and 1490 nm regions. The high-loss region atabout 1100 nm is called the mode transition region. Thisis where the fiber changes from multimode to single-mode characteristics.

In order to make use of the low-loss properties of agiven region in the fiber, the optic light source mustgenerate light at that wavelength. For multimodefiber, light sources are used in the 850 and 1310 nmwavelengths. In single-mode fiber, light sources aretypically at 1310 and 1550 nm. CWDM lasers are in the1470–1610 nm range. The curve in Figure 6.10-11shows that the fiber has low loss and a flat spectrum atthese wavelengths. Corning introduced a CWDMmetro fiber that eliminated the high water peak or thehigh-loss region centered at about 1380 nm. Mostsingle-mode fibers, on new installation, use this flat-spectrum fiber with a usable spectrum from about1270–1610 nm. The new fiber gives the ability to haveup to 18 CWDM wavelengths on one single-modefiber.

Most video fiber-optic systems take advantage ofthe 18 usable wavelengths. CWDM is far less expen-sive than its 42 wavelength counterpart, DWDM. Withthe fiber-optic systems available with up to 8 channelsof video per wavelength, when combined with the

capabilities of CWDM optical multiplexing, more than144 channels of video can be transported over onefiber.

Plastic fiber is used over short distances due to highattenuation. The visible light region at around 650 nmis used over plastic fiber. Optical attenuation is con-stant at all bit rates and modulation frequencies. Theattenuation in copper cable increases at higher bitrates and modulation frequencies. In a copper cable, a100 MHz signal will be attenuated more per foot thana 50 MHz signal. This results in distances and band-width limitation. In a fiber cable, the 100 Mhz and 50MHz signals are attenuated the same.

SYSTEM TESTING, TROUBLESHOOTING, AND MAINTENANCE

There are simple procedures to test, troubleshoot, andmaintain a fiber-optic system. For basic proceduresonly simple inexpensive equipment is required. Moresophisticated equipment can be used for advancedanalysis.

Measuring Optical Power

The optical power output from a fiber-optic transmit-ter or fiber cable can be measured with a simple andinexpensive light meter. The light meter is calibratedfor each of the three optical windows—850, 1310, and1550 nm. The meters are available with interchange-able connectors so that systems with any fiber connec-tor type can be tested. The meter gives a reading inmilliwatts or dBm.

When troubleshooting a fiber-optic system, the firststep is to see if the transmitter unit is sending any opti-cal power. The technician will attach the meter to thetransmitter with a fiber patch cord. The output opticalpower of the transmitter can then be confirmedagainst the manufacturer’s datasheet. If the transmit-ter is within specification, the next step is the see iflight is making it through the fiber to the receiver side.

If the light level output of the transmitter does notmeet specifications, this indicates the source of a pos-sible failure. After reconnecting the transmitter back tothe fiber, the optical meter is connected to the receiverside of the fiber. Measure the output fiber from the endof the fiber. The theoretical attenuation for the fiberlength can be calculated. Using the theoretical attenua-

TABLE 6.10-4Typical Fiber-Optic Bandwidth

Fiber Bandwidth-Distance Product (MHz × km)

Size (μm) Type 850 nm 1300 nm 1550 nm

9/125 SM 2000 20,000+ 4000–20,000+

50/125MM

200–800 400–1500 300–1500

62.5/125 100–400 200–1000 150–500

FIGURE 6.10-11 Single-mode fiber attenuation curve.(Courtesy of Corning Glass Works.)

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tion of the fiber and subtracting it from the transmitteroptical output power, the power level that should bepresent at the fiber end near the receiver can be calcu-lated. As long as the optical power level at thereceived side is higher than the receiver optical sensi-tivity, the fiber link should operate. If there is a low orno optical signal at the end of the fiber on the receiverside, the fiber may be damaged or have faulty or dirtyconnectors. Lint-free optical wipes, isopropyl alcohol,and a can of compressed air can be used to clean alloptical connectors.

A test of a fiber-optic cable can be performed priorto the purchase of fiber equipment. If a calibrated opti-cal source is not available, handheld calibrated lightsources are available as a companion device to theoptical power meter. The calibrated light source can beattached to one side of the fiber and the optical meterto the other end.

Optical Time Domain Reflectometer

More extensive tests can be performed with an opticaltime domain reflectometer (OTDR). An OTDR is asophisticated device that sends a calibrated lightsource at a specific wavelength down one end of afiber. The unit is extremely sensitive and measures theextremely low levels of light reflected back throughthe fiber.

OTDR works very much like sonar. In sonar anaudio tone is bounced off objects. The size of thereflection and the delay determine the size and dis-tance of the objects. As the downstream light beamfrom the OTDR hits connectors, splices, and otherdefects in the fiber, it reflects small amounts of lightback to the OTDR. Based on the size of the reflectionand the time it takes for the reflection to return to theOTDR, the system will provide a calibrated represen-tation of the attenuation and flaws in an optical fiber.The OTDR analysis can be performed at differentwavelengths with various modulation schemes. AnOTDR analysis is typically only necessary on verylong fiber runs with many optical connectors, patchpanels, and splices. It is easier to predict optical lossesand bandwidth on the majority of fiber-optic cableruns since there will be a minimal number of connec-tors and splices. Most applications will not require anOTDR.

Cleaning and Maintaining Optical Connectors

Fiber-optic connectors should be cleaned with lint-freeoptical wipes, and 100% pure isopropyl alcoholshould be used with the wipes. Compressed air is alsouseful to clean any dirt and debris from connectors orreceptacles. There are cassette-type cleaning devicesthat have an advanceable cleaning ribbon. The tips ofa male fiber-optic connector are typically ceramic. Aprotective cap should always be applied to the con-nector on the fiber cable as well as to the connector onthe fiber equipment. This prevents damage and dirtbuild up.

FIBER-OPTIC TRANSMISSION SYSTEMS

Digital Modulation

The digital bit is the basic unit of digital information.This unit has two values: one or zero. The bit repre-sents the electronic equivalent of the circuit being onor off where a zero equals off and a one equals on. Onebit of information is limited to these two values. Thedigital information is transmitted through the fiberserially one bit at a time.

A digital pulse train represents the ones and zerosof digital information. The pulse train can also depicthigh and low electrical voltage levels or the presenceand absence of a voltage.

Digital in the Television and Video Industries

A digital signal can mean different things to video andcable TV system engineers, causing much confusion.The most common types of digital video and audioare:

• 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

The process of digitizing a standard NTSC videosignal is straightforward. The typical bandwidth of avideo signal is 4.5 MHz. Typically a sample rate of fourtimes the video bandwidth is used or about 18 mega-samples per second. The analog-to-digital (A/D) con-verter typically has a sampling resolution of 8, 10, or12 bits.

This process generates a serial digital data stream ofabout 144–270 Mbps. The video signal is typicallyencoded in a digital format at the video source or inthe video camera. Depending on the digital video for-mat, the analog video will be encoded in one of sev-eral standard formats such as 4:2:2, 4:1:1, or 4:2:0.

While these encoding schemes are not referred to ascompression, they omit or remove certain informationto reduce the systems bandwidth requirement. In theencoding schemes above, the three digits refer to thethree common components of video. The first compo-nent is luminance (Y) or the light intensity of the videosignal. The second is the color signal of red minusluminance (R–Y). The third component is the color sig-nal of blue minus luminance (B–Y). These three com-ponents are referred to as YUV. The numbers 4:2:2have to do with the fact that twice the bandwidth isused for the Y channel than the two color channels.This technique is a form of compression that will beaddressed later in the chapter. HDTV or high-defini-tion video requires a data rate of 1.485 Gbps for oneuncompressed signal.

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The most efficient means of analog video transportutilizes analog to digital conversion. Once video andaudio signals are converted to digital information,many channels can be combined into one high-speeddata stream using TDM. The high-speed serial digitaldata stream is then converted to light via a laser orLED.

The receiver unit performs the reverse function. Thelight or optical signal is received by a PIN photo detec-tor. The optical signal is converted back into a serialdata stream. The data stream is demultiplexed usingTDM. The digital data is then converted back to videoand audio via digital-to-analog (D/A) converters.

Digital video transmission has many advantagesover analog transmission. An analog fiber-optic sys-tem requires high-linearity optical components thatare expensive and require fine tuning and complexcalibration procedures. Once a video or audio signalhas been digitized, it can be transported via fiberusing readily available digital telecom optical compo-nents for both multimode and single-mode applica-tions. A digital system has a higher immunity to noiseand superior performance characteristics compared toan analog system. A digital signal can be regeneratedand repeated virtually indefinitely without signal orperformance degradation.

Compressed Digital Video

When compression is introduced into a video trans-port system, a substantial reduction in bandwidth canbe implemented. A digital composite signal requires144 Mbps and an HD-SDI signal requires 1.485 Gbps.When considering a system that will transport manychannels of digital video, an enormous amount ofbandwidth is required. A compression systemremoves redundant or repetitive information from thedigital data stream. A compression or transmissionencoding scheme will take advantage of limitations inthe human eye. The human eye has lower sensitivityor resolution to color detail. Many compression orencoding schemes take this into account and compressor omit certain color details.

There are two basic types of compression systems:lossless and lossy. A lossless compression system doesnot degrade the video or audio quality. The receiverunit recovers the original uncompressed information.A lossless compression system strictly removes repeti-tive information from the data stream. Most videocontent has repetitive information from one videoframe to the next. For example, the background imagemay not change from frame to frame. Therefore, thereis no need to send this information repetitive times.Unfortunately, a lossless compression scheme does notoffer significant bandwidth savings. A compressionrate of three to four times can be expected.

A lossy compression scheme can achieve very highlevels of compression but at the cost of image or signalquality. A lossy compression algorithm removes detailfrom the original image. Once the information hasbeen removed, it cannot be reconstructed. There aremany compression and encoding schemes used in

video transport. The 4:2:2, 4:1:1, and 4:1:0 encodingschemes mentioned earlier are a technique used toreduce bandwidth. Since the human eye has less sensi-tivity or less resolution for color, these encodingschemes have less bandwidth for the color informa-tion. The human eye has higher resolution horizon-tally than vertically. When taking this into account,most video formats have a higher horizontal resolu-tion than vertical resolution.

QAM Digital Encoding

Quadrature amplitude modulation (QAM) is a widelyused modulation technique for video transport appli-cations, particularly in digital cable TV systems. In aserial digital modulation scheme there are only twoinformational states: 1 and 0, or on and off. With 256-QAM there are 256 states. The information is encodedby a varying 360 degree quadrature phase and ampli-tude. This modulation scheme can provide an enor-mous amount of data throughput in a limited amountof bandwidth, but a higher signal-to-noise band ratiois required. Figure 6.10-12 is the phase constellationfor a 16-QAM signal.

MULTIPLEXING

In communication, there are many techniques totransport multiple signals over one transmissionmedium. These techniques apply to fiber-optic trans-port.

Time-division Multiplexing

Time-division multiplexing (TDM) is an encodingtechnique that combines many data streams into one

FIGURE 6.10-12 16-QAM encoding phase constella-tion.

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high-speed serial digital stream by combining eachdata stream in turn on a time basis and converting itinto a single data stream. As a result, the single datastream is a sum of the total data from the streams to bemultiplexed, plus some overhead bits to organize thedata stream.

To produce a serial data stream from a parallel datasource, a parallel to serial converter or serializer maybe employed that takes, for example, an 8-bit paralleldata word and converts it into a 1-bit serial digital sig-nal. If a system with eight 150 Mbps serializers is to bemultiplexed together in a TDM system, the outputserial data stream will be 8 times 150, or 1200 Mbps.

Optical Multiplexing

Optical multiplexing techniques can be used in addi-tion to the TDM techniques mentioned above. If bothoptical multiplexing and TDM are combined, verylarge bandwidths of information can be transportedby one fiber.

Wave-Division Multiplexing

Wave-division multiplexing (WDM) is the techniqueof taking two or more wavelengths or colors of lightand combining them onto one fiber. On one end of thefiber the two wavelengths are combined, and then onthe other end separated. Basic WDM uses two wave-lengths. In multimode the 850 and 1310 nm wave-lengths are used. In single mode, 1310 and 1550 nmare used. The two wavelengths can travel in the samedirection or in opposite directions.

Coarse Wave-Division Multiplexing

Coarse wave-division multiplexing (CWDM) gives theability to combine up to 18 wavelengths onto one fiber.The 18 wavelengths are evenly spaced from 1270–1610nm in 20 nm increments. Each laser source is preciselytuned to a given wavelength to within ±1 nm. Whatmakes CWDM technology possible is extreme temper-ature stability in laser light sources from 0–70ºC with-out active cooling. CWDM lasers are relativelyinexpensive and provide very high and scalable band-width. A system could be initially designed andinstalled using only one wavelength. At any point inthe future, up to 17 more wavelengths can be added toincrease the system capacity. Components are avail-able to provide both multimode and single-modeCWDM systems.

Dense Wave-Division Multiplexing

Dense wave-division multiplexing (DWDM) takesoptical bandwidth and throughput to a higher level.DWDM permits up to 80 wavelengths to share onefiber. The DWDM spectrum is spaced very tightlyover a narrow range. The laser systems are complex inorder to provide precise wavelength accuracy and sta-bility over temperature. If the wavelength of a laserdrifts, it will interfere with an adjacent channel.

The typical DWDM channel spacing is 0.8 nm. Thetight spectrum of DWDM permits optical amplifica-tion using EDFA. DWDM technology is expensive andis seldom used in video applications. It is rare that anapplication will require the use of 80 wavelengths.DWDM technology is typically used for fiber-opticsystems with long fiber-optic cables between conti-nents and on the ocean floor. DWDM technology isonly available for single-mode fiber.

APPLICATIONS FOR VIDEOFIBER-OPTIC TRANSPORT

There are many applications for fiber-optic communi-cations. Any application that requires high bandwidthor high bit rate communications is ideally suited forfiber-optic transport. The television and video indus-tries are a perfect application for fiber-optic transport.Analog television is a relatively high bandwidth sig-nal of more than 5 MHz. Digital television (in particu-lar HDTV) has bit rates of more than 1.5 Gbps. High-resolution computer graphics can have a bandwidthexceeding 500 MHz. All of these television and videoapplications are ideal for fiber.

Broadcast Television Transmission

As mentioned earlier, television production andbroadcast engineers have always sought out the besttechnology for media events such as the Olympics. Inthe mid-1980s, fiber-optic transport was introducedinto the television industry. Fiber optics are used in allaspects of production and distribution of video andaudio signals. The state of the art for the transport ofanalog video is to use 12-bit video digital encoding.The serial digital bit rate can vary from about 144–300Mbps.

With the introduction of digital video in the 1990s,fiber-optic transport continued to enjoy growth in thebroadcast industry. Digital video was encoded intodata rates ranging from 144–360 Mbps. These high bitrate video signals could only travel over copper up toabout 300 meters. Transport distance beyond 300meters with a coax required a repeater (which needspower) or a fiber optic system.

The transition to 100% DTV/HDTV has created aneed to transport signals with a bit rate as high as 1.5Gbps. HDTV using an SDI interface (HD-SDI), in itsnative or uncompressed form, is 1.485 Gbps. HD-SDIcan better reach about 150 meters over a coax. Onceagain, fiber is the only choice to reach distancesbeyond 150 meters.

Systems can be designed using many of the tech-nologies described above. Analog and digital signaltransport can be mixed. Time-division and opticalmultiplexing can be combined.

A broadcast television station may typically residein a downtown metropolitan area. The televisiontransmitter and satellite up and down links may be ona distant mountaintop outside the city. This situationis a perfect application for fiber transport. The system

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may require both analog video and digital video sincethe station may be in the midst of its conversion fromanalog to digital broadcast. Signals in both directionswill be required to support downlink satellite video.

Another typical application is that of back-haulfeeds, where many channels of video and audio aretrunked together over one fiber. Such a system can useTDM to combine groups of eight channels of videowith audio into single wavelengths. The optical multi-plexing or CWDM technology is used to combine thewavelengths with groups of eight videos onto onefiber. The combined technique of TDM and CWDMprovides a fiber transport capacity of more than 144video channels on one fiber.

High-Resolution Graphics and Video Transmission

The quality and fidelity of an analog signal over longdistances are difficult to maintain over copper. As sig-nals increase in bandwidth and bit rate, it becomesmore and more difficult for systems to transport thesehigh bandwidth signals even a short distance overcopper. This becomes very apparent when workingwith high-resolution video and graphics.

A computer-generating RGB-HV or UXGA signal at1,600 × 1,200 pixels requires an analog bandwidth ofclose to 500 MHz. If the signal is digitized, it requires adata transport bit rate of 3–4 Gbps. There are manycopper-based products that will transport these sig-nals but at a cost in performance and video quality.Figure 6.10-13 shows a typical RGB/UXGA high-reso-lution video and audio fiber-optic link.

Many applications today require the same video orgraphical signal to displayed on a series of monitors.

An example may be an airport terminal where arrivaland departure information is displayed every 100 ft.This application requires a long daisy-chain of unitsthat can drop and repeat the same signals to eachmonitor every 100 ft.

Systems are available with the drop-and-repeat ordaisy-chain feature. As shown in Figure 6.10-14, onetransmitter can send the video signal to the firstreceiver. The first receiver decodes the optical signaland generates an output for the local monitor. Thereceiver also repeats and regenerates the optical signalto send to the next receiver in the chain. This tech-nique saves on installation and equipment costs. Thealternative would be to run a fiber from each monitorback to the control room. Instead, one fiber can feedmany monitors.

Optical Repeaters and Distribution Amplifiers

There are applications in fiber-optic communicationswhere a signal requires regeneration and replication.The function required is similar to that of a distribu-tion amplifier or digital signal reclocker. A passivesplitter can be used to split an optical signal, but eachsignal is significantly weaker after the split. A devicecalled an optical repeater or distribution amplifier canbe used to repeat or regenerate a weak optical signal.This is helpful on long fiber-optic runs where a fibersignal is reaching its limit. The repeater can be used toregenerate the signal for further distribution.

The same device can be used to replicate an opticalsignal. One optical signal can be replicated up to eighttimes with one device. Unlike the passive split werethe optical output is diminished, the output opticalsignals are regenerated to full optical power.

The device can also be used as a mode converter orwavelength remapper. The device can be configuredwith a single-mode input and multimode outputs.This gives the ability to convert from multimode tosingle mode or from one wavelength to another wave-length. The devise can convert optical signals toCWDM wavelengths.

Broadband Cable Television Transport

Broadband cable television signals are traditionallytransported over a hybrid system of fiber and coax.Hundreds of video sources are modulated onto indi-vidual carriers and combined into one broadband RFsignal. These RF signals can have a frequency band-width of 48–870 MHz. This is an enormous bandwidthto transport strictly over coax. Cable systems aredesigned to transport the high bandwidth signals overfiber to each residential community. The last mile ofdistribution is then accomplished over coax for deliv-ery to each home. Multiple line amplifiers are used totransport the high bandwidth signals over coax thelast mile.

Another application is in a campus environmentsuch 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-resolutionvideo and audio fiber-optic link. (Photo courtesy ofMultidyne Video & Fiber Optic Systems.)

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sion system. These facilities will receive a commercialcable TV signal from a local provider. They will stripout the channels of interest, such as news and educa-tional programming. They will then combine thesechannels with their own internal programming, suchas human resource and training channels. The chan-nels are then modulated and combined into onebroadband RF signal.

Many of the buildings on the campus may be sev-eral miles away. Broadband fiber-optic links can beused to get the cable TV signal to each building. Thecable TV is then distributed in each building via coaxthe last several 100 ft.

Broadband RF and Satellite Link Transport

There are many broadband RF applications for fiber.One important application is satellite uplinks anddownlinks. Commercial satellites, for long wave-length applications, typically use an intermediate fre-quency (IF) signal as a means of communication. TheIF signal is typically 70 or 140 MHz. The signal has alimited transport distance over copper coax. The noiselevel and sensitivity of a satellite system will sufferwith the use of copper coax. The relatively high band-width and noise sensitivity issues make IF signaltransport ideal for fiber.

Consumer and residential satellite service is findingits way into more and more commercial, corporate,and military applications. Consumer satellite equip-

ment is used in many applications as a source of infor-mation and news. A large corporation may useconsumer satellite instead of cable for news and infor-mational content. In a corporate building or militarybunker, the satellite dish may be on the roof manyfloors above the control or conference room. The L-band signal has a bandwidth from 950–2,150 MHz. Acoax run will not transport the L-band satellite signalvery far. A consumer satellite dish is small due to ashorter wavelength. It typically has a device called alow-noise block downconverter (LNB). The LNB is anactive device that receives the satellite signal andtranslates and amplifies it to be sent down the coax tothe receiver. The LNB requires a DC voltage, which istypically generated by the receiver and sent up thecoax. If a fiber link is used, the DC voltage needs to begenerated on the dish side of the fiber link. Whendesigning an L-band fiber link, the system needs toprovide the appropriate LNB DC power.

FIBER-OPTIC ROUTING SWITCHERS

Just about every broadcast and audio visual systemtoday has some sort of a video and audio routingswitcher. The switcher gives the user the ability to con-trol the source and destination of a given video andaudio signal. As more and more video and communi-cations migrate from copper to fiber, it makes sensethat the need for an optical routing switcher arose. Theoptical routing switcher is a new concept for the video

FIGURE 6.10-14 Daisy-chain or drop-and-repeat video fiber transport.

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market, but it has been used for many years in the tele-communications industry. It has been used to routeand control telephone traffic.

As video and broadcast television industriesbecome more and more complex with dozens of dif-ferent video and encoding formats, optical switchingstarts to make more sense. In broadcast or video appli-cation there may be analog video, component video,SDI, and HD-SDI. An optical switch can switch mostsignals in the optical domain. There are two basictypes of optical switching.

Photonic Fiber-Optic Switcher

The first is 100% optical switching using 3D micro-electromechanical mirror (MEMS) technology. It useselectronically controlled mirrors to route opticalsignals. This type of switch has an optical input, anoptical cross-point, and an optical output. The abbre-viation for this technology is OOO. An OOO switchprovides only point-to-point switching. One inputcannot be multicast to many outputs. The mirrors can-not point to more than one output at a time. The use ofmirrors does permit multiple wavelengths and wave-lengths in both directions. Switches are available issizes from 8 × 8 to 256 × 256. Pure optical switching isavailable for multimode and single-mode applica-tions. Optical switching supports both analog and dig-ital optical signals.

Pure optical switching is performed using 3D MEMSarrays. Tiny mirrors are fabricated out of silicon. Themirrors are positioned and controlled with electrostaticcharges. The core of the optical switch is a 1 inch squarecube. The cube has an array of up to 256 input fibers onthe left side as shown in Figure 6.10-15. Each fiber has alens that focuses the optical light onto a MEMS mirror.Each input has its own mirror. On the right side is anarray of output fibers. Each output has a MEMS mirror.An optical connection is made when one input mirroraligns with one of the output mirrors.

Fiber-optic switching is ideal for video broadcast,production, security, and other video applicationsrequiring transmission, switching, and replication ofhigh-quality optical signals. The fiber-optic switcherrevolutionizes how video is distributed and managed.It is based on state-of-the-art field proven photonicswitching technology. Laser light is switched in a pureoptical format, without electrical conversion, allowingit to support transparent connections compatible withany video or data format including uncompressed HDvideo at 1.5 Gbps. Also, since the switching is doneoptically, the switch eliminates video degradation.With a traditional electrical switcher, electrical-to-opti-cal (EO) and optical-to-electrical (OE) conversions arerequired that cause signal degradation and jitter.

An optical switch supports a wide range of formatsfrom 19.4 Mbps ATSC through 1.5 Gbps HDTV as wellas NTSC, PAL, SECAM, SMPTE 259M serial digital(SDI) video, broadband analog, L-band, IF, and manymore. The optical switcher will also transparentlyswitch CWDM and DWDM signals.

Optical switcher technology can be used in the fieldto support applications requiring reliable, high-qualityvideo distribution such as mobile production trucks,sports venues, and professional video facilities; cam-pus video and surveillance networks; remote videomonitoring; as well as government and military. Opti-cal layer protection and fault tolerant switching can beconfigured for mission critical, nonstop applications.

Optical switching is cost effective for any applica-tions requiring 32 or more switched optical ports. Iteliminates the need for expensive video transceiversto convert signals between electrical and optical for-mats. Switching the signals in optical format can sub-stantially reduce the cost per port in fiber-optictransport equipment costs.

Electro-Optical Switch

The second type is the electro-optical switch. The elec-tro-optical switch uses a hybrid approach. The input isoptical, the cross-point is electrical, and the output isoptical. The abbreviation for this technology is OEO.An OEO switch supports point-to-multipoint or multi-cast switching. Any input can be switched to everyoutput if necessary. Since the optical signal is con-verted to electrical, only one wavelength can beswitched at a time. Also an electrical cross-point onlyoperates in one direction. Therefore, only one wave-length in one direction is supported.

FIGURE 6.10-15 Three dimensional MEMS pure opticswitching element. (Photo provided by Calient Net-works.)

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THE FUTURE OF VIDEO FIBER-OPTIC TRANSPORT

Systems are currently in development for the trans-port of high-resolution video at bit rates exceeding 10Gbps. Digital cinema and the proliferation of HDTVtelevision will demand fiber-optic transport systemswith high bandwidth capabilities. Fiber transport tothe home of video, telephone, and Internet traffic isslowly becoming a reality in many North Americancommunities. This will fuel the demand for high-speed content delivery and distribution throughoutthe globe.

BibliographyA Brief History of Fiber Optic Technology, at http://www.fiber-

optics.info/fiber-history.htm.Corning website, at http://www.corning.com/.Fiber Optics website, at http://www.wetenhall.com/Physics/

History.html.Goff, David R. Fiber Optic Video Transmission: The Complete Guide, 1st

ed. Boston: Focal Press, 2003.Goff, David R. Fiber Optic Reference Guide, 3rd ed. Boston: Focal

Press, 2002.A Short History of Fiber Optics, at http://www.sff.net/people/

Jeff.Hecht/history.html.Multidyne website, http://www.multidyne.com.Optical Cable Corporation website, at http://www.occfiber.com/.Sterling, Donald J. Technicians’ Guide to Fiber Optics, 4th ed. Delmar

Learning, 2004.Fiber Optics Communications Handbook, 2nd ed. Blue Ridge Summit,

PA: TAB Books, April 1991.

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