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C H A P T E R 3

Fibre Optic Communications

3.1 INTRODUCTION

With the advent of optical media for the purposes of transferring digital data, an era of expansionin the data communications field was born. Light rays are high-frequency electromagnetic waveswith a short wavelength in the micron (micrometer) range. For the optical fiber communications,light is made to travel through a medium of glass. Light in the visible range exhibits wavelengthsbetween 0.4 and 0.7 micrometer or (0.4 × 10-6 and 0.7 × 10-6 meter) and is highly attenuated byglass. On the other hand, light waves with wavelengths between 0.85 and 1.6 micrometer (infraredrange) travel very efficiently through glass. Fiber optic cables can manage data rates exceeding1000 Mbps due to increased bandwidth.

The huge capacity and digital proficiency of optical fibers have made them a natural for com-puter communications. They are being used in local area networks to connect central processorswith peripheral equipment and other computers, creating integrated computer systems that span aroom, a building, a campus or a city. Developers are incorporating data highways made of opticalfibers into the design of skyscrapers; intelligent buildings, as they are called, have nervous systemsof optical fibers that carry information about temperatures, lighting, breaches of security or firealarms to a central computer for action and control.

3.2 OPTICAL SOURCE

Two commonly used optical sources are Light-emitting diodes (LEDs) and Injection LaserDiodes. The LEDs emit a lower level of light (-15 dBm power level) but concentrate light into atighter cone pattern. The laser diodes emit light at -6 dBm. The pattern of light is shown in Figure3.1.

From the above Figure it is evident that the light-emitting diode pattern is broader, since lightis emitted from the surface of the diode. Lasers project light from their edge, forming a moreintense and narrow cone.

An optical transmission system has three components:(a) Light source(b) The transmission medium(c) The detector

Fibre Optic Communications 69

Figure 3.1(a) LED(b) Inject laser

Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a zero bit.The transmission medium is an ultra-thin fiber of glass. The detector generates an electrical pulsewhen light falls on it. By attaching a light source to one end of an optical fiber and a detector to theother, we have a unidirectional data transmission system that accepts an electrical signal, convertsand transmits it by light pulses, and then reconverts the output to an electrical signal at the receiv-ing end.

3.3 PROPAGATION IN FIBER

In an optical fiber, a central core of glass is surrounded by so-called cladding (cutaway), a similarmaterial with a lower refractive index; light pulsed through the fiber will be bent at the interfacetowards the material with the higher refractive index the core. The diameter of the core and thedifference between the refractive indices of core and cladding determine the clarity of the signalreceived at the other end of an optical fiber.

Fiber optic cables are constructed to operate in one of the following three modes:(a) Single Mode Optical Fiber(b) Multimode Step Index Optical Fiber(c) Graded Index Multimode Optical Fiber

3.3.1 Single Mode Optical Fiber

Systems using laser diodes as the light-emitting sources employ single mode optical fiber. Thesingle mode concentrates the passage of light to the center of the fiber core. Where the center isvery narrow about 2 to 10 micro meters in diameter, the ray concentrates at the centre moves thequickest through the cable with the least distortion and attenuation (See Figure 3.2). A core only 2to 10 micrometers wide confines light to one path, making the fiber transmission rate, by somemeasurements, at least four times that of multimode fibers.

(a) LED (Microwatts)

(b) Injection Laser (Milliwatts)

70 Data Communication

Figure 3.2Single modeoptical fiber

In a single-mode fiber, the glass core is extraordinarily narrow. The rays of a light pulse,therefore, have little space to bounce from side to side. As a result, the pulses of light retain theirdefinition, permitting as many as 30 times the number of pulses per second to be transmittedthrough a single-mode fiber as through the multimode variety. For the same reason, pulses cantravel much farther in a single-mode fiber before the signal requires regeneration. The lesseneddemand for expensive regenerators more than compensates for the extra cost of buying and instal-ling single-mode fibers and the premium paid for laser light sources, which the narrow core in thistype of fiber requires.

3.3.2 Multimode Step Index Fiber

While the single mode fiber accepts only one light ray at a time, in a narrow diameter, the multi-mode fiber allows more than one ray of the light at a moment. With each ray at a slightly differentangle from the other in a wider core. (See Figure 3.3)

Figure 3.3Step indexfiber

The first kind of multimode fiber is the step index. In the step index core, the incident rayenters the core, is refracted slightly and travels through the core as it is reflected from one side ofthe cladding to the other.

Cladding

Cladding

Core

LightRay

8 - 12 mµ

125 mµ

Multimode Step Index


Step-index fibers are characterized by a sharp transition of refractive index at the boundarybetween the core and the cladding. Light rays traveling straight through the core arrive sooner thanrefracted rays that have taken longer paths, causing a smearing of the digital signal.

� The main disadvantage of step-index fibers is that every time the ray strikes thecladding, the cladding absorbs some of its energy, resulting in a little of attenuation.

3.3.3 Graded Index Multimode Fiber

The disadvantage of the step index core is removed by the graded index core. The incident rayenters the cable in the same way as in the case of the step index. However, instead of beingreflected straight from the cladding, it is refracted in small increments as it travels through thecore. The refraction bends the ray away from the cladding back towards the core. Thus, there is noloss due to the absorption of light by the cladding. (See Figure 3.4)

In this mode, the gradual paths of refraction are shorter so light rays arrive more nearly simul-taneously, resulting in much less distortion of the information.

In graded-index fibres, the index of refraction is highest in the center of the core and tapersgradually to a lower value in the cladding.

Figure 3.4Multimodegraded index fiber

3.4 LIGHT DETECTOR

On the receiving end of the fiber-optic cable is a light-sensitive detector encased in a specialadapter. The connection must hold the alignment of the end of the cable tightly to the lens of thedetector to allow the maximum amount of light to be sensed by the device.

A photosensitive device like a photo-diode or photo-transistor operates on the principle thatlight striking an exposed section of a PN junction of semiconductor material will cause electronand hole flow activity to begin. The more light applied, the higher the current flow. In essence, asthe light increases, the effective resistance of the semiconductor is reduced by the increased activ-ity. Electrical current from an external current source increases in inverse proportion to the reduc-tion of the device’s resistance. A typical detector of Motorola’s make is shown in Figure 3.5.

Multimode Graded Index


Figure 3.5An OpticalDetector

For longer fiber cable application, a photodarlington transistor such as Motorola’s MFOD2302is used. (See Figure 3.6) This device detects infrared light sources operating at medium-frequencyranges over fiber cable distances up to 1,000 meters.

Figure 3.6Long-rangeInfrared Detector

3.5 FIBER DISTRIBUTED DATA INTERFACE

FDDI (Fiber Distributed Data Interface) is another ring-based network, FDDI, unlike Token Ring,

1 2

0.280.

32

N Region

P Region

Intrinsic Region

Fiber Optics

NPN SiliconPhotodarlington

Transistor


is implemented without hubs, although you can use devices called concentrates to perform a simi-lar function. FDDI uses fiber-optic cables to implement very fast, reliable networks.

FDDI is a high performance fiber optic token ring LAN running at 100 Mbps over distances upto 200 km with up to 1000 stations connected . It can be used in the same way as any of the otherLANs, but with its high bandwidth, another common use is a backbone to connect copper LANs,as shown in Figure 3.7.

Figure 3.7AN FDDI ringbeing used as abackbone toconnect LANsand computers

FDDI uses multimode fibers because the additional expense of single mode fibers is notneeded for networks running at only 1000 Mbps. It also used LEDs rather than lasers, not only dueto their lower cost, but also because FDDI may sometimes be used to connect directly to userworkstations.

Figure 3.8(a) FDDI con-sisting of twocounterrotating rings(b) Two ringsjoined together

Token ring

FDDI ring

Ethernet

Bridge

Token bus

Ethernet

Computer

Bridge

(a) (b)


The FDDI cabling consist of two fiber rings, one transmitting clockwise and the other trans-mitting counterclockwise, as illustrated in Figure 3.8(a). If either one break, the other can be usedas a backup. If both break at the same point, for example, due to a fire or other accident in thecable duct, the two rings can be joined into a single ring approximately twice as long, as shown inFigure 3.8 (b). Each station contains relays that can be used to join the two rings or bypass thestation in the event of station problems. Wire centers can also be used, as in 802.5.

3.6 ADVANTAGES OF FIBER OPTICS CABLES

Both multimode and single-mode fibers have several steps on conventional copper wires, in addi-tion to their huge information-carrying capacity. Stray electromagnetic impulses do not affectglass as they do wires, so optical fibers are immune to errors in data caused by electricalinterference. Thus, they are ideally suited for use in places like machine shops and factories, whereconventional wires often require shielding. Furthermore, optical fibers offer tight security becausethey are extremely difficult to tap, making them attractive for military as well as banking applica-tions. Fiber-optic cables are much smaller and lighter than copper-wire cables, one reason whytelephone companies have begun to install them in cities, where they can snake through conduitscrowded almost to capacity.

3.7 FIBER OPTIC NETWORKS

Fiber optics can be used for LANs as well as for long-haul transmission,. Two types of interfacesare used. These are:(a) A passive interface(b) An active repeater

3.7.1 Passive Interface

A passive interface consists fo two taps fused onto the main fiber. One tap has an LED or laserdiode at the end of it (for transmitting), and the other has a photo-diode (for receiving), The tapitself is completely passive and is thus extremely reliable because a broken LED or photo-diodedoes not break the ring. It just takes one computer off-line.

3.7.2 Active Repeater

In Figure 3.9, another type of interface is shown. It is the active repeater. The incoming light isconverted to an electrical signal, regenerated to full strength if it has been weakened, and retrans-mitted as light. The interface with the computer is an ordinary copper wire that comes into thesignal regenerator.

If an active repeater fails, the ring is broken and the network goes down. On the other hand,since the signal is regenerated at each interface, the individual computer-to-computer links can bekilometers long, with virtually no limit on the total size of the ring. The passive interfaces loselight at each junction, so the number of computers and total ring length are greatly restricted.


Figure 3.9A fiber opticring withactive repeat-ers

A ring topology is not the only way to build a LAN using fiber optics. It is also possible tohave hardware broadcasting using the passive star construction of Figure 3.10. In this design, eachinterfaces has a fiber running from its transmitter to a silica cylinder, with the incoming fibersfused to one end of the cylinder. Similarly, fibers fused to the other end of the cylinder are run toeach of the receivers. Whenever an interface emits a light pulse, it is diffused inside the passivestar to illuminate all the receiver, thus achieving broadcast. In effect, the passive star combines allthe incoming signals and transmits the merged result out on all lines. Since the incoming energy isdivided among all the outgoing lines, the number of nodes in the network is limited by the sensi-tively of the photodiodes.

Comparison of Fiber Optics and Copper Wire

Fiber has the following advantages:

(a) It can handle much higher bandwidths than copper. This alone would require its use inhigh-end networks.

(b) Due to the low attenuation, repeaters are needed only about every 30 Km on long lines,versus about very 5 Km for copper, a substantial cost saving.

(c) It is not affected by power surges, electromagnetic interference, or power failures. Nor it isaffected by corrosive chemical in the air, making it ideal for harsh factory environments.

(d) Fibers do not leak light and are quite difficult to tap. This gives them excellent securityagainst potential wiretappers.

Computer

Fiber

Opticalreceiver

(photodiode)

Signalregenerator(electrical)

Opticaltransmitter

(LED)

propagationof light

Direction

Opticalfiber

Interface

Detail ofinterface

To / from computer

Copper wire


Figure 3.10A fiber opticring withactive repeat-ers

Disadvantages of Fiber cable over Copper wires

(a) Fiber is an unfamiliar technology requiring skills most engineers do not have.(b) Since optical transmission is inherently unidirectional, two-way communication requires

either two fibers or two frequency bands on one fiber.(c) Fiber interfaces cost more than electrical interfaces.

REVIEW QUESTIONS WITH ANSWERS

State True or False.

1. Twisted wire pair is effected by the electromagnetic interference.2. Attenuation in a bounded media changes the power value at the receiving end.3. Coaxial cable can be used for data rates over 10 Mbps and frequencies up to 400 MHz.4. Single mode used in fiber optics does not have any dispersion problem.5. When the same number of channels are to be multiplexed for transmission, FDM always

requires a greater bandwidth than TDM.

Answers

1. True 2. True 3. True 4. True 5. False

Each incomingfiber illuminates

Transmitter

Receiver

the whole star

sees light from allEach outgoing fiber

the incoming fibers

inte

rfac

esC

ompu

ter


Select the correct answer.

1. PCs in a computer communication networks are usually connected by:

(a) Telephone lines only (b) Satellite only(c) Either satellite or telephone line or (d) None of the above

both

2. The meaning of a digital channel means that the channel:

(a) is digitized (b) is carrying digital data(c) accepts digital modulation tech- (d) None of the above

niques

3. Data networks for the efficiency of communication reasons, uses:

(a) Simplex transmission (b) Half-duplex transmission(c) Full-duplex transmission (d) None of the above

4. Coaxial cables can be used for:

(a) Telephone networks only (b) Cable TV networks only(c) Both in telephone and cable TV (d) None of the above

networks

5. Evesdropping is not possible in:

(a) UTP (b) STP(c) Coaxial cable (d) Fiber optics

Answers

1. (c) 2. (b) 3. (c) 4. (c) 5. (d)

TEST PAPER

Time: 3 Hrs. Marks: 100

Note: Answer all questions.1. Compare the following transmission media:

(a) Twisted pair and optical fiber.(b) Terrestrial microwave link and satellite microwave(c) STP and coaxial cable

2. A fiber optic system requires 5 micro watts of power for proper functioning at thereceiver. The cable is 10 km long and has an attenuation loss of 2 dB/km. There isa loss of 2 dB at both the source and the receiver. Calculate the required level ofoptical power at the optical source.

3. Answer the following:(a) Why do you connect the outer conductor of a coaxial cable to the ground?


(b) A band of frequencies range from 100 to 190 KHz is being allocated forchannels. Each channel is 5 KHz wide with a 1-KHz guard band. Sketchthe channel assignments from 100 to 112 KHz.

(c) Why the digital communication systems are more resistant to channelnoise than analog systems.

(d) Draw a block diagram of a simplex communication system such as TVtransmission and reception system and briefly explain the function ofeach block.

4. (a) Illustrate with the help of a schematic diagram the different componentsof a typical fiber optic link. Mention the various components of signalloss.

(b) State the advantages of semiconductor laser diode over light-emittingdiodes (LED) for fiber transmission?

(c) State the mechanism by which a optical pulse travelling along a opticalfiber suffers from dispersion.

(d) With a diagram show the structure of an optical fiber cable.(e) What are the advantages and disadvantages of s single mode optical fiber

over multimode optical fiber.5. (a) Explain the term full-duplex as applied to telephony.

(b) What is the type of multiplexing used in telephone trunk circuits to trans-mit a large number of voice channels.

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