Optical Communications.pdf

7/28/2019 Optical Communications.pdf

1/57

Optical Communications


2/57

Communication system with light as the carrier and fiber ascommunication medium

Propagation of light in atmosphere impractical: the great attenuation of the light due to atmospheric effects such as water

vapor, oxygen, particles.

Optical fiber is used, glass or plastic, to contain and guide light

waves

Capacity Microwave at 10 GHz with 10% utilization ratio: 1 GHz BW

Light at 100 Tera Hz (1014 ) with 10% utilization ratio:10 THz(10,000GHz)

Communication using light


3/57

A Fiber-Optic System

The components of a typical fiber-optic com-municationssystem are illustrated in following Fig. 13-1. The

information signal to be transmitted may be voice, video,or computer data. The first step is to convert theinformation into a form compatible with thecommunications medium. This is usually done byconverting continuous analog signals such as voice andvideo (TV) signals into a series of digital pulses. An A/D

converter is used for this purpose. Computer data isalready in digital form. These digital pulses are then usedto flash a powerful light source off and on very rapidly.


4/57

4

Optical Fiber Fibers of glass

Usually 120 micrometers in diameter

Used to carry signals in the form of light overdistances up to 50 km without any repeater.

Core thin glass center of the fiber where lighttravels.

Cladding outer optical material surrounding thecore

Buffer Coating plastic coating that protects thefiber.


5/57

5

Optical Fiber: Advantages

Fiber-optic cable has several advantages over metallic cable (twisted-pairor coaxial).

Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths(and hence data rates) than either twisted-pair or coaxial cable. Currently, data ratesand bandwidth utilization over fiber-optic cable are limited not by the medium but

by the signal generation and reception technology available. Less signal attenuation. Fiber-optic transmission distance is significantly greater

than that of other guided media. A signal can run for 50 km without requiringregeneration. We need repeaters every 5 km for coaxial or twisted-pair cable.

Immunity to electromagnetic interference. Electromagnetic noise cannot affectfiber-optic cables.

Resistance to corrosive materials. Glass is more resistant to corrosive materials

than copper. Light weight. Fiber-optic cables are much lighter than copper cables.

More immune to tapping. Fiber-optic cables are definitely more immune to tappingthan copper cables. Copper cables create antennas that can easily be tapped.


6/57

6

Optical Fiber: Disadvantages

There are some disadvantages in the use of optical fiber.

Installation/maintenance. Fiber-optic cable is a relatively new

technology. Installation and maintenance need expertise that isnot yet available everywhere.

Unidirectional. Propagation of light is unidirectional. If weneed bidirectional communication, two fibers are needed.

Cost. The cable and the interfaces are relatively more

expensive than those of other guided media. If the demand forbandwidth is not high, often the use ofoptical fiber cannot be justified.


7/57

Fiber Optic Communication: Block

Diagram


8/57

8

Fiber Optic Communication System

Light source: LED or ILD (Injection Laser Diode): amount of light emitted is proportional to the drive current

Source-to-fiber-coupler (similar to a lens): A mechanical interface to couple the light emitted by the

source into the optical fiber

Light detector: PIN (p-type-intrinsic-n-type) or APD (avalanche photo

diode) both convert light energy into current


9/57

9

Fiber-Optic Cable The fiber, which is called the core, is usually surrounded by a protective

cladding.

The cladding is also made of glass or plastic but has a lower index of

refraction. This ensures that the proper interface is achieved so that the light waves

remain within the core.

In addition to protecting the fiber core from nicks and scratches, the claddingadds strength.

Some fiber optic cables have a glass core with a glass cladding. Others have aplastic core with a plastic cladding. Another common arrangement is a glass

core with a plastic cladding. It is called plastic clad silica (PCS) cable.


10/57

10

Fiber-Optic Cable


11/57

11

Characteristics and Behavior of Light

Light waves travel in a straight line like microwaves do.

Like electricity, these light rays travel at the speed of light,which is generally considered to be 300,000,000 m/s infree space.

The speed of light depends upon the medium throughwhich the light passes.

When light passes through another material such as glass,its speed is slower.

Index of refraction is n=c/v.


12/57

12

Total Internal Reflection in Fiber

Total internal reflection is an opticalphenomenon that occurs when a ray of lightstrikes a medium boundary at an angle largerthan the critical angle with respect to the normalto the surface.

If the refractive index is lower on the other sideof the boundary no light can pass through, soeffectively all of the light is reflected.

The critical angle is the angle of incidenceabove which the total internal reflection occurs.

Check Snells law and relevant problems fromViswanathan.


13/57

13

Fiber-Optic Cable Optical fibers use reflection to guide light through a

channel.

A glass or plastic core is surrounded by a cladding of lessdense glass or plastic.

The difference in density of the two materials must be suchthat a beam of light moving through the core is reflected offthe cladding instead of being refracted into it.


14/57


15/57

Types of Fiber-Optic Cable There are two basic ways of classifying fiber-

optic cables.

The first way is an indication of how the index of refractionvaries across the cross section of the cable.

The second way of classification is by mode. Mode refers tothe various paths that the light rays can take in passing throughthe fiber.

Usually these two methods of classification are combined to

define the types of cable. There are two basic ways of defining the index of refraction

variation across a cable. These are Step index

Graded index


16/57

Step Index Step index refers to the fact that there is a sharply

defined step in the index of refraction where the fibercore and the cladding interface.

It means that the core has one constant index ofrefractionN1, while the cladding has another constantindex of refractionN2. They are related by theexpressionN2= N1.( 1-), here is a constant calledindex difference.

Where the two come together, there is a distinct step as

illustrated in Fig. 13-11. If you were to plot a curve showing how the index of

refraction varies vertically as you move from left toright across the cross section of the cable, there would

be a sharp increase in the index of refraction as thecore is encountered and then a sharp decline in the

index of refraction as the cladding is encountered.


17/57

Graded Index

In this type of cable, the index of refraction

of the core is not constant.

Instead, the index of refraction variessmoothly and continuously over the diameter

of the core as shown in Fig. 13-12.

As you get closer to the center of the core,

the index of refraction gradually increases,

reaching a peak at the center and then

declining as the other outer edge of the core

is reached. The index of refraction of the

cladding is constant.


18/57

Propagation Modes Mode refers to the number of paths for the light rays in the

cable.

There are two classifications: Single mode

Multimode.

In single mode: light follows a single path through the core.

In multimode: the light takes many paths through the core.

Each type of fiber-optic cable is classified by one of these

methods of rating the index or mode. In practice, there are threecommonly used types of fiber optic cable.

These are multimode step index,

single mode step index

multimode graded index.


19/57

Multimode step index fiber

The density of the core remains constant from the

center to the edges. A beam of light moves through

this constant density in a straight line until it reachesthe interface of the core and the cladding. At the

interface, there is an abrupt change to a lower density

that alters the angle of the beam's motion.


20/57

Multimode step index fiber The multimode step-index fiber cable is probably the most

common and widely used type.

It is also the easiest to make and, therefore, the least expensive.

It is widely used for short to medium distances at relatively lowpulse frequencies.

The main advantage of a multimode step-index fiber is the largesize. Typical core diameters are in the 50- to 1000 m range.

Such large diameter cores are excellent at gathering light and

transmitting it efficiently. This means that an inexpensive light source such as an LED can

be used to produce the light pulses.

The light takes many hundreds or even thousands of pathsthrough the core before exiting.


21/57

Modal DispersionFor example, in Fig. 13-13. a short light pulse isapplied to the end of the cable by the source. Lightrays from the source will travel in multiple paths.At the end of the cable, those rays which travel the

shortest distance reach the end first. Other raysbegin to reach the end of the cable later in time untilthe light ray with the longest path finally reachesthe end concluding the pulse. In Fig. 13-13 ray Areaches the end first, then B, then C. The result is a

pulse at the other end of the cable that is lower in

amplitude due to the attenuation of the light in thecable and increased in duration due to the differentarrival times of the various light rays. Thisstretching of the pulse is referred to asmodaldispersion or pulse spreading. The differencebetween the travel times of the fastest and slowestlight rays is called pulse spreading constant t andis ex ressed in nanoseconds/km.


22/57

Modal Dispersion Because the pulse has been stretched, input

pulses cannot occur at a rate faster than theoutput pulse duration permits.

Otherwise, the pulses will essentiallymerge, as shown in Fig. 13-14. At theoutput, one long pulse will occur and will

be indistinguishable from the three separatepulses originally transmitted. This meansthat incorrect information will be received.


23/57

Cure of Modal DispersionFor multimode step index

1. Reduce the pulse repetition rate or thefrequency of the pulses. When this is done,

proper operation occurs. But with pulses ata lower frequency, less information can behandled.

2. Bit rate can be increased with the help of Return to zero coding. Maximum bit ratesfor RZ and NRZ signals are given as:

BRZ=(tL) bps and BNRZ=(tL)/2 bpswhere L is the fiber length.

OR

REPLACE THE FIBER WITH EITHER SINGLE MODE STEP INDEX OR MULTIMODE GRADED INDEX FIBER.


24/57

Single Mode Step Index In a single mode, or mono mode, step-index fiber cable the core is so small

that the total number of modes or paths through the core are minimized andmodal dispersion is essentially eliminated. See Fig. 13-15. Typical core sizes

are 2 to 15 m. Virtually the only path through the core is down the center. With minimum

refraction, no pulse stretching occurs. The output pulse has essentially thesame duration as the input pulse.

The single-mode step-index fibers are by far the best since the pulserepetition rate can be high and the maximum amount of information can be

carried. For very long distance trans-mission and maximum informationcontent, single mode step index fiber cables should be used.


25/57

Single Mode Step Index

The main problem with this type of cable is thatbecause of its extremely small size, it is difficult to

make and is, therefore, very expensive. Handling, splicing, and making interconnections

are also more difficult.

Finally, for proper operation an expensive, super-

intense light source such as a laser must be used. For long distances, however, this is the type of

cable preferred.


26/57

Multimode Graded Index Multimode graded index fiber cables have several modes or

paths of transmission through the cable, but they are much moreorderly and predictable.

Figure 13-16 shows the typical paths of the light beams. Becauseof the continuously varying index of refraction across the core,the light rays are bent smoothly and converge repeatedly atpoints along the cable.

The light rays near the edge of the core take a longer path buttravel faster since the index of refraction is lower.

All the modes or light paths tend to arrive at one pointsimultaneously. The result is that there is less modal dispersion.

It is not eliminated entirely, but the output pulse is not nearly as

stretched as in multimode step index cable.


27/57

Multimode Graded Index

The output pulse is only slightly elongated. As a result, this

cable can be used at very high pulse rates and, therefore, a

considerable amount of information can be carried on it. This type of cable is also much wider in di-ameter with core

sizes in the 50 to 100 m range.

Therefore, it is easier to splice and interconnect, and cheaper,

less intense light sources may be used.


28/57

Propagation Modes


29/57

Fiber typesFiber types

Type Core Cladding Mode

50/12550/125 50 125 Multimode, graded-index

62.5/12562.5/125 62.5 125 Multimode, graded-index

100/125100/125 100 125 Multimode, graded-index

7/1257/125 7 125 Single-mode


30/57

Optical Transmitters and Receivers

In an optical communications system,

transmission begins with the transmitter, which

consists of a modulator and the circuitry thatgenerate the carrier.

In this case, the carrier is a light beam that is

modulated by digital pulses which turn it on and

off.

The basic transmitter is nothing more than a light

source.


31/57

Optical Transmitters Conventional light sources such as incandescent lamps

cannot be used in fiber optic systems.

The reason for this is that they are simply too slow. Anincandescent light source consists of a filament that heatsup and emits light. Such a light source cannot be turned offand on fast enough because of the thermal delay in thefilament. In order to transmit high speed digital pulses, avery fast light source must be used.

The two most commonly used light sources are LEDs

Semiconductor lasers.


32/57

Light Emitting Diode (LED) LED is a PN junction semiconductor device that emits

light when forward biased. When a free electronencounters a hole in the semiconductor structure, the twocombine, and in the process they give up energy in theform of light. Semiconductors such as gallium arsenide(GaAs) are superior to silicon in light emission.

Most LEDs are GaAs devices optimized for producing redlight. They are used for displays indicating whether a

circuit is off or on, or for displaying decimal and binarydata. However, because LED is a fast semiconductordevice, it can be turned off and on very quickly. Therefore,it is capable of transmitting the narrow light pulsesrequired in a digital fiber-optic system.


33/57

Light Emitting Diode (LED) Light-emitting diodes can be designed to emit virtually any

color of light desired. Red LEDs are the most common, butyellow, green, and blue LEDs are also available. The LEDsused for fiber-optic transmission are usually in the red andlow infrared ranges.

Typical wavelengths of LED light commonly used are 0.82,0.94, 1.3, and 1.55 m. These wavelengths are all in the near-infrared range just below red light. The light is not visible to

the naked eye. These wavelengths have been chosen primarilybecause most fiber-optic cables have the lowest losses in thesewavelength ranges.

The most commonly used wavelength is 1.3 m becausemany fiber-optic cables have minimum loss at that frequency.


34/57

Light Emitting Diode (LED) Special LEDs are made just for fiber optic applications.

These units are made of GaAs indium phosphide (GaAsInP) and emit lightat 1.3 m.

They come with a fiber-optic "pigtail" already attached for optimumcoupling of light. The pigtail usually has a connector that attaches to themain cable.

There are two basic ways that digital data is formatted in fiber-opticsystems. These are the return-to-zero {RZ) andnon-return-to-zero (NRZ)formats.

Both of these are illustrated in Fig. 13-22.


35/57

LED Transmitter The Digital data to be transmitted is converted into a serial pulse train and then into

the desired RZ or NRZ format. These pulses are then applied to light transmitter.

The light transmitter consists of the LED and its associated driving circuitry.

A typical circuit is shown in Fig. 13-23. The binary pulses are applied to a logic

gate which, in turn, operates a transistor switch Q1 that turns the LED off and on. A positive pulse at the NAND gate input causes the NAND output to go to zero.

This turns offQ1, so the LED is forward-biased through R2 and turns on. With zeroinput, the NAND output is 1, so Q1 turns on and shunts current away from the LED.


36/57

LED Transmitter Very high current pulses are used to ensure a brilliant high-

intensity light. High intensity is required if data is to betransmitted reliably over long distances. Most LEDs arecapable of generating power levels up to approximatelyseveral hundred microwatts.

With such low intensity, LED transmitters are good foronly short distances. Further, the speed of the LED islimited. Turn-off and turn-on times are no faster than

several nanoseconds, and, therefore, transmission rates arelimited. Most LED-Ike transmitters are used for shortdistance, low-speed digital fiber-optic systems.


37/57

Lasers The other commonly used light transmitter is a laser.

A laser is a light source that emits coherent monochromaticlight. Monochromatic means a single light frequency.

Although an LED emits red light, that light covers anarrow spectrum of red frequencies.

Monochromatic light has a pure single frequency.Coherent refers to the fact that all the light waves emittedare in phase with one another.

Coherent light waves are focused into a narrow beam,which, as a result, is extremely intense. The effect issomewhat similar to that of using a highly directionalantenna to focus radio waves into a narrow beam whichalso increases the intensity of the signal.


38/57

Lasers There are many different ways to make lasers.

Some lasers are made from a solid rod of special material which is capableof emitting light when properly stimulated.

Certain types of gases are also used. However, lasers used in fiber optic

systems are usually nothing more than specially made LEDs which arecapable of operating as lasers.

The most widely used light source in fiber optic systems is an injection laserdiode (ILD).

Like the LED, it is a PN junction diode usually made of GaAs.


39/57

Lasers At some current level, it will emit a brilliant light. The physical

structure of the ILD is such that the semiconductor structure iscut squarely at the ends to form internal reflecting surfaces.

One of the surfaces is usually coated with a reflecting materialsuch as gold. The other surface is only partially reflective.

When the diode is properly biased, the light will be emitted andwill bounce back and forth internally between the reflectingsurfaces.

The distance between the reflecting surfaces has been carefully

measured so that it is some multiple of a half wave at the lightfrequency.

The bouncing back and forth of the light waves causes theirintensity to reinforce and build up. The structure is like a cavityresonator for light. The result is an incredibly high brilliance,single-frequency light beam that is emitted from the partiallyreflectin surface.


40/57

Lasers Injection laser diodes are capable of developing light

power up to several watts. They are far more powerful thanLEDs and, therefore, are capable of transmitting over

much longer distances. Another advantage ILDs have over LEDs is their ability to

turn off and on at a faster rate. High-speed laser diodes arecapable of gigabit per second digital data rates.

Injection lasers dissipate a tremendous amount of heat and,therefore, must be connected to a heat sink for properoperation. Because their operation is heat-sensitive, mostinjection lasers are used in a circuit that provides somefeedback for temperature control. This not only protectsthe laser, but also ensures proper light intensity andfrequency.


41/57

Lasers A typical injection laser transmitter circuit is shown in Fig. 13-

25. When the input is zero, the AND gate output is zero, so Q1is off and so is the laser. CapacitorC2 charges throughR3 to the

high voltage. When a binary 1 input occurs, Q1 conducts,connecting C2 to the ILD. Then C2 discharges a very highcurrent pulse into the laser, turning it on briefly and creating anintense light pulse


42/57

Optical Amplifiers: EDFA


43/57

Optical Receivers The receiver part of the optical communications system is relatively simple.

It consists of a detector that will sense the light pulses and convert theminto an electrical signal.

This signal is then amplified and shaped into the original serial digitaldata. The most critical component, of course, is the light sensor.

The most widely used light sensor is a photodiode.

This is a silicon PN junction diode that is sensitive to light. This diode isnormally reverse-biased as shown in Fig. 13-26


44/57

Optical Receivers The only current that flows through it is an extremely small reverse

leakage current.

Whenever light strikes the diode, this leakage current will increasesignificantly.

It will flow through a resistor and develop a voltage drop across it.

The result is an output voltage pulse.

The reverse current in a diode is extremely small even when the diodeis exposed to light. The resulting voltage pulse is very small, so it must

be amplified.

This can be done by using phototransistor. The base-collector junction

is exposed to light. The base leakage current produced causes a larger emitter-to-collector

current to flow. Thus the transistor amplifies the small leakage currentinto a larger, more useful output.


45/57

Optical Receivers The sensitivity and response time of a photodiode can be

increased by adding an undoped or intrinsic (I) layer between theP and N semiconductors to form a PIN diode.

The I layer is exposed to the light. The diode is reverse biased.

A widely used photosensor is the avalanche photodiode (APD).It is the fastest and most sensitive photodiode available, but it isexpensive and its circuitry is complex. Like the standardphotodiode, the APD is reverse-biased; However, the operationis different. The APD uses the reverse breakdown mode ofoperation that is commonly found in zener diodes. When asufficient amount reverse voltage is applied, an extremely highcurrent will flow due to the avalanche effect


46/57

Optical Receivers

Normally, several hundred volts of reverse bias,just below the avalanche threshold, is applied.

When light strikes the junction, breakdownoccurs and a large current flows.

This high reverse current requires lessamplification than the small current in a

standard photodiode.

The APDs are also significantly faster and arecapable of handling the very high gigabit persecond data rates possible in some systems.


47/57

Transmission Challenges

Dispersion


48/57


Dispersion


49/57


Dispersion


50/57



51/57


Dispersion

Two bands have developed in the third transmission window the Conventional, or C-band,

from approximately 1525 nm 1565 nm, and theLong, or L-band, from approximately

1570 nm to 1610 nm.


52/57

Wavelength Division Multiplexing


53/57

Fiber-Optic Data Communications Systems

Multiplexing on Fiber-Optic Cable

Data is most easily multiplexed on fiber-optic cable by using TDM as in theTl system. However, developments in optical components make it possible touse FDM on fiber-optic cable (called wavelength division multiplexing, orWDM), which permits multiple channels of data to operate over the cable'slight-wave bandwidth.

WDM uses separate lasers to transmit serial digital data simultaneously ontwo or more different light wavelengths. Current systems use light in the1550-nm range. A typical four-channel system uses laser wavelengths of1534, 1543, 1550, and 1557.4 nm. Each laser is switched off and on with thedesired data. The laser beams are then optically combined and transmittedover a single fiber cable.

Figure 2.5 illustrates the WDM system. A separate serial data source controlseach laser.


54/57

WDM System


55/57

WDM System At the receiving end of the cable, special optical

filters are used to separate the light-beams intoindividual channels. Each light beam is detected with

an optical sensor and then converted into the fourindividual data streams.

WDM significantly increases the data-handlingcapacity of fiber-optic cable. When WDMmultiplexer/demultiplexer units are added to existing

systems, more data channels and/or higher dataspeeds can be accommodated. Systems with 8, 16, or32 channels are available.

Example of multiplexer/demultiplexer: Prismrefraction, waveguide grating etc.


56/57

a) Coarse WDM and b) Dense WDM

Originally, the term "coarse wavelength division multiplexing" was fairly generic,

and meant a number of different things. In general, these things shared the fact that

the choice of channel spacings and frequency stability was such that erbium doped

fiber amplifiers (EDFAs) could not be utilized. Prior to the relatively recent ITU

standardization of the term, one common meaning for coarse WDM meant two (or

possibly more) signals multiplexed onto a single fiber, where one signal was in the

1550 nm band, and the other in the 1310 nm band.

Dense wavelength division multiplexing (DWDM) refers originally to optical signalsmultiplexed within the 1550 nm band so as to leverage the capabilities (and cost) of

erbium doped fiber amplifiers (EDFAs), which are effective for wavelengths

between approximately 15251565 nm (C band), or 15701610 nm (L band). EDFAs

were originally developed to replace SONET/SDH optical-electrical-optical (OEO)

regenerators, which they have made practically obsolete.

WDM System: Classifications


57/57

Problems

Bellamy and Vishwanathan

Documents

Optical Communications.pdf