Chapter 1 Historical Development and Recent Situation of

8/7/2019 Chapter 1 Historical Development and Recent Situation of

1/31

Chapter 1 Historical Development and Recent Situationof Optical Communications

1.1 Historical Development of Optical

Communications

1.2 General Optical Fiber Communication Systems

1.3 Advantages of Optical Fiber Communications

1.4 Applications of Optical Fiber Communications


2/31

1.1 Historical Development of

Optical Communications The use of light for communication purposes dates back to

antiquity if we interpret optical communications in a broad

sense.

Most civilizations have used fire and smoke signals

to convey a single piece of information (such as victory

in a war).

Essentially the same idea was used up to the endof the 18th century through signaling lamps, flags,

and other semaphore devices.

The advent of telegraphy in the 1830s replaced the

use of light by electricity and began the era of electrical


3/31


Optical Communications The bit rate B could be increased to ~ 10 b/s by the use of

new coding techniques such as the Morse code.

The use of intermediate relay stations allowed communication

over long distances (~1000 km). Indeed, the first successful

transatlantic telegraph cable went into operation in 1866.

The invention of the telephone in 1876 brought a majorchange inasmuch as electric signals were transmitted in the

"analog" form through a continuously varying electric current.


4/31


Optical Communications The development of worldwide telephone networks during

the 20th century led to many advances in the design of

electrical communication systems.

The first coaxial-cable system, put into service in 1940, was a

3-MHz system capable of transmitting 300 voice channels or a

single television channel.

The bandwidth of such systems is limited by the frequency-

dependent cable losses which increase rapidly for frequencies

beyond 10 MHz.


5/31


Optical Communications This limitation led to the development of microwave

communication systems in which an EM carrier

wave with frequencies ~1-10 GHz is used to transmit the signalby using suitable modulation techniques.

The first microwave system operating at the carrier frequency

of 4 GHz was put into service in 1948.

The most advanced coaxial system, put into service in 1975,

operates at a bit rate of 274 Mb/s.


6/31


Optical Communications Microwave communication systems generally

allow larger repeater spaces, but their bit rate is

also limited by the carrier frequency of suchwaves. The capacity of a communication system is often

measured through the bit rate-distance productBL, where B is the bit rate and L is the repeaterspacing.


7/31


Optical Communications

Fig. 1.1 Increase in bit rate-distance product

during 1850-2000.


8/31


Optical Communications It was realized during the second half of the 20th century

that an increase of several orders of magnitude in the BL

product would be possible if optical waves were used ascarrier.

However, neither a coherent optical source nor a suitable

transmission medium was available during the 1950s. The

invention of the laser and its demonstration in 1960 solvedthe first problem.


9/31


Optical CommunicationsAttention was then focused on finding ways for

using the laser light for optical communications.

Many ideas were advanced during the 1960s, the

most noteworthy being the idea of light confinement

by using a sequence of gas lenses.

It was suggested in 1966 that optical fibers might be

the best choice, as they are capable of guiding the

light in a way similar to the guiding of electrons in

copper wires.


10/31


Optical CommunicationsThe main problem was the high loss of optical fibers; fibers

available during the 1960s had losses in excess of 1000

dB/km.A breakthrough occurred in 1970 when the fiber loss could be

reduced to about 20 dB/km in the wavelength region near 1-Qm.

At about the same time, GaAs semiconductor lasers,

operating continuously at room temperature, weredemonstrated.

The simultaneous availability of a compact optical source and

a low-loss optical fiber led to a worldwide effort for

developing fiber-optic communication systems.


11/31


Optical CommunicationsFigure 1.2 shows the progress in the performance oflightwave systems realized after 1974 through five

generations of development stages.

Fig. 1.2 Progress in lightwave communicationtechnology over the period 1974-1992.


12/31


Optical CommunicationsThe commercial deployment of lightwave systems followed

the research and development closely.After many field trials,

the first generation lightwave systems operating near 0.8 Qm

began to be deployed in 1978.

They operated at a bit rate in the range 50-100-Mb/s range

and allowed a repeater spacing of about 10 km [BL ~ 500

(Mb/s)-km].

It was clear during the 1970s that the repeater spacing could

be increased considerably by operating the lightwave system in

the wavelength region near 1.3 Qm.


13/31


Optical Communications Furthermore, optical fibers exhibit minimum dispersion in

this wavelength region.

This realization led to a worldwide effort for the development

of InGaAsP semiconductor lasers and detectors operating near

1.3 Qm. Such a laser was demonstrated in 1977.

The second generation of fiber-optic communication systemsbecame available in the early 1980s and allowed a repeater

spacing in excess of 20 km.


14/31


Optical CommunicationsA laboratory experiment in 1981 demonstrated

2-Gb/s transmission over 44 km of single-mode fiber.

The introduction of commercial systems soon followed.

By 1987, second-generation 1.3 Qm lightwave systems

operating at bit rates up to 1.7 Gb/s with a repeater spacing of

about 50 km were commercially available.

The repeater spacing of the second-generation lightwave

systems is limited by the fiber loss at the operating wavelength

near 1.3 Qm (typically 0.5 dB/km).


15/31


Optical CommunicationsThe introduction of third-generation lightwave systems

operating at 1.55 Qm was considerably delayed

by large fiber dispersion near 1.55 Qm.

Conventional InGaAsP semiconductor lasers could not be

used because of pulse spreading occurring as a result of

simultaneous oscillation of several longitudinal modes.

By 1985 the laboratory transmission experiments showed the

possibility of communicating information at bit rates up to 4

Gb/s over distances in excess of 100 km.


16/31


Optical Communications The third-generation 1.55-Qm systems operating at 2.4

Gb/s became available commercially in 1990.

Such systems are capable of operating at bit rates in excess of

10 Gb/s with a careful design of semiconductor lasers and

optical receivers.

The development of 10-Gb/s lightwave systems is under way

in several laboratories.


17/31


Optical CommunicationsCoherent systems have been under development worldwide

during the 1980s, and their potential advantage has been

demonstrated in many system experiments.

In one experiment 100 channels of 622 Mb/s were

multiplexed by using a star coupler and transmitted over

50 km of fiber length with negligible interchannel crosstalk.

The use of coherent detection is not a prerequisite for

lightwave systems employing optical amplifiers.


18/31


Optical CommunicationsThese devices were originally fabricated from alloys of

AlGaAs which emitted optical wavelength.

Recently this wavelength range has been extended to the

region 1.1-1.6 Qm by the use of other semiconductor alloys.

The light emitting diodes (LEDs) and photodiodes (PDs) alsocontributed to the realization of optical fiber

communication systems.


19/31


Optical CommunicationsThe radio communication was developed to higher

frequencies leading to the introduction of the even higher.

The relative frequencies and wavelengths of these types

of electromagnetic wave can be observed from the

electromagnetic spectrum.

In additional benefit of the use of high carrier

frequencies is the general ability of the communication

system.


20/31


Optical CommunicationsOptical communication systems were developed with

the invention of the solid state laser.

This device provided a coherent light source together

with the possibility of modulation at high frequency.

The invention of the laser instigated a tremendousresearch effort in the study of optical components to

achieve reliable information transfer using a lightwave

carrier.


21/31

1.2 The General Optical Fiber

Communication Systems The optical fiber communication systems are

similar to all the other communication systems. In general, the communication systems

therefore consist of a transmitter, thetransmission medium, and a receiver.

For optical fiber communication system, theinformation source provides an electrical signal

to a transmitter comprising optical source andoptical modulator.


22/31


Communication Systems The transmission medium may consist of a pair of wires, a

coaxial cable or a radio link through free space down which the

signal is transmitted to the receiver.

In any transmission medium the signal is attenuated and is

subject to degradations due to contamination by random signals

and noise.

In any communication system there is a maximum

permitted distance between the transmitter and the receiver.


23/31


Communication Systems The optical carrier may be modulated by using either an

analog or digital information signal.

A

nalog modulation involves the variation of the lightemitted from the optical source in a continuous manner.

Thus the analog optical fiber communication links are

generally limited to shorter distances and lower bandwidths

than the digital optical fiber communication links.


24/31


Communication Systems Optical communication systems differ in principle from

other communication systems only in the frequency range of

the carrier wave used to carry the information.

The optical carrier frequency is typically ~100 THz, which

should be compared with the microwave carrier frequency of

~1-10 GHz.


25/31


Communication Systems

Fig. 1.3(a) The general communication system


26/31


Communication Systems

Fig. 1.3(b) The optical fiber communication


27/31

1.3 Advantages of Optical Fiber

Communications Low Loss

Now the losses optical fibers have been achieved as low as 0.2dB/km.

It increases the repeater spacing of optical fiber

communication systems.

Broad Band

The frequency of optical carrier in the range 10 MHz to 10 THz.

The information-carrying capacity of optical fiber systems isalready proving far superior to the best copper cable systems.


28/31


Communications Electrical Isolation

Optical fibersare fabricated from glasssometimesaplasticpolymer.

Small Size And Light WeightWhensuch fibersarecovered withprotective

coatingstheyare farsmallerandmuchlighterthanthecorresponding coppercables.

Non-crosstalk CharacteristicTheoperationofanoptical fibercommunication

systemisunaffectedbytransmission.


29/31


CommunicationsFFlexibility

The coated optical fibers are manufactured with hightensile strengths.

The coated fibers may also be bent to quite small radii ortwisted without damage.

Nature Resource ConservationThe optical fiber is made from sand. So, in comparison

with copper conductors.In the future it will become as cheap to use optical fibers

with their superior performance than almost any type ofelectrical conductor.


30/31

1.4 Applications Of Optical Fiber

Communications Voice CommunicationApplication

The first applications were for public and privatetelephone communication system links.

The expanded service and the suitability of optical fibersare for the voice communication to the design and test oftelephone equipments.

Video CommunicationApplication

Optical Visual Information System (OVIS )

Cable Television (CATV) System.


31/31

1.4 Applications Of Optical Fiber

Communications

Data CommunicationApplication

Connections between the central processing unit (CPU)and terminal equipments and between CPUS can be made alocal area network (LAN).

SensorApplication

The application of optical fibers in sensing externalstimuli such as current, pressure, temperature, rotationselectric and magnetic fields.

Date post:	09-Apr-2018
Category:	Documents
Upload:	gajjar-gaurang
View:	221 times
Download:	0 times

Chapter 1 Historical Development and Recent Situation of

Documents