Overview OFC

8/2/2019 Overview OFC

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Chapter 1

Overview of Optical Fiber Communications

Introduction

Fundamentals of communicationsFundamentals of communications

Terahertz communications : the potentialTerahertz communications : the potential

Limitations ofLimitations ofelectronic communicationselectronic communications

Optical Communication SystemsOptical Communication Systems

Novel TechniquesNovel Techniques

Transmitter ReceiverChannel

Information

Information

Fundamentals of Communications

The information is usually transferred over the communication

channel by superimposing the information onto a sinusoidally

varying electromagnetic wave known as thecarrier.

A Historical Perspective

18371837 -- Morse demonstrates telegraphMorse demonstrates telegraph

18781878 -- Bell invents telephoneBell invents telephone

18781878 -- MaxwellMaxwells equations describe propagations equations describe propagation

of Electromagnetic Wavesof Electromagnetic Waves

18881888 -- HertzHertzs demonstration of long radio wavess demonstration of long radio waves

18951895 -- MarconiMarconis demonstration of wireless radios demonstration of wireless radio

communicationscommunications

The amount of information that can be

transmitted is directly related to the frequency

of the carrier.

Early radio (15 KHz voice signals in the 0.5Early radio (15 KHz voice signals in the 0.5

--2 MHz range)2 MHz range)

Television (6 MHz bandwidth, CarrierTelevision (6 MHz bandwidth, Carrier

frequencies in the 100 MHz range)frequencies in the 100 MHz range)

Microwaves (GHz domain)Microwaves (GHz domain)

Optical Communications (THz ?)Optical Communications (THz ?)

The trend has been to employ progressively

higher frequencies for the carrier.

The optical spectrum ranges from about 50 nm to

100 m.

UltravioletUltraviolet 50 nm50 nm

Visible lightVisible light 400 to 700 nm400 to 700 nm

Near InfraredNear Infrared about 800 to 5,000 nmabout 800 to 5,000 nm

Mid InfraredMid Infrared about 5,000 to 30,000 nmabout 5,000 to 30,000 nm

Far InfraredFar Infrared 30,000 nm and longer30,000 nm and longer

Optical fiber communications usually operate in the

800 to 1,600 nm wavelength band.


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Key technical problems

Highest speeds for electronic devicesHighest speeds for electronic devices

((picosecondpicosecond range = 10range = 10--1212 secssecs))

There are no devices currently that can react at the frequency of

light for communication purposes. Detection is at much slower

rates (relative to the light carrier) by intensity modulation.

We need a reliable and consistent transmission medium.

Optical mediums include

free space

optical waveguides

THz Communications...

Note : 1 % ofNote : 1 % of vvruby = 5 x 10= 5 x 101212 Hz ( 5 THz!)Hz ( 5 THz!)

Can carry 10Can carry 1066

commercial video channelscommercial video channels Can carry 10Can carry 109 telephone calls at 5 kHz per calltelephone calls at 5 kHz per call

First laser (Ruby) operated at a wavelength of 694 nmFirst laser (Ruby) operated at a wavelength of 694 nm

This wavelength corresponds to a carrier frequencyThis wavelength corresponds to a carrier frequency

of 5 x 10of 5 x 101414 HzHz

So why isnSo why isnt this bandwidth being fully utilized ?t this bandwidth being fully utilized ?

Appropriate light sources that can be modulated anywhere nearAppropriate light sources that can be modulated anywhere nearthat fast.that fast.And detectors that can react to the frequency rather than theAnd detectors that can react to the frequency rather than theintensity of the light.intensity of the light.

The Role of the Optical Fiber

One reason for an optical fiber is the limitations of free space

optical communication.

In free space communication, no one owns the channel. The

user is subject to the whims of weather, passing beam

obstructions (birds, dust, new buildings) and so on.

A free space channel might be a hazardous to people and objects

in high optical power conditions.

A free space channel is inherently line of sight. No over the

horizon communication!

The advantages of guided wave propagation

The channel is well-defined with reliable and repeatable

performance.

Long distance communication is now possible within the limits

of channel attenuation and distortion.

No horizon problem.

Early fibers had large attenuation (1000 dB/km !)

Elimination ofElimination of impurities has dramatically lowered attenuationhas dramatically lowered attenuation

to a low of 0.2 dB/km at a wavelength of 1500 nm.to a low of 0.2 dB/km at a wavelength of 1500 nm.

Advantages of Optical Fiber Communications

Capacity for 25 THz information bandwidthCapacity for 25 THz information bandwidth

Low loss.Low loss.

Signal can travel for very long distances without repeaters orSignal can travel for very long distances without repeaters or

regenerators.regenerators.

Light weight.Light weight.

A fiber bundle is much lighter than an equal sized metalA fiber bundle is much lighter than an equal sized metal

conductor cable.conductor cable.

The signal capacity within the same size optical fiber bundle isThe signal capacity within the same size optical fiber bundle is

much greater than for metal conductors.much greater than for metal conductors.

Immunity from EMI and crosstalk.Immunity from EMI and crosstalk.

The optical signal is not usually affected by nearbyThe optical signal is not usually affected by nearby

electromagnetic fields even large ones.electromagnetic fields even large ones.

Advantages of Optical Fiber Communications

Compatible family of devices exist.Compatible family of devices exist.

There are now lasers, detectors and optoelectronic integratedThere are now lasers, detectors and optoelectronic integrated

circuits (circuits (OEICOEICss) from many vendors that are compatible at) from many vendors that are compatible at

specific wavelengths.specific wavelengths.


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Types of Optical Fiber Communication Systems

Long Distance TelecommunicationLong Distance Telecommunication

Point to point links (between cities)Point to point links (between cities)

Telephone communicationsTelephone communications

Purchasing and installing fiberPurchasing and installing fiber

Data CommunicationsData Communications Local Area NetworksLocal Area Networks

Computers, Databases, WorkstationsComputers, Databases, Workstations

Hardware costs (TXR, Connectors, Switches,Hardware costs (TXR, Connectors, Switches,Filters, RCVRS)Filters, RCVRS)

What is required ?

SwitchesSwitches

AmplifiersAmplifiers

FiltersFilters ConnectorsConnectors

Better modulation schemes (FM or PM)Better modulation schemes (FM or PM)

Novel Techniques

Wavelength Division MultiplexersWavelength Division Multiplexers

Optical AmplifiersOptical Amplifiers

Optical Filters & SwitchesOptical Filters & Switches

Coherent Modulation SchemesCoherent Modulation Schemes

Section 1.1 Basic Network Information Rates

Various services require different data rates for useful

communication.

Historically these various services were time divisionmultiplexed onto higher capacity transmission channels.

Fig. 1-2: Digital transmission hierarchy

Example of telecommunication multiplexing scheme Section 1.1 Basic Network Information Rates

There are many competing and complimentary formats andschemes in use including

SONET synchronous optical network

SDH synchronous digital hierarchy

ATM asynchronous transfer mode

The trend is to communicate at very high data rates to

accommodate various purposes rather than characterizing the

channel by the source (such as voice, fax, video)


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Section 1.2 The Evolution of Fiber Optic Systems

The bit-rate-distance product (BL) measures the transmission

capacity of optical fiber links.

Since 1974, the transmission capacity has increased by 10-fold

every four years!

The transmission capacity increase has resulted from innovation

in the four key components of an optical link.

The optical fiber

Light sources

Photodetectors

Optical amplifiers

Section 1.2 The Evolution of Fiber Optic Systems

Optical fiber improvements resulted in wider repeater spacing

due to improvements in

Attenuation at specific wavelengths

Dispersion (distortion of the light pulse)

Light sources have improved in reliability, modulation rate,

power consumption and wavelength availability.

Photodetectors also saw improvement in noise performance and

low light detection capability.

Optical amplifiers have reduced the need to detect/regenerate/re-

transmit light signals. This has fairly dramatically increased

repeater separation distances.

Fig. 1-3: Operating ranges of components

Section 1.3 Elements of an Optical Fiber Transmission Link

An optical fiber transmission link comprises:

Light transmitter and associated drive circuitry.

Optical fiber in a cable for mechanical and environmental

protection.

Receiver consisting of a photodetector plus amplification and

signal-restoring circuitry.

Frequently the fiber cable may contain copper wires for

powering optical amplifiers or signal regenerators.

Fig. 1-5: Major elements of an optical fiber linkFig. 1-6: Optical fiber cable installations


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One of the principal characteristics of an optical fiber is its

attenuation as a function of wavelength.

Historically, 800 to 900 nm was the first band for fiber

transmission. This is called thefirst window.

Continued fiber development resulted in very low loss in the

1,100 to 1,600 nm region.

Centered at 1,310 nm is the second window.

Centered at 1,550 nm is the third window.

Fig. 1-7: History of attenuation


Once the fiber cable is installed, a light source that is

dimensionally compatible with the fiber core is used to launch

optical power into the fiber.

Semiconductor light-emitting diodes (LEDs) and laser diodes

are suitable for this purpose.

Their light output can be modulated rapidly by simply varying

the bias current at the desired transmission rate.

At high data rates (> 1 GHz), direct modulation of the source

can lead to unacceptable signal distortion. Thus an externalmodulator is frequently used.

Section 1.4 Simulation and Modeling Tools

Hand analysis results in ball-park answers.

Increasingly, photonic design automation (simulation) is used to

refine designs to be more efficient, cost-effective and robust.

The author has included the student edition of one such

simulation program thePhotonic Transmission Design Suite

(PTDS) by Virtual Photonics, Inc.

The department also has a copy of LINKSIM by RSoft, as well

as NIs labVIEW. We will use various combinations of the

software during the semester.

End of Chapter 1

Overview of Optical Fiber Communications

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Overview OFC