Introduction to Optical Communications

Thanks is due to slides from

Neal S. BerganoTycom, Inc.

Eatontown, NJ 07724

Neal S. BerganoTycom, Inc.

Eatontown, NJ 07724

Course Objectives

To develop a practical understanding of the components and techniques used in current optical communication systems and to have an analytical understanding of their capabilities and limitations.

Overview of the module

Outline -

Historical backgroundWhy Optical communications?Transmitting bits across the ocean using WDM techniques

A Brief History

1820: Oersted - Electricity deflects a magnet1831: Joseph Henry and Michael Faraday – elucidate laws of induction1838: Morse – telegraphy1858: First undersea cable Worked for 27 days

History

1865: Undersea cable laid between Canada and Ireland1876: Alexander Graham Bell invents the telephone1897: Marconi patents a wireless system

History

The Great Eastern deploys the first successful transatlantic telegraphcable in 1865. The cost of a message was $5 per word.

The Great Eastern deploys the first successful transatlantic telegraphcable in 1865. The cost of a message was $5 per word.

Bern Dibner, The Atlantic Cable, Burndy Library 1959

History

Undersea cable milestones:–1956 First Transatlantic Telephone Cable (TAT-1)

–1988 First Transatlantic Fiber Optic Cable (TAT-8)

–1998 First WDM Undersea Cable (AC-1)

Modern cable ship

Fiberoptics

Q Why fiberoptics?A Practical system designs have enormous

capacity (Hundreds of Gigabits per second)

Q Why do fiber systems support such large capacity?

A Optics provide very high frequency & bandwidth.

Old Trans-Atalantic cable

RESEARCH

DEPLOY

MANUFACTURE

TEST

DESIGN

MAINTAIN/ OPERATE

BranchingBranchingUnitUnit

RepeaterRepeater

CableCable

Traffic

Terminal

LineTerminatingEquipment

Power FeedEquipment

Undersea Network

Management Equipment

Cable geometry

OPTICALFIBER

UNITFIBER

STRUCTURE

STRENGTHWIRES

COPPERSHEATH

INSULATIONJACKET

ARMORED PROTECTIONLAYER

Cumulative Installed: Trans-Atlantic Capacity

Bit Rate Circuit Count*

(Gb/s) (# 64Kb/s)0.155 1,890 2.5 30,2405.0 60,48010 120,96040 483,840100 1,209,600160 1,935,360320 3,870,720

1000 12,096,000

Bit Rate Circuit Count*

(Gb/s) (# 64Kb/s)0.155 1,890 2.5 30,2405.0 60,48010 120,96040 483,840100 1,209,600160 1,935,360320 3,870,720

1000 12,096,000

1955 1965 1975 1985 1995

100

1K

10K

100K

1M

10M

NSB Atlanti2

1960 19801970 20001990

Dig

ital

Reg

ener

ator

s Opt

ical

Am

plifi

ers

20% AnnualGrowth

50% AnnualGrowth

64Kb/s Digital Circuits3KHz Analog Circuits

Analog

Repeaters

>100% Annual Growth

* Assuming:• 30 circuits per 2Mb/s E1• 63 E1s in an STM-1

YEAR

Four Generations of Lightwave Systems

First Generation: Digital regenerator, 1.3µm FP lasers, 1.3µm λ0 fiber. Example TAT-8, 0.3Gb/sFirst Generation: Digital regenerator, 1.3µm FP lasers, 1.3µm λ0 fiber. Example TAT-8, 0.3Gb/s

Second Generation: Digital regenerator, 1.55µm DFB lasers, 1.3µm λ0 fiber. Example TPC-4, 0.6Gb/sSecond Generation: Digital regenerator, 1.55µm DFB lasers, 1.3µm λ0 fiber. Example TPC-4, 0.6Gb/s

Third Generation: EDFA repeater, Single Channel, 1.55µm λ0 fiber. Example TAT12/13, 5 Gb/sThird Generation: EDFA repeater, Single Channel, 1.55µm λ0 fiber. Example TAT12/13, 5 Gb/s

Fourth Generation: EDFA repeater, Multi Channel, 1.58µm λ0 fiber. Example AC1, 16x2.5 Gb/sFourth Generation: EDFA repeater, Multi Channel, 1.58µm λ0 fiber. Example AC1, 16x2.5 Gb/s

The Early 1990’s Capacity Dilemma

Optical fibers have a large intrinsic capacity, but fiber opticsystems using electro-optic regenerators do not.

Optical fibers have a large intrinsic capacity, but fiber opticsystems using electro-optic regenerators do not.

Optical amplifier repeaters(Remove capacity bottleneck)

Wavelength division multiplexing(Use more bandwidth)

Dispersion management(Reduce channel interactions)

New transmission formats(Allow for higher data rates)

Forward error correcting codes(Improve error performance)

Time

Undersea fiberoptic cable systems make the Web “Worldwide”

Data Waveforms

Bit Slot = 94 psec (10.7 Gb/s signal)Bit Slot = 94 psec (10.7 Gb/s signal)

Equivalent distance traveled in a fiber (one bit is 0.75in long in a fiber)

Equivalent distance traveled in a fiber (one bit is 0.75in long in a fiber)

Non-Return-to-Zero (NRZ) and Return-to-Zero (RZ) is a uni-polar binary code. Binary “1” is represented with a pulse, and “0” is represented by the absence of a pulse.

T=1/B

T=1/B

NRZRZ

Soliton

Inte

nsity

Time

Data streams

NRZ

CRZ

Soliton

Time

1 1 1 0 11 101

Bit Slot

Frequency/Bandwidth

540kHz 1600kHz

AM radio(10KHz)

Sports radio 660 Bloomberg 1130

. . .

Infrared

Light(10GB/s)

230 THz(1300nm)

190 THz(1600nm)

450 THz(660nm)

Red

Ora

nge

Yello

wG

reen

Blue

Indi

goVi

olet

100 Million times

Capacity

Light

40 THz

230 THz(1300nm)

190 THz(1600nm)

450 THz(660nm)

Today’s optical fiber has a potential of about 40Tera Hz of “Bandwidth” {40,000,000,000,000Hz}If we could use 1/2 of the available bandwidth, the ultimate capacity of a fiberoptic transmission system would be about 20TB/s20TB/s is equivalent to 1/2 Billion 45KBit/s modem connections

Red

Ora

nge

Yello

wG

reen

Blue

Indi

goVi

olet

C-Band EDFA191.7 - 196.7 THz

(5THz)

Chromatic Dispersion

Different wavelengths travel at different speed, or group velocity.

Example: 50km of standard single mode fiber

Time Delay

Wavelength

244.4 usec

1550.0 1550.8

Equivalent fiber length

50 km

50 km + 14cm244.40068usec

Signal Distortion Caused by Chromatic Dispersion

Eye Diagram Optical Spectrum

Chromatic Dispersion: Different wavelengths travel at different speed, or group velocity.

The Early 1990’s Capacity Dilemma

Optical fibers have a large intrinsic capacity, but fiber opticsystems using electro-optic regenerators do not.

Optical fibers have a large intrinsic capacity, but fiber opticsystems using electro-optic regenerators do not.

T R

Electro-Optic Repeater

Erbium-doped Fiber Amplifier

PumpLaser

Pump/SignalCombiner

ErbiumFiber

(10-50 m)

Distortionless Amplification of Light

Bit-Rate Independent

Signal Remains in Fiber

Amplifies Many Wavelengths Simultaneously

Transmitting bits through a fiberoptic line

OpticalTransmitter

OpticalReceiverBPF

Data source

DataSink

1 1 1 0 1 0 10 0 11

Optical Amplifier Transmission System

T RBPF

Single Channel

Wavelength Division MultiplexingλN

λ3

λ2

λ1

RBPF

RBPF

RBPF

RBPFλN

λ3

λ2

λ1T

T

T

T

Wavelength Division Multiplexing (WDM)

Data bits in the fiber

λN

λ3

λ2

λ1T

T

T

T

Project Yellow:• 32 x 10Gb/s channels• Channel spacing is

0.6nm (or 75GHz)

Optical Spectrum

Intensity

(or brightness)

Wavelength (color)

Amplified Transmission Line

g

1/g

g

1/g

g

1/g

Operating Point

AmplifierGain

Output Power

(Span Loss)-1

Gain Compression

3.0

2.5

2.0

1.5

1.0

0.5

0.00 2000 4000 6000 8000 10000

Distance (km)

Opt

ical

Pow

er (m

W)

Signal Power

Noise Power

Noise Accumulation In Amplifier Systems

gg ++2nsp(g-1)hvB0

Pin Pout

30 dB Amplifier: 400µW/A in 10,000km

150 km10 dB Amplifier: 12µW/A in 10,000km

Transmission Formats

Old Thinking:

NRZ had the advantage of compatibility, with adequate performance.

Solitons had the advantage of single channel capacity.

Use NRZ first, than switch to a “higher” capacity format.

Old Thinking:

NRZ had the advantage of compatibility, with adequate performance.

Solitons had the advantage of single channel capacity.

Use NRZ first, than switch to a “higher” capacity format.

What has happened over the past 5 years:

WDM removed the capacity advantage of solitons.

NRZ migrated to synchronously modulated NRZ.

Solitons migrated to guided, dispersion managed solitons.

What has happened over the past 5 years:

WDM removed the capacity advantage of solitons.

NRZ migrated to synchronously modulated NRZ.

Solitons migrated to guided, dispersion managed solitons.

Present Thinking: Increase capacity using WDM

Reduce and/or manage the fiber’s nonlinear index

Take advantage of fiber’s nonlinear index

Present Thinking: Increase capacity using WDM

Reduce and/or manage the fiber’s nonlinear index

Take advantage of fiber’s nonlinear index

WDM Transmitter

Chirped Return-to-Zero (CRZ) Signal

...

Com

bine

rC

ombi

ner

Com

bine

r111

333

636363

DataDataData

time

Phase

Intensity

time

220-1222020--11FECFECFEC

...

Com

bine

rC

ombi

ner

Com

bine

r222

444

646464

DataDataData AMAMAM PMPMPM

215-1221515--11FECFECFEC

AMAMAM PMPMPM

CRZ Waveform: More tolerant to large accumulated dispersion

CRZ Waveform: More tolerant to large accumulated dispersion

λ

... ...Orthogonal polarization launch: Reduced channel interaction

Basic definitions

Losses in dB

X dB = -10 log10 (Pout/Pin)

Beer’s law : Pout/Pin = e-αL

dB/Km =X/L= 10 log10 (e) α

Power (dBm) = 10 log10 (P(mW)/1 mW)

Log(Pin)

Distance

Signal digitizing

Sampling theorem due to Nyquist

sample frequency

fs > 2 ∆f

∆f is the signal bandwidth

Digital signal has finite accuracy,

say m bits. The number of quantized

levels is M = 2m

Digitized Signal

Bandwidth of digital signal B = m fsSignal to noise ratio (SNR)

SNR (dB) = 20 log10 (Amax/AN)

M > Amax/AN

AN is the average noise amplitude

B > 2 ∆f log10(M)/ log10(2) > ∆f SNR/3