OPT-1101(USA2006) - Optical Fundamentals

1 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicOPT-110112512_04_2006_c2

Optical Fundamentals

OPT-1101


Agenda

Introduction and Terminology

Optical Propagation and Fiber Characteristics

Attenuation and Compensation

Dispersion and Dispersion Compensation

Non Linearity

SM Optical Fiber Types

Simple SPAN Design

Summary


Introduction


Modern Lightwave Eras

0

1

10

100

1,000

10,000

1985 1990 1995 2000Year

C

a

p

a

c

i

t

y

(

G

b

/

s

)

FiberizationDigitization

SONET Rings and DWDM Linear Systems

Optical NetworkingWavelength Switching

Research Systems

Commercial Systems


Optical Spectrum

LightUltraviolet (UV)VisibleInfrared (IR)

Communication wavelengths

850, 1310, 1550 nmLow-loss wavelengths

Specialty wavelengths980, 1480, 1625 nm

UV IR

Visible

850 nm980 nm

1,310 nm1,480 nm

1,550 nm1,625 nm

125 GHz/nm

Wavelength: (Nanometers)Frequency: (Nerahertz)

C = x


Terminology

Decibels (dB): unit of level (relative measure) X dB is 10X/10

Decibels-milliwatt (dBm): decibel referenced to a milliwatt

dBm used for output power and receive sensitivity (absolute value)dB used for power gain or loss (relative value)X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW

Wavelength (Lambda): length of a wave in a particular medium; common unit: nanometers, 109m (nm)

Frequency (): the number of times that a wave is produced within a particular time period

Wavelength x frequency = speed of light x = C


TerminologyFiber Impairments

Attenuation = Loss of power in dB/km

Chromatic Dispersion = Spread of light pulse in ps/nm-km

Optical Signal-to-Noise Ratio (OSNR) = Ratio of optical signal power to noise power for the receiver


ITU Wavelength Grid

The International Telecommunications Union (ITU) has divided thetelecom wavelengths into a grid; the grid is divided into bands;the C and L bands are typically used for DWDM

ITU Bands

1530.33 nm 1553.86 nm0.80 nm

195.9 THz 193.0 THzChannel Spacing = 100 GHz

O E S C L U (nm)

0 1 n

1

2

6

0

1

3

6

0

1

4

6

0

1

5

3

0

1

5

6

5

1

6

2

5

1

6

7

5


Bit Error Rate (BER)

BER is a key objective of the optical system designBER = P(1)P(0 received for 1 transmitted) + P(0)P(1 received for 0 transmitted)

Goal is to get from Tx to Rx with a BER < BER threshold of the Rx

BER thresholds are on data sheets

Typical minimum acceptable rate is 1012


Eye Diagram

An eye diagram shows a relative performance of the signal The opening of the eye provides valuable information about

the ability of the receiver to detect the signal correctly

Consider All Eight Possible Combination of 3-Bit Sequence

Superimpose All CombinationsEye Pattern


Eye Diagram

The vertical eye opening shows the ability to distinguish between a 1 and a 0 bit

The horizontal opening gives the time period over which the signal can be sampled without errors


Eye Diagram

For a good transmission system, the eye opening should be as wide and open as possible

Eye diagram also displays information such as maximum signal voltage, rise and decay time of pulse, etc.

Extinction ratio (ratio of a 1 signal to a 0 signal) is also calculated from eye diagram


Eye Mask Testing

It is difficult to measure a numeric value that can describe the openness of the eye

A process called eye-mask testing is used

Eye Mask is a collection of polygons that represents where the eye waveform cannot exist

Eye diagram is not allowed to cross or intersect the mask

Minimum acceptable opening is defined by the shape and size of polygon


Optical Power

Optical Power Measurements: Power is measured in watts; however, a convenient way to

measure optical power is in units of decibels (dB)

The power measured on a particular signal is measured in dBm

The gain/loss measured between two points on a fiber is in dBPower loss is expressed as negative dB

Power gain is expressed as positive dB

Definition: Optical Power Is the Rate at Which

Power Is Delivered in an Optical Beam


Optical Power Budget

Calculate using minimum transmitter power and minimum receiver sensitivity

Attenuation/loss in the link, greater than the power budget, causes bit errors (dB)

Allow for two to three dBm power loss budget; so if the power budget is 15 dB, design for 12

Design networks with power budgets, not distances

The Optical Power Budget is:Optical Power Budget = Power Sent Receiver Sensitivity


Optical Power BudgetExample

Transmitter maximum power = 2 dBm Receiver sensitivity = 28 dBm

2 dBm 28 dBmTransmitter Receiver

Power Budget = 26 dB

Common Power Budgets

Short Reach (SR) 6 dB (75% Power Loss)

Intermediate Reach (IR) 13 dB (95% Power Loss

Long Reach (LR) 26 dB (99.75% Power Loss)

Calculate Power Budget = ??


A Few Words on Optical Safety

Think optical safety at all times

Optical power is invisible to the human eye

Never stare at an optical connector

Keep optical connectors pointed away from yourself and others

Glass (fiber cable) can cut and puncture


Laser Classifications/Safety IconsClass 1Lasers that are incapable of causing damage when the beam is directed into the eye under normal operating conditions. These include helium-neon lasers operating at less than a few microwatts of radiant power.

Class 2Lasers that can cause harm if viewed directly for second or longer. This includes helium-neon lasers with an output up to 1 mW (milliwatt).

Class 3ALasers that have outputs less than 5 mW. These lasers can cause injury when the eye is exposed to either the beam or its reflections from mirrors or other shiny surfaces. As an example, laser pointers typically fall into this class.

Class 3BLasers that have outputs of 5 to 500 mW. The argon lasers typically used in laser light shows are of this class. Higher power diode lasers (above 5 mW) from optical drives and high performance laser printers also fall into this class.

Class 4Lasers that have outputs exceeding 500 mW. These devices produce a beam that is hazardous directly or from reflection and can produce skin burn. Many ruby, carbon dioxide, and neodymium-glass lasers are class 4.

SR and IR Optics, Some LR

Many LR Optics, CWDM GBICS

Some LR Optics, Amplifier Outputs


Protective Eyewear Available

Protective goggles or glasses should be worn for all routine use of Class 3B and Class 4 lasers

Remember: eyewear is wavelength specific, a pair of goggles that effectively blocks red laser light affords no protection for greenlaser light

Laser Safety Equipment Can Be Investigated in Greater Detail at the

Following Link:http://www.lasersafety.co.uk/frhome.html


Some Final Words on Optical Equipment Safety

Remember: Optical cabling is constructed from strands of glass, about the size of a human hair. Never handle exposed fiber strands unless you have the proper training. Fiber fragments that becomeembedded under the skin can be extremely painful, and are very difficult (in some cases impossible) to remove.

Remember: Optical equipment is typically powered from either 120vac or 48vdc. Always remove jewelry such as rings, watches, bracelets, etc. when working with energized equipment. Care should be taken to never come in contact with exposed electrical connections or buswork.

Remember: Be careful not to damage equipment through ESD (Electrostatic Discharge). Ensure that all equipment is properlygrounded, wrist straps are used when removing modules with active components and always handle equipment modules by their edges.

Note: Many Companies Have Documented ESD Guidelines to Their Environment. Please Consult Those Documents for Additional Detail as Appropriate.


Optical Propagation in Fibers


Attenuation: Reduces power level with distance

Dispersion and nonlinearities: Erodes clarity with distance and speed

Signal detection and recovery is an analog problem

Analog Transmission Effects


Fiber Geometry

The core carries thelight signals

The cladding keeps the lightin the core

The coating protects the glass

Coating

An Optical Fiber Is Made of Three Sections:

CladdingCore


Fiber Dimensions

Fiber dimensions are measured in m

1 m = 0.000001 meters (10-6)

1 human hair ~ 50 m

Refractive Index (n)n = c/v

n ~ 1.46

n (core) > n (cladding)

Cladding(125 m)

Core(245 m)

Coating(862.5 m)


Geometrical Optics

1 = Angle of incidence 1r = Angle of reflection 2 = Angle of refraction 1 1r

2n2n1

cIs the Critical AngleIf Angle of Incidence Is Greater Than Critical Angle, All the Light Will Reflect (Instead of Refract); This Is Called Total Internal Reflection

c

2=90

n2

>

n1

n1 n2

c>

Light Is Reflected/Refracted at an Interface


Propagation in Fiber

Light propagates by total internal reflectionsat the core-cladding interface

Total internal reflections are lossless

Each allowed ray is a mode

1

n2

n1

Cladding

0 CoreIntensity Profile


n2

n1

Cladding

Core

n2

n1

Cladding

Core

Different Types of Fiber

Multimode fiberCore diameter varies

50 mm for step index

62.5 mm for graded index

Bit rate-distance product> 500 MHz-km

Single-mode fiberCore diameter is about 9 mm

Bit rate-distance product> 100 THz-km


Attenuation


Attenuation in Fiber

Light loss in fiber is caused by two thingsAbsorption by the fiber material

Scattering of the light from the fiber

Light loss causes signal attenuation

Rayleigh Scattering

Scattering

850 nm Highest

1310 nm Lower

1550 nm Lowest


Other Causes of Attenuation in Fiber

MicrobendsCaused by small distortions of the fiber in manufacturing

MacrobendsCaused by wrapping fiber around a corner with too small a bending radius

Back reflectionsCaused by reflections at fiber ends, like connectors

Fiber splicesCaused by poor alignment or dirt


T T

Pi P0

Optical Attenuation

Pulse amplitude reduction limits how far

Attenuation in dB=10xLog(Pi/Po) Power is measured in dBm:

P(dBm)=10xlog(P mW/1 mW)

Examples

10 dBm 10 mW0 dBM 1 mW

3 dBm 500 uW10 dBm 100 uW30 dBm 1 uW

)


Attenuation Response at Different Wavelengths

850nm Region 1310 nm Region 1550 nm Region


Attenuation: Compensated by Optical Amplifiers

Erbium-doped fiber amplifiers (EDFA) are the most commonly deployed optical amplifiers

Commercially available since the early 1990sWorks best in the range 1530 to 1565 nmGain up to 30 dB (1000 photons out per photon in)

Optically transparentWavelength transparentBit rate transparent

Input

1480 or 980 nm Pump Laser

Erbium Doped Fiber

Output

IsolatorCoupler


Energy = h .

Fundamental State

Excited State

Pump Photonat 980 nm

Fundamental State

Transition to a Lower Energy StateMetastable State

Telecom SignalPhoton at 1550 nm

Energy = h .

Einstein Tells Us That the Photon Generated by the Decay of the

Erbuim Ion Back to Its Fundamental State Will Be the Same Phase as the

Signal Photon that Triggered the Stimulated Emission

+ =Amplified Telecom

SignalPhoton at 1550 nm

The Photoelectric Effect in Erbium


Dispersion


Types of Dispersion

Polarization Mode Dispersion (PMD) Single-mode fiber supports two polarization states Fast and slow axes have different group velocities Causes spreading of the light pulse

Chromatic Dispersion Different wavelengths travel at different speeds Causes spreading of the light pulse


A Snapshot on Chromatic Dispersion

Affects single channel and DWDM systems A pulse spreads as it travels down the fiber Inter-symbol Interference (ISI) leads to performance

impairments Degradation depends on:

Laser used (spectral width)Bit-rate (temporal pulse separation)Different SM types

Interference


60 Km SMF-28

10 Gbps

t

40 Gbps

t

Limitations from Chromatic Dispersion

Dispersion causes pulse distortion, pulse smearing effects

Higher bit-rates and shorter pulses are less robust to Chromatic Dispersion

Limits how fast and how far

4 Km SMF-28


Combating Chromatic Dispersion

Use DSF and NZDSF fibers(G.653 and G.655)

Dispersion compensating fiber

Transmitters with narrow spectral width


Industry StandardNot Cisco Specific

Transmission Over SM FiberWithout Compensation

Transmission Rate Distance

2.5 Gb/s 980 km

10 Gb/s 60 km

40 Gb/s 4 km


How Far can I Go Without Dispersion?

Distance (Km) =Specification of Transponder (ps/nm)

Coefficient of Dispersion of Fiber (ps/nm*km)

A Laser Signal with Dispersion Tolerance of 3400 ps/nm Is Sent Across a Standard SM Fiber,

Which Has a Coefficient of Dispersion of 17 ps/nm*km

It Will Reach 200 Km at Maximum Bandwidth

Note that Lower Speeds Will Travel Farther


Dispersion Compensation

Dispersion Shifted Fiber Cable

+1000

100200300400500

Distance from Transmitter (km)

No CompensationWith Compensation

Transmitter

Dispersion Compensators

C

u

m

u

l

a

t

i

v

e

D

i

s

p

e

r

s

i

o

n

(

p

s

/

n

m

)

Total Dispersion Controlled


Polarization Mode Dispersion

Caused by ovality of core due to:Manufacturing process Internal stress (cabling) External stress (trucks)

Only discovered in the 90s Most older fiber not characterized for PMD


Polarization Mode Dispersion (PMD)

The optical pulse tends to broaden as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of some relevance at bit rates of 10Gb/s or more

nx

nyEx

Ey

Pulse as It Enters the Fiber Spreaded Pulse as It Leaves the Fiber


Combating Polarization Mode Dispersion

Factors contributing to PMDBit rateFiber core symmetryEnvironmental factorsBends/stress in fiberImperfections in fiber

Solutions for PMDImproved fibers RegenerationFollow manufacturers recommended installation techniques for the fiber cable

PMD does not need compensation up to 10G in systems up to about 1600km optical transmission, while compensation is required for longer systems or 40G


Nonlinearity


From Linear to Non-Linear Propagation

As long as optical power within an optical fiber is small, the fiber can be treated as a linear medium

Loss and refractive index are independent of the signal power

When optical power levels gets fairly high, the fiber becomes a nonlinear medium

Loss and refractive index depend on the optical power


Effects of Nonlinearity Self-Phased Modulation (SPM) and Cross Phase Modulation (XPM)

Interference

Interference

Multiple Channels Interact as They Travel (XPM)

A Single Channels Pulses Are Self-Distorted as They Travel (SPM)


Out of Fiber1 221-2 22-11 2

Into Fiber

Four-Wave Mixing (FWM)

Channels beat against each other to form intermodulation products

Creates in-band crosstalk that cannot be filtered (optically or electrically)


Four-Wave Mixing (FWM)

If you have dispersion the beat signal will not fall on a real signal

Hence some dispersion is really good

Dispersion (some) helps to prevent FWM

Out of Fiber1 2 22-11 2

Into Fiber21-2


Channel Spacing (nm)

0.0 0.5 1.0 1.5 2.0 2.550

30

10

0

20

40

D=0

D=17

D=2

D=0.2

FWM and Dispersion

F

W

M

E

f

f

i

c

i

e

n

c

y

(

d

B

)

Dispersion Washes Out FWM Effects


Re-Shape DCU

The Three Rs of Optical Networking

Pulse as It Enters the Fiber Pulse as It Exits the Fiber

Re-Gen to Boost the Power

tts Optimum Sampling Time


Phase Variation

Re-TimeO-E-O

Re-gen, Re-Shape, andRemove Optical Noise


Phase Re-Alignment*

*Simplification

The Options to Recover the Signal from Attenuation/Dispersion/Jitter Degradation Are:


SM Optical Fiber Types


Types of Single-Mode Fiber

SMF (standard, 1310 nm optimized, G.652)Most widely deployed so far, introduced in 1986, cheapest

DSF (Dispersion Shifted, G.653)Intended for single channel operation at 1550 nm

NZDSF (Non-Zero Dispersion Shifted, G.655)LS

For WDM operation in the 1550 nm region onlyTrueWave, FreeLight, LEAF, TeraLight, etc.

Latest generation fibers developed in mid 90sFor better performance with high capacity DWDM systems

MetroCor, WideLight

Low PMD ultra long haul fibers

TrueWave Is a Trademark of Lucent; TeraLight Is a Trademark of Alcatel; FreeLight and WideLight Are Trademarks of Pirelli; MetroCor Is a Trademark of Corning


Fiber Dispersion Characteristics

Normal fiber Non-Dispersion Shifted Fiber (NDSF) G.652 > 90% of deployed plant

20

15

10

5

0

5

10

15

20

25

1350 1370 1390 1410 1430 1450 1470 1490 1510 1530 1550 1570 1590 1610 1630 1650

DS NZDS+NZDS- SMF

Wavelength (in nm)

D

i

s

p

e

r

s

i

o

n

(

i

n

p

s

/

n

m

-

k

m

)

DSF G.653NZDSF G.655


The Primary Difference Is in the Chromatic Dispersion Characteristics

Different Solutions for Different Fiber Types

SMF(G.652)

Good for TDM at 1310 nm

OK for TDM at 1550

OK for DWDM (with Dispersion Mgmt)

DSF(G.653)

OK for TDM at 1310 nm


Bad for DWDM (C-Band)

NZDSF(G.655)



Good for DWDM (C + L Bands)

Extended Band (G.652.C)

(Suppressed Attenuation in the Traditional Water Peak Region)



OK for DWDM (with Dispersion Mgmt

Good for CWDM (> Eight wavelengths)


SPAN Design


Span Design LimitsAttenuation

Source and receiver characteristicsTx: 0dBmRx sensitivity: 28dBm Dispersion tolerance: 1600ps/nmOSNR requirements: 21dB

Span characteristicsDistance: 120kmSpan loss: .25dB/km (30dB total)Dispersion: 18ps/nm*km

Tx

Rx

Time Do

main 0dBm

Wavelen

gth

Domain

20km

5dBm

25dBm

100km

30dBm

120km


Span Design LimitsAmplification



Tx

Rx

Time Do

main

+12dBm

20km

8dBm

100km

Wavelen

gth

Domain

13dBm

120km

EDFA

-6dBm0dBm 6dB

+17dBm EDFA characteristics

Gain: 23dB (max = 17dBmNoise figure: < 6dBMax input: 6dBm


Span Design LimitsDispersion



Tx

Rx

Time Do

main 0ps/nm

Wavelen

gth

Domain

20km

360ps/nm

100km

1800ps/nm

120km

2160ps/nm


Span Design LimitsDispersion Compensation



EDFA characteristicsGain: 23dB (Max +17dBm)Noise figure: < 6dBMax input: 6dBm

DCF characteristicsDispersion: 600ps/nmLoss: 10dB

Tx

Rx

Time Do

main

+12dBm 360ps/nm

20km

8dBm 1800ps/nm

100km

Wavelen

gth

Domain

EDFA

6dBm0dBm 6dB

+17dBm0ps/nm

DCF

(10dB)

13dBm 2160ps/nm

120km

23dBm 1560ps/nm


Span Design Limits of Amplification (OSNR)


Span characteristicsDistance: 60km x 4 SpansSpan loss: .25dB/km (15dB/span)Dispersion: 18ps/nm*km

EDFA characteristicsGain: 23dB (Max +17dBm)

Noise figure: < 6dB

Max input: 6dBm

Tx

Rx

Time Do

main

60km

Wavelen

gth

Domain

EDFA

0dBm 6dB

EDFA

EDFA

EDFA

EDFA

+17dBmOSNR 39dB

Noise

Noise

+17dBm OSNR 21dB

Noise

Noise

+17dBm OSNR 27dB

Noise

Noise

+17dBm OSNR 15dB

Noise

Noise

DCF

DCF

60km

60km

60km

Noise

Noise

+17dBm OSNR 33dB

DCF

DCF

Too Low


Real Network Design Challenges

Complicated multi-ring designs

Multiple wavelengths

Any to any demand

Nonlinearities

Advanced modulation

Simulation and Network Design Software Is Used

to Simplify Design


Rack diagrams

Step-by-step interconnect

Smooth transition from design to implementation

Any-to-any demand

Comprehensive analysis = first-time success

GUI-based network design entry

Bill of materials

Network Design Tools?Concept to Creation Easier


Summary


Summary

AttenuationCompensated by amplifiers

Dispersioncompensated by DCUs

OSNRLimits of amplification

Multi-channel considerations

Non-linear effects

Use design software for advanced designs


Q and A


Recommended Reading

Continue your Cisco Networkers learning experience with further reading from Cisco Press

Check the Recommended Reading flyer for suggested books

Available Onsite at the Cisco Company Store


Complete Your Online Session Evaluation

Win fabulous prizes; Give us your feedback

Receive ten Passport Points for each session evaluation you complete

Go to the Internet stations located throughout the Convention Center to complete your session evaluation

Drawings will be held in theWorld of Solutions

Tuesday, June 20 at 12:15 p.m.

Wednesday, June 21 at 12:15 p.m.

Thursday, June 22 at 12:15 p.m. and 2:00 p.m.


Optical FundamentalsAgendaModern Lightwave ErasOptical SpectrumTerminologyTerminologyFiber ImpairmentsITU Wavelength GridBit Error Rate (BER)Eye DiagramEye DiagramEye DiagramEye Mask TestingOptical PowerOptical Power BudgetOptical Power BudgetExampleA Few Words on Optical SafetyLaser Classifications/Safety IconsProtective Eyewear AvailableSome Final Words on Optical Equipment SafetyAnalog Transmission EffectsFiber GeometryFiber DimensionsGeometrical OpticsPropagation in Fiber Different Types of FiberAttenuation in FiberOther Causes of Attenuation in FiberOptical Attenuation Attenuation Response at Different WavelengthsAttenuation: Compensated by Optical AmplifiersThe Photoelectric Effect in ErbiumTypes of Dispersion A Snapshot on Chromatic DispersionLimitations from Chromatic Dispersion Combating Chromatic DispersionTransmission Over SM FiberWithout CompensationHow Far can I Go Without Dispersion?Dispersion CompensationPolarization Mode DispersionPolarization Mode Dispersion (PMD)Combating Polarization Mode DispersionFrom Linear to Non-Linear PropagationEffects of Nonlinearity Self-Phased Modulation (SPM) and Cross Phase Modulation (XPM) Four-Wave Mixing (FWM)Four-Wave Mixing (FWM) FWM and DispersionThe Three Rs of Optical NetworkingTypes of Single-Mode FiberFiber Dispersion CharacteristicsDifferent Solutions for Different Fiber TypesSpan Design LimitsAttenuationSpan Design LimitsAmplificationSpan Design LimitsDispersionSpan Design LimitsDispersion CompensationSpan Design Limits of Amplification (OSNR)Real Network Design ChallengesNetwork Design Tools?Concept to Creation EasierSummaryRecommended ReadingComplete Your Online Session Evaluation

OPT-1101(USA2006) - Optical Fundamentals

Documents

Transcript of OPT-1101(USA2006) - Optical Fundamentals