Wdm Basics Issue1

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Optical Network Curriculum

Development Section

Optical Network Curriculum

Development Section

ISSUEISSUE

OTC000601 WDM BasicsOTC000601 WDM Basics

1.01.0

Confidential Information of Huawei. No Spreading without Permission.

Security Level: Internal

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Why WDM technology ？Why WDM technology ？



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Optical Network Capacity ExpansionOptical Network Capacity Expansion



4

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ADM REG ADMREGREGREGADM REG ADMREGREGREG

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...... ......

Before the use of WDM

DWDMDWDM ADM

ADM

ADMADMADMADM

.........

ADM

ADMADMADM

ADMADM

... After the use of WDM

DWDM Reduces the Cost of NetworkingDWDM Reduces the Cost of NetworkingDWDM Reduces the Cost of NetworkingDWDM Reduces the Cost of Networking



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Course ObjectiveCourse Objective

Grasp the basic characteristics and types of

optical fiber;

know the light source and photodetector,

know the working principle and characterist

ics of various types of optical amplifiers;

Know the basic knowledge of passive optic

al components;

Master the basic knowledge's and key tech

nologies of WDM system.

Upon completing of this course, you will…



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Course ContentsCourse Contents

Chapter 1 Optical Fiber

Chapter 2 light Source and Photodetector

Chapter 3 Optical Amplifier

Chapter 4 Passive components

Chapter 5 WDM System



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Chapter 1 FiberChapter 1 Fiber

1.Single-Mode Fiber (SMF)

2.Properties of Fiber

3.Classifications of Single Mode Fiber

4.Nonlinear Effects of Single Mode Fiber



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Structure of Single-Mode Fiber and Refractive Index Profile

Structure of Single-Mode Fiber and Refractive Index Profile

Structure of a fiber

coating

n2 cladding

n2 cladding

coating

core n1 d1 d2

d

n2

n1

d

n2

n1

Refractive

index

profile



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Fiber AttenuationFiber Attenuation

Attenuation in optical fiber is mainly determined by three types

of loss: absorption loss, scattering loss and bend loss.

Attenuation coefficient （ unit:dB/km ）

λ(nm)15501310

OH ion Absorption

Atte

nuat

ion(

dB)

C_band L_band

red

orange

yellow

green

blue

cyan

purple

White light

DispersionDispersion



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Dispersion in Single Mode Fiber (SMF)Dispersion in Single Mode Fiber (SMF)

pulse

λ 1λ2λ3. . .

pulse

λ1λ2λ3. . .

Fiber coreFiber core

Fiber cladding

Fiber cladding

Dispersion in fiber refers to a physical phenomenon of signal

distortion caused when various modes carrying signal energy or

different frequencies of the signal have different group velocity

and disperse from each other during propagation.

Dispersion in SMF is classified into chromatic dispersion and

polarization mode dispersion (PMD)



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Chromatic DispersionChromatic Dispersion

Input laser is not momochromatic, it is composed of many wavelength or colour.

The different wavelengths arrive at different times to BROAD, separated, or DISPERSED output pulse.

The chromatic dispersion in the fiber causes different wavelengths to travel at different speeds, and propagation delay.

Inputlaser

Optical

receiver

L

DATA IN DATA OUT



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Chromatic DispersionChromatic DispersionD

ispe

rsio

n(p

s/nm

Km

)

G.653

λ(nm)15501310

G.652 fiber G.65517

Generally, two kinds of dispersion exist in single mode optical fiber,they

are material dispersion and waveguide dispersion.

Dispersion coefficient （ unit ： ps/km∙nm ）



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Polarization Mode Dispersion － PMD Polarization Mode Dispersion － PMD

Detectorpower

Signal response

Polarization mode dispersion （ PMD ）

Polarization mode dispersion coefficient ：

ps/km1/2

Fiber profile

Ellipse fiber core

Slow in propagation

Fast in propagation

Delay time



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λ3 λ1 λ3 λ1 λ3 λ3λ1 λ1

T

T+ΔT

Inter-symbol InterferenceInter-symbol Interference

Broaden pulse caused by dispersion will bring the adjacent

consecutive pulses to overlap.



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Cut-off WavelengthCut-off Wavelength

Cutoff wavelength is the shortest wavelength at which a fiber

can support single mode operation. If we work with light at a

wavelength shorter than cut-off wavelength, two, three or more

modes will propagate along the fiber.

1) Cutoff wavelength of primary coating fiber in jumper

cable shorter than 2m.

2) Cutoff wavelength of 22m cable optical fiber.

3) Cutoff wavelength of 2~20m jumper cable.



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Mode Field Diameter and Effective AreaMode Field Diameter and Effective Area

Mode field diameter （ MFD ） describes the concentrate l

evel of optical energy in the single mode fiber.

Fiber core

MFD



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Dis

pers

ion

(ps/

nm K

m)

G.653

λ(nm)15501310

G.652 fiber G.655

OH ion Absorption

17Att

enua

tion

(dB

)

C_band L_band

Types of Optical FiberTypes of Optical Fiber

G.654fiber



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Nonlinear Effects in Single Mode FiberNonlinear Effects in Single Mode Fiber

Stimulated Nonelasticity Scattering

Stimulated Brillouin Scattering(SBS)

Stimulated Raman Scattering(SRS)

Kerr effect

Self-phase Modulation(SPM)

Cross-phase Modulation(XPM)

Four-wave Mixing(FWM)



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Stimulated Nonelasticity ScatteringStimulated Nonelasticity Scattering


Limiting the injection power, threshold power is100mw 。

Stimulated Brillouin Scattering(SBS)

Limiting the optical power of single wavelength ， thre

shold power is smaller for spectrum line lasers



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Kerr EffectKerr Effect

Self-phase Modulation (SPM)

Broaden the signal's spectrum,

the influence of dispersion

becoming bigger.

Cross-phase Modulation

Limiting the input optical power

and the wavelength spacing.



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Four-Wave MixingFour-Wave Mixing

Four-wave mixing (FWM) occurs in the case that two or three

light waves with different wavelength interact and cause new

light waves at other wavelengths.

1 2

f113

F123,213

F112 F223

F132,312

F221

F231,F321

F332 F331

F F F3

Uneven and relatively large channel spacing can reduce FWM.



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QuestionsQuestions

What are the properties of optical fibers?

In the propagation of optical pulse signal,

what is the influence of the dispersion in the

optical fiber?

What are the types of optical fibers? What

features do they have?

What are the phenomena of the non-

linearity effect in single-mode optical fiber?



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SummarySummary

optical fiber consists of core, cladding and coating, the

refractive index of core must be bigger than that of the

cladding’s.

The properties of optical fiber are attenuation coefficient,

dispersion coefficient, cutoff wavelength, mode field diameter

and effective area.

Dispersion in single mode optical fiber consists of chromatic

dispersion and polarization mode dispersion. In the digital

communication, dispersion will result in inter-symbol

interference , and finally causes bit errors.

Non-linear effects consists of stimulated nonelasticiy scatter,

Kerr effect and FWM.



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Chapter 2 Optical Source and Photoelectric Detector






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Chapter 2 Light Sources and Photo DetectorsChapter 2 Light Sources and Photo Detectors

Light sources

Laser modulation modes

Types and characteristics

of photo detectors



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Types and Characteristics of Light SourceTypes and Characteristics of Light Source

LED;

FP-LD (MLM)

DFB-LD (SLM) 。

Low output power,

Poor beam focus,

Wide spectrum, low bit rate,

Inexpensive,

Suit for short distance communications

High output power

Good beam focus

Narrow spectrum, high bit rate

expensive

Suit for long distance



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Modulation TechniquesModulation Techniques

Direct modulation (internal modulation) ；Indirect modulation (external modulation) ：

Electro-Absorption modulation

Mach-Zehnder modulation



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Direct ModulationDirect Modulation

Direct modulation is: Output laser is controlled by input current

Advantages: simple structure,low loss and low cost

Disadvantages: modulation chirp transmission distance ≤100km

transmission rate ≤2.5Gbit/s

LDCurrent Laser



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What is chirp?What is chirp?

Chirp(as in bird chirping) is the deviation of laser frequency from its radiation-center frequency.

positive chirp negative chirp

no chirp

fore edge back edge



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Electro-Absorption ModulatorElectro-Absorption Modulator

E-A modulation modulates the laser indirectly and adding

an external modulator in its output path to modulate the

light intensity.

Support long haul transmission (2.5Gb/s >600km)

Less chirp

High reliability

Complex technology

LD EA



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Mach-Zehnder ModulatorMach-Zehnder Modulator

Advantages:chirp can be almost zero,

suit for long transmission distance

Disadvantages: Expensive

LD



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Comparision of ModulationsComparision of Modulations

Types Direct

Modulator

EA Modulator M-Z Modulator

Max.dispersion

toleration

(ps/nm)

1200~4000 7200~12800 >12800

Cost moderate expensive Very expensive

Wavelength

Stability

good better best

For STM-16 Signal



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Photo DetectorPhoto Detector

The function of photoelectric detector is to convert the rece

ived optical signal to corresponding electric signal.

Positive Intrinsic Negative （ PIN ） Avalanche Photo Diode （ APD ）

Dynamic ranges ： The difference of overload power and receiving sensitivity is c

alled dynamic ranges, generally about 20dB.

Types Spectrum

response

Overload

Power

Optical

Sensitivity

PIN 1100 ～ 1600n

m

0dBm -20dBm

APD 1000 ～ 1600n

m

-9dBm -28dBm



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QuestionsQuestions

How many different types of LD modulation?

In the high rate and technology single mode

communication, which type of laser can be selected?

How many types of photoelectric detector? What are

features of them?



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Chapter 2 Light Source and Photodetector






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Chapter 3 Optical AmplifierChapter 3 Optical Amplifier

Summary of Optical Amplifiers

EDFAEDFA

Raman Fiber Amplifier



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Optical AmplifierOptical Amplifier

The optical power is increased by optical amplifier.

Amplified optical signalInput optical signalOA



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Common Parameters of Optical AmplifierCommon Parameters of Optical Amplifier

Gain

Noise Figure

Gain bandwidth

Saturated output

power 0

10

20

λ

Gai

n(d

B)

30 3dB

λbλa

Pin

Gai

n(d

B)

3dB

PT

Pou

t (dB

m)

PS

Pout

Gain



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Types of Optical AmplifierTypes of Optical Amplifier

Erbium Doped Fiber Amplifier (EDFA)

Raman Fiber Amplifier (RFA)

Semiconductor Optical Amplifier (SOA)



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EDFA Energy Level DiagramEDFA Energy Level Diagram

The stimulated radiation and ASE of Er3+ ions in the EDF

ASE accumulation is resource of noise

PumpE2 metastable state

E3 excited state

1550nm

E1 ground state

1550nm

Decay

light

signal lightsignal light



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Advantages and Disadvantages of EDFAAdvantages and Disadvantages of EDFA

Major advantages of EDFA:

Its operating wavelength is consistent with the minimum loss

window of the SMF.

High coupling efficiency. Active Medium is in fiber

High energy conversion efficiency.

High gain, low noise figure, large output power and low cross-talk.

Stable gain characteristics.

Major disadvantages of EDFA:

The gain wavelength range is fixed

Gain bandwidth unflatness.

Optical surge problem.



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The Operating principle of Raman Fiber AmplifierThe Operating principle of Raman Fiber Amplifier


Pump

Gain

30nm

70~100nm



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Characteristics of Raman Fiber AmplifierCharacteristics of Raman Fiber Amplifier

Its gain wavelength is determined by the pump wavelength.

The gain medium is the transmission fiber itself.

Low noise. PUMP1 PUMP3

70~100nm30nm

GAINPUMP2

EDFA

Span 1

Raman Pump

transmittingReceiving

EDFA

Span k

Raman Pump



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Classifications of Raman Fiber AmplifierClassifications of Raman Fiber Amplifier

Discrete Raman Fiber Amplifier

Distributed Raman Fiber Amplifier



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RFAs and EDFAsRFAs and EDFAs



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Advantages of RFAAdvantages of RFA

Advantages:

Gain wavelength is determined by the pumping light

wavelength ;

Simple structure of amplifier;

Nonlinear effects can be reduced;

Low noise;

PUMP1 PUMP3

70~100nm30nm

GAINPUMP2



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Disadvantages of RFADisadvantages of RFA

Disadvantages:

High pump power,low efficiency and high cost;

Instantaneous gain, adopting backward pump fashion,

Optical components and optical fiber undertake high

optical power,

Characteristics of gain online are not consistent;



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Principle of Semiconductor Optical AmplifiersPrinciple of Semiconductor Optical Amplifiers

Coupling optics

Injecton （ pump ）

Active region

Transmission fiber

Transmission fiber

cladding

core

Amplified optical signal

Optical inputsignal



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Advantages and Drawbacks of SOAsAdvantages and Drawbacks of SOAs

Advantages:

Operating at the 1300nm and 1550nm wavelengths-even

simultaneously.

Wide bandwidth(up to 100nm has been achieved)

Easy to integrate, along with other semiconductor and

photonic devices, into one monolithic chip called opto-

electronic integrated circuit (OEIC)

Drawbacks:

Relatively high crosstalk,

polarization sensitivity

High temperature sensitivity;



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Comparison of Three Types of AmplifiersComparison of Three Types of Amplifiers

Type EDFA SOA Raman

Maturity Very maturity Not maturity maturity

Gain high normal normal

Bandwidth wider wide Very wide

Coupling

efficiency

high low high

Cost moderate high Very high



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QuestionsQuestions

Now, what are two types of main optical amplifiers ？ Wh

ich type of optical amplifier is in commercial?

Compared to regenerator, what are optical amplifiers' ad

vantages?

Summarize the operating principle and advantages of E

DFA.

What is the main noise of EDFA?

Summarize the operating theory and advantages of RFA.



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Chapter 4 Passive ComponentsChapter 4 Passive Components

1. Dielectric film filter type

2. Optical Grating type

3. Integrated Waveguide type: AWG

4. Comb Filter

5. Coupler and Splitter

6. Isolator and Circulator

7. Optical Switch



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Dielectric Thin Film FilterDielectric Thin Film Filter

The main features of dielectric film filter DWDM component are as

follows: miniaturization and structural stability of the component can

be implemented via design, the signal pass-band is flat and

polarization-independent, and its insertion loss is low and channel

isolation is good.

θ θ

λ/4

n1

n2



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Dielectric Thin Film FilterDielectric Thin Film Filter

¦ Ë1-4

¦ Ë

¦ Ë4

¦ Ë2

3

Self-focusing lens

¦ Ë1 filter

¦ Ë3 filter

Glass

¦ Ë1



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Fiber Grating FilterFiber Grating Filter

Periodic variation of the refractive index (grating)

Ultraviole light interference

¦ Ë1¦ Ë2¦ Ë3 ¦ Ë2



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Integrated Waveguide type :AWGIntegrated Waveguide type :AWG

λ 1 λ 2¡ ¡

Wavegui degrati ng

Free space

Fan- l i kewavegui de

Fan- l i kewavegui de

Small spacing

Large number of channels

Flat pass-band



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Coupler and SplitterCoupler and Splitter



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Multiplexer using CouplerMultiplexer using Coupler

multiplexer



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Comb FilterComb Filter

The basic indices are insertion loss, bandwidth and

crosstalk.



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Isolator and CirculatorIsolator and Circulator



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CirculatorCirculator

Fiber grating



64

Optical SwitchOptical Switch

A passive component possessing one or more ports which

selectively transmits, redirects, or blocks optical power in an

optical transmission line.



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QuestionsQuestions

How many classifications of common

passive components?

What are their functions?



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SummarySummary

All components for WDM systems can be divided into active

and passive. The list of passive components includes

couplers, isolators, and filters. passive components work

without power supply. Optical switches implement logical

action in optical domain.



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Chapter 5 WDM SystemChapter 5 WDM System

Related Standards on WDM

Transmission Modes of WDM System

Application Modes on WDM

WDM Key technologies

Management of Laser safety

Optical Supervisory Channel



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Related ITU-T Recommendations Related ITU-T Recommendations

G.652 Characteristics of a single-mode optical fiber cableG.655 Characteristics of a non-zero dispersion-shifted SMFG.661/G.662/G.663… Relevant recommendations of OAG.671 Characteristics of passive optical componentsG.957 Optical interfaces relating to SDH systemG.691 Optical interfaces for single channel STM-64,

STM-256 systems and other SDH systems with OAG.692 Optical interfaces for multi-channel systems with OAM.3100 Generic network information moduleG.709 Interface for Optical transport networkG.694-1 Spectral Grids for WDM Applications: DWDM Wavelength GridG.694-2 Spectral Grids for WDM Applications: CWDM Wavelength Grid



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Single fiber unidirectional transmission

Operating Modes of WDM SystemOperating Modes of WDM System

Uni-directional WDMUni-directional WDM



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Single fiber bi-directional transmission

Bi-directional WDMBi-directional WDM

Operating Modes of WDM SystemOperating Modes of WDM System



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Application modes-open systemApplication modes-open system

OTU OTUDMUX MUX

Optical amplifier

OSC

Acc

ess

chan

nels



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Application modes-integrated systemApplication modes-integrated system

DEMUX MUX

Optical amplifier

OSC

Acc

ess

chan

nels



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WDM Key Technologies - Light Source WDM Key Technologies - Light Source

The light source of the DWDM system has two outstanding features:

1) Larger dispersion tolerance;2) Standard and stable wavelength.

Technical measures:

DFB-LD+Temperature control

E-A Modulator

M-Z Modulator



75

Key Technologies of WDM System-Multiplexing and Demultiplexing

Key Technologies of WDM System-Multiplexing and Demultiplexing

• Large number of channels

• Low insertion loss, less polarization dependence,

• High isolation, low crosstalk



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WDM System Key TechnologiesWDM System Key Technologies

Type Advantages Disadvantages Product

Coupling typemultiplexer

Simple mechanism,high stability, low cost.

High insertion loss,only used for multiplexer

16 and 32

Wavelength system

Dielectric Film type

low insertion loss, High isolation.

Small number of channels

Above 16

Wavelength system

AWG Good flat spectral

response, suitable

for integration

Temperaturesensitive

Above 32

Wavelength system



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Gain Characteristics of EDFAGain Characteristics of EDFA

Gain Flatness --

utilizing heavily aluminum plus

erbium-doped optical fiber and

Gain Equalization Filter (GEF)

optimizing the optical structure

Wavelength

1525nm-1565nm non-aluminum-doped EDFA

Gain

1525nm-1565nmaluminum-doped EDFA

Gain

Wavelength



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Impact of Gain Flatness in Long Haul TransmissionImpact of Gain Flatness in Long Haul Transmission

Cascading amplification of amplifier gain unflatness

Cascading amplification of amplifier gain flatness



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Gain LockingGain Locking

>1dB

<0.5dB

Drop



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Gain LockingGain Locking

>1dB

Add



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1. Non-linearity problem

Although enhanced by adopting EDFA, the optical

power is not the higher the better. When it reaches a

certain level, the optical fiber will generate nonlinear

effects (including Raman scattering and Brillouin

scattering).

2. Optical surge problem

3. Dispersion problem

Problems after Applications of EDFAProblems after Applications of EDFA



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Laser SafetyLaser Safety

ALS (Automatic Laser Shutdown)

APR (Automatic Power Reduce)

Refer to ITU-T G.664

West Multiplex

East Multiplex

T2R1

OTS

OMS

LA LA LA LA

T1 R2

ALA LA LA LA



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Optical Supervisory ChannelOptical Supervisory Channel

Transmit related management , supervision information

Operating wavelength 1510nm

Monitoring rate:2Mb/s

add/extract OSC

Requirement: Not limit pump wavelength of OA, not limit

1310nm service, available when OA fails, long distance

transmission



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TS0: FASTS1: E1TS2: F1 TS3-TS14: D1-D12(DCC channel)TS15:E2 TS16-TS31: reserved

0 1 3114 15 162 3

Typical Frame Structure of OSCTypical Frame Structure of OSC



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QuestionsQuestions

What are the applications of EDFA?

What are the requirements of DWDM system for the performance

of EDFA? Why is optical monitoring channel necessary? What are the

wavelength, code rate and code pattern of optical monitoring

channel. What are the requirements of DWDM system for optical monitoring

channel?



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SummarySummary

WDM system can be divided into unidirectional WDM and

bi-directional WDM.

In practice, We can adopt open WDM system or integrate

d system.

WDM system key technologies include light source, multi

plexing/demultiplexing and optical amplification techolog

y.

OSC can transport the network management information.



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Wdm Basics Issue1