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Understanding DWDM and ROADM NetworksCrosstalk (XT)

Four Wave Mixing (FWM)

Crosstalk occurs in devices that �lter and separate wavelengths. A pro-portion of optical power intended for a speci�c channel is found in anadjacent or di�erent channel.

E�ects: generation of additionalnoise a�ecting optical signal tonoise ratios (OSNR), leading tobit errors.

Solutions: use appropriateoptical channel spacing, forexample 0.4 nm → 10 Gbps.

This interference phenomenon produces unwanted signals from threefrequencies (fxyz = fx + fy − fz) known as ghost channels. As three channelsautomatically induce a fourth, the term four wave mixing is used. FWMis problematic in systems using dispersion-shifted �bers (DSF). Wave-lengths traveling at the same speed at a constant phase over long periods increase the e�ect of FWM.

E�ects: power transfer to newsignal frequencies (harmonics),channel crosstalk, and bit errors.

Solutions: use of �berswith CD and irregular channelspacing.

Chromatic Dispersion (CD)

CD refers to the phenomenon when di�erent wavelengths of an optical pulse travel at di�erent velocities along a �ber and arrive at di�erenttimes in the receiver.

E�ects: decrease of peak power,pulse broadening, and bit errors.

Solutions: use of �bers ormodules with reverse CDvalues (DCF/DCM).

Positive chromatic dispersion X

Polarization Mode Dispersion (PMD)

PMD refers to the e�ect when di�erent polarization modes (fast axis and slow axis) of a signal statistically travel at di�erent velocities due to �berimperfections. The time di�erence is called Di�erential Group Delay (DGD).

E�ects: decrease of peak power,distortion of pulse shape, andbit errors.

Solutions: lay �ber carefully (no stress), use new �ber with low PMD values, exact �ber geometry.

DWDM signal

Crosstalk

λ1-λ3

λ1

λ2

λ3

DEM

UX

Original channels

Interference products

f113

f132

f223

f231

f312

f221

f223

f112

f321

f331

f231

f332

f

DGD

Slow

Fast

Optical Bands

40G Modulation Techniques

SSMF standard single-mode �ber AWF all wave �ber

O original band C conventional band E extended band L long band S short band U ultra long band

Att

enua

tion

(dB/

km)

AWF

SSMF

0.0

0.1

0.2

0.3

0.4

0.5

1200 1300 1400 1500 1600 1700

Wavelength (nm)

12601360

14601530

15651625

1675

O E S L UC

C-band 22 45 90 180 360

L-band 35 70 140 280 560

Maximum Number of Channels

Channel Spacing [GHz] 200 100 50 25 12.5

nm 1.6 0.8 0.4 0.2 0.1

Maximum Number of Channels (at 1550 nm)

GHz 200 100 50 25 12.5

δυ = {c/λ2} ∆λ

NRZ Eye pattern

Opt

ical

spe

ctru

m

High CD Low CD

High PMDLow PMD

CD

PMD

Com

plex

ity/c

ost

Dis

tanc

e

NRZ-DPSK RZ-DPSK

NRZ-DQPSK RZ-DQPSK

NRZ-DB

NRZ

NRZ

New modulation techniques are used in high- speed 40G networks to shift dispersion limitations.

NRZ formats are used to overcome large CD.

RZ formats are used to handle high PMD.

Phase modulation is used to increase transmission distances that a�ect the complexity and cost of the system.

Modulation techniques directly impact the optical spectrum and the eye pattern.

NRZ non-return-to-zeroRZ return-to-zeroDB duo-binary

DPSK di�erential phase shift keyingDQPSK di�erential quadrature phase shift keying

Understanding DWDM and ROADM Networks

Optical Transport Networks

Span Loss andDispersion Management of a Link

Tx

Power

DGD

OSNR

0

OAM OAM OAM RamanAmpli�er

RxManaging CD can reduce FWM crosstalk in long-distance high-speed networks. Optical ampli�ers with integrated dispersion compensators (OAM) are distributed along the link to recover the optical power and to overcome the positive dispersion of the �ber. Each ampli�er will reduce the OSNR due to the ASE noise.

OSNR = Optical signal power Optical noise power

Glossary

ASE ampli�ed spontaneous emission (noise) in an optical ampli�erCD chromatic dispersionCWDM coarse wavelength division multiplexingDCF dispersion compensation �berDCM dispersion compensation moduleDemux optical demultiplexerDFB distributed feedback laserDGD di�erential group delayDWDM dense wavelength division multiplexingEDFA erbium-doped �ber ampli�erFBG �ber Bragg gratingFWM four wave mixingMUX optical multiplexerOAM optical ampli�er module (incl. dispersion compensation)OSNR optical signal-to-noise ratioPLC planar lightwave circuitPMD polarization mode dispersionROADM recon�gurable optical add-drop multiplexerWB wavelength blockerWSS wavelength selective switchWXC wavelength cross-connectXT crosstalk

ROADM Types

Wavelength Blocker (WB) Small Switch Array (PLC) Wavelength Selective Switch (WSS) Wavelength Cross Connect (WXC)

Block Diagram

Ports

Network Function

Application

In Out WavelengthBlocker

Combiner Splitter

In Out

Drop Add

DEM

UX

MU

X

Add/Drop

In Out

DEM

UX

MU

X

λ1 λn

In/Add

Out/Drop

2 DWDM ports(1 In, 1 Out)

Dynamic channel equalizer+ wavelength blocking

Long-haul, ultra long-haulPoint to point→ 2 degree ROADM

Metro/EdgeLowest cost→ 2 degree ROADM

Metro/EdgeRing structure→ ≥2 degree ROADM

Not colorlessDynamic Thru and Add channel balancing

Colorless→ switches λs from In to Out/Drop and Add to Out

Colorless→ switches λs from In or Add to Out or Drop

Ring interconnectionMesh cross-connect→ ≥3 degree ROADM

2 DWDM ports + N single λ ports (1 In + 1 Out + N Add + N Drop)

N+1 DWDM ports (1 In + 1 Out + N-1 Add/Drop)

2N DWDM ports (N-1 In + N-1 Out + 1 Add + 1 Drop)

TxTxTx

Tx

λ1

λ2

λ3

λn

MU

X

EDFA

ROADM

WXC

Power spectrum of DFB-Tx

ROADM

ROADM ROADM

WSS

WSS

WSS

OAM ModuleROADM

FBG EDFA

E�ect of ch Add/Drop with ROADM→ needs in-band OSNR testing

Raman Ampli�er

DEM

UX

λ1

λ2

λ3

λn

RxRxRx

Rx

Pump

Drop Add

PLC

E�ect of CD comp. with FBG �lters

Power spectrum of 27 ch system

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Fiber Optic Testing

Volume 2