Introduction to WDM

7/22/2019 Introduction to WDM

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Introduction to WDM

Training manual

8AS 90200 0667 VH ZZA Ed.02

Edition 2003

All rights reserved. Passing on and copying of thisdocument, use and communication of its contents not

permitted without written authorization from Alcatel


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Alcatel University - 8AS 90200 0667 VH ZZA Ed.02 Page 0.2

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Note : Please print this document with comments pages


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1 - TDM versus WDM1.1The telecommunication market

1.2 TDM technique

1.3 WDM technique

2 - Optical fibre

2.1 Principle of light guiding

2.2 Penalties induced by optical fibre

2.3 Non-linear effects

2.4 Types of fibre

3 -Technical Solutions

3.1 DCUs

3.2 PMD compensation devices

3.3 FECs

3.4 TEQs and SEQs

3.5 Modulation formats

4 - Optical Components

4.1 Receivers

4.2 Lasers

4.3 Modulators

4.4 Optical filters, multiplexers and de-multiplexers

4.5 Wavelength adapters

4.6 Miscellaneous devices

4.7 Optical amplifiers

5 - WDM Optical Networks

5.1 Optical network elements

5.2 Optical network structure

5.3 Protections of optical networks

5.3 Supervision of WDM networks

5.5 WDM applications

5.6 Alcatel references in WDM

Contents


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Self assessment of the objectives

Instructional objectivesYes (orGlobally

yes)

No (orglobally

no)Comments

1. To be able to compare the advantages of TDM

and WDM techniques.

2. To be able to quote the limiting factors inWDM.

3. To be able to describe the technical solutionsused to compensate for optical fibreimpairments.

4. To be able to describe the function of opticalcomponents used inWDM systems.

5. To be able to identify the systems used inWDM networks and the architecture of thesenetworks.

Contract number :

Course title : Introduction to WDM

Client (Company, centre) :

Language : English dates from : to :

Number of trainees : Location :

Surname, First name :

Did you meet the following objectives ?

Tick the corresponding box

Please, return this sheet to the trainer at the end of the training


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Instructional objectivesYes (orGlobally

yes)

No (orglobally

no)Comments

Self assessment of the objectives (continued)

Thank you for your answers to this questionnaire

Other comments


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Alcatel University - 8AS 90200 0667 VT ZZA Ed.021.1

1 TDM versus WDM


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1.2

1 - TDM versus WDMSession presentation

Objective: to be able to compare the advantages of TDM andWDM techniques.

Program:

1.1The telecommunication market

1.2 TDM technique

1.3 WDM technique


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1 TDM versus WDM

1.1 The telecommunication market


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1.4

1 - TDM versus WDMThe Telecommunication Market

Based on demand by end customers for high bandwidth, optical fiber networks capacity is increasing exponentially. Synchronousoptical networks have paved the way for ultra high bit rates and ultra high bandwidth. Nevertheless, as the bandwidth keepsincreasing, a question arises : what is the best way to adapt the transmission medium to face the demand?


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1.5

1 - TDM versus WDMTraffic Increase Expectation

Example of

Trans-Atlantic

traffic

Need for easy upgrade capacity of transmission equipment

It is expected that the demand for capacity will keep increasing over the next few years at a pace at least equal to the current one.This means that from to-days capacities, that are close to 1Tbit/s per fiber, there will be a need for further increases to 10 Tbit/s andeven more per fiber.

Another important measure of capacity is the spectral density, that is, the number of bit/s per hertz. Typical commercial systems with1 Gbit/s per channel and 100 GHz spacing have a spectral density of 0.1 bit/s/Hz. The challenge for research is to improve that by anorder of magnitude.


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1.6

1 - TDM versus WDMSolutions for a Larger Throughput

Space Division Multiplexing (SDM)

increasing the fiber count

Time Division Multiplexing (TDM)

increasing the bit rate

Wavelength Division Multiplexing (WDM)

increasing the bandwidth efficiency

There are two approaches to increase the network throughput : install more fibers or increase the bandwidth of the existing fiber.

To increase the bandwidth of a fiber, there are two options :

- increase the bit rate. Currently, optical interfaces modulated at 10 Gbit/s are available and in the near future, 40Gbit/s bit rates will be used in optical networks. However, the electronic circuitry of such systems is neither trivialnor cheap.

- increase the number of wavelengths transported in the same fiber. Several wavelengths carrying data at 10 or 40

Gbit/s would increase the bandwidth by a factor equal to the number of wavelengths.


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1 TDM versus WDM

1.2 TDM technique


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1.8

1 - TDM versus WDMWhat is Optical Transmission ?

O P T I C A L T R A N S M I S S I O N

LaserFibre Cable

Optical Amplifier

Receiver

Electrical

Output

Electrical

Input


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1.9

1 - TDM versus WDMTDM Principle

STM-16 # 1

STM-16 # 3

STM-16 # 2

STM-16 # 4

2.5 Gbit/s digital streams

400 ps

Time

400/4 = 100 ps

10 Gbit/s digital stream

Electrical synchronous multiplexer4 to 1

Opticaltransmitter10-Gbit/s

a

a

a

a

b

b

b

bc

c

c

c

d

d

d

d

abc ad b ad c b ad cd c b a

The first optical systems designed by transmission equipment manufacturers were based on TDM principle. But an electricalmultiplexer is only able to combine signals having the same bit rate . Consequently, if these signals are plesiochronous (they comefrom systems working with different clocks) , they need to be synchronized before being multiplexed.


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1.10

1 - TDM versus WDMTDM : Plesiochronous Hierarchy

2048

E1564992

8448

E2

34368

E3

139264

E4

1544

DS 1274176

32064 397200

6312

DS 2

44736

DS 3

97728

EUROPE

USA

JAPAN

G742 G751 G751

G755

G752

G752

G753 G752

X4 X4 X4 X4

X4X3

X6X7X4


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1.11

1 - TDM versus WDMTDM : Synchronous Hierarchy

STS1

OC-1

STM-0EUROPE

USA

STM-1 STM-4 STM-16 STM-256STM-64

STS192

OC-192

STS3

OC-3

STS12

OC-12

STS48

OC-48

155.520 622.080 2448.320 9953.28051.840


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1 TDM versus WDM

1.3 WDM technique


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1.13

1 - TDM versus WDMWDM Principle

STM-16 # 1

1 2 3 4

2

3

4

Time

1

Time

Opticaltransponder

2,5-Gbit/s

Opticaltransponder2,5-Gbit/s

Opticaltransponder

2,5-Gbit/s

Optical

transponder2,5-Gbit/s

WDM

MUX

STM-16 # 2

STM-16 # 3

STM-16 # 4

TDMWDM

a1b1c1d1

a4b4c4d4

a3b3c3d3

a2b2c2d2

a1

a2

b1

b2

c1

c2

d1

d2

d3 a3b3c3

c4d4 b4 a4

There are two types of WDM systems :

- Opaque systems : they receive a photonic information. After de-multiplexing the incoming signals, they turn the photonicinformation into an electrical information. Each channel is individually processed (error monitoring, switching, routing,)and then converted back to an optical signal.

- Transparent systems : the optical signals received are never converted into electrical signals. Switching, de-multiplexing,multiplexing functions are purely optical.


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1.14

1 - TDM versus WDMWDM Systems Format Independence

SDH

IP

Leased line

ATM

...

W

DM

MUX


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1.16

1 - TDM versus WDMTDM and WDM Capacity

0.1

1

10

100

1000

1984 1988 1992 1996 2000 2004

140 Mbit/s

565 Mbit/s

2.5 Gbit/s

10 Gbit/s

40 Gbit/s

1994 1998 20021986 1990

2 x 2.5 Gbit/s

4 x 2.5 Gbit/s

8 x 2.5 Gbit/s

8 x 10 Gbit/s

64 x 2.5 Gbit/s

128 x 2.5 Gbit/s

40 x 10 Gbit/s

32 x 2.5 Gbit/s

16 x 10 Gbit/s

Anne

Capacittotaleparfibre(Gbit/s)

10 Gbit/s

100 Gbit/s

200 Gbit/s

WDM + TDM

N x 2.5 Gbit/sN x 10 Gbit/s

TDM(mono

canal)16 x 2.5 Gbit/s

Globalcapacityperfib

er(Gbits/s)

Year

SingleChannel


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1.17

1 - TDM versus WDMOptical bands in WDM

Wavelength (m)

Loss(dB/km

)

0.8 1.0 1.2 1.4 1.6 1.8

1

0.5

10

5

Absorption spectrum of the optical fiber

CS L

Based on optical power loss of fibers, spectrum ranges have been characterized for compatibility purposes with light sources,receivers and optical components, including the optical fiber.

Three optical windows have been used in optical transmission : the first window at 850 m, the second window at 1300 nm and thethird window at 1550 nm. According to the broad absorption minimum, third window is best suited for WDM technique

For WDM transmission systems, three optical bands are defined in the third optical window :

- the S band : 1460 to 1490 nm

- the C band : 1530 to 1565 nm- the L band : 1565 to 1595 nm


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1.18

1 - TDM versus WDMFrom 80 to 240 Channels

LS = Line Shelf

LS LS LS

1530 nm - 1565 nm(C band)

1530 nm - 1565 nm(C band)

Channel # 1

Channel # 80

Trib Shelfs

1565 nm - 1595 nm(L band)

1565 nm - 1595 nm(L band)

LS LS LS

Channel # 81

Channel # 160

1460 nm - 1490 nm(S band)

1460 nm - 1490 nm(S band)

LS LS LS

Channel # 161

Channel # 240

Trib Shelfs

= Combiner/Splitter


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1.19

1 - TDM versus WDMC Band and ITU-T Grid

RED BAND

(LONG BAND)

BLUE BAND

(SHORT BAND)

C Band

1528 15471542 1565

191.0191.5192.0192.5193.0193.5194.0194.5195.0195.5196.0

1530 1535 1540 1545 1550 1555 1560 1565

Frequency (THz)

WDM GRID at 100-GHz ITU-T G.692

Wavelength(nm)


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1.20

Time allowed :

10 minutes

1 - TDM versus WDMExercise

1- What is the parameter that has boosted the traffic increase in

telecommunication networks ?

2- What is the maximum bit rate of current TDM systems in PDH and

in SDH ?

3- Which optical window is used by WDM systems ?

4- What is the name of the optical band used to-day in WDMtransmission ? What are the lower and the upper wavelength

limits in this band ?

5- What is the channel spacing of current WDM systems ?

6- What is the pitch of a an optical grid of which two adjacentchannels have the following wavelengths : 1554.13 nm and1553.33 nm ?

7- Quote three advantages of WDM technology in comparison with

TDM technology.

1- The increase of internet traffic has contributed a lot to the increase of traffic in telecommunication networks.

2 - The maximum standardised bit rate of PDH TDM systems is 140 Mbit/s. In SDH, the maximum TDM bit rate is 10 Gbit/s (STM-64 OC-192)

3- The third optical window is used in WDM systems.

4- Current WDM system use the so-called C-band in the range 1530nm to 1565nm.

5- The spacing of WDM is expressed in GHz. At the beginning of WDM, the spacing was 200 GHz. To-day the spacing is 50 GHzfor DWDM systems and a spacing of 25 GHz is envisaged for future systems

6- = c/f => f =c/

f1 = 3 x 108/1554.13 x 10-9 = 193.034 Thz

f2 = 3 x 108/1554.33 x 10-9 = 193.133 Thz

So the channel spacing is : 193133 - 193034 = 99 GHz. The pitch of the system is 100 GHz.

Advantages of WDM

independence to tributary format multiplexing of non-synchronised tributaries

reliability improved by the reduction of pieces of equipment

Remove this rectangle to discover the solution


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1.21

Thank you for answeringthe self-assessment

of the objectives sheet

1 - TDM versus WDMEvaluation

Objective: to be able to compare theadvantages of TDM and WDMtechniques.


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1.22

1 - TDM versus WDMNotes


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2- Optical Fibre


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2.2

2 - Optical fibreSession presentation

Objective: to be able to quote the limiting factors in WDM.

program:

2.1Principle of light guiding



2.4 Types of fibre


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2- Optical fibre

2.1 Principle of light guiding


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2.4

2 - Optical fibreBasic Spectrum of Electromagnetic Radiation

in m f in Hz

1 km = 103

1 m = 100

blue1 nm = 10-9

106 = 1 MHz

101

102

400 nm

1 m = 10-6

1 mm = 10-3

1 pm = 10-12

10-1

10-2

10-4

10-5

10-8

10-7

10-11

10-10

107

108

1010

1011

1013

1014

1017

1018

1020

1021

1019

1016

1015

109 = 1 GHz

1012 = 1 THz

C=

f

IR-Radiation

electric waves

x-Ray radiation

- radiation

visual range

600 nm

700 nm

800 nm

500 nm

green

yellow

red

WDM


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2.5

2 - Optical fibrePropagation of Light in an Optical fibre

n1

n1

n2

n2>n1

crit

Criticalcone

Light is transmitted through guided modes in an optical fibre. These modes are labeled TEmn or TMmn depending of the value of thetransverse electric field or the transverse magnetic field. In a multimode fibre, the diameter of the fibre in on the order of 50 m andseveral modes are propagated whereas in a monomode fibre, the diameter is around 9 m, and only one mode is propagated and inthat case.

The critical angle is the maximum angle of incidence of light at which the light stops being refracted an is totally reflected. This angledepends on the refractive index : sin crit=n1/n2.

The refractive index is given by the following equation : n = c/v where c is the speed of light in vacuum and v the speed of light in the

medium.

The cutoff wavelength of a monomode fibre is defined as the shortest wavelength that can travel through the fibre in single mode.Below this wavelength, the fibre behaves like a multimode fibre. ITU-T G650 gives a definition of this cutoff wavelength.


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2- Optical fibre



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2.7

2 - Optical fibreOptical Attenuation

Absorption

Scattering

Impurities

Rays of light

Wavelength (m)

Loss(dB/km)

0.8 1.0 1.2 1.4 1.6 1.8

1

0.5

10

5

Absorption spectrum of the optical fibre

During the manufacturing process, all impurities cannot be removed from the material. These unwanted elements have either anabsorptive effect on the light or a scattering effect, thus reducing the optical throughput of the fibre.

One of the most difficult impurity to remove is OH radical causing a strong attenuation in the 1400 nm range


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2.8

2 - Optical fibreModal Dispersion

n1

n1

n2

crit

Tmin

Tmax

Input pulse Output pulse

t t

Different optical

modes in the fibre

result in a variationof the time, the

light remains in the

fibre.

t3 2 1

Power

The rays transmitted in the fibre are not parallel since they are transmitted through a small cone. Consequently, the length of the pathfollowed by these rays are not equal. Thus the initial pulse is widened. This phenomenon is called Modal Dispersion.

This parameter is negligible with a monomode fibre since, in theory,only one mode can be propagated in the core.

In the former optical transmission systems, a way to reduce this penalty consisted in manufacturing a fibre with a graded-index fibre.In this case the refraction index of the fibre varies from the center to the periphery according to a pre-defined profile.


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2.9

2 - Optical fibreChromatic Dispersion (GVD)

Positivechromatic

dispersion

Time

The propagation speed of light in a medium with a given refractive index depends on its wavelength. Since the pulse of light launchedinto the fibre is not purely monochromatic, the travel time of each wavelength is different even if these wavelengths are guided in thesame straight path. As a result, the different wavelengths dont arrive at the same time. This is known as chromatic dispersion d orGVD (Group Velocity Dispersion). It is an intra-modal dispersion.

d is expressed in ps/nm.km

To-day, 80% of optical fibres installed in the world are monomode fibres complying with ITU-T recommendation G652. Due to theavailability of optical components in the second optical window (1310 nm), this fibre was designed so as to have a minimal chromatic

dispersion at this wavelength.

So, for very high bit rate and very long optical links, the terminal equipment has to face a very high chromatic dispersion ( +18ps/nm.km for a G652 fibre). The consequence is the widening of the optical pulses transmitted through the fibre, creating aninterference inter-symbols which makes more difficult the recognition of a succession of binary symbols.


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2.10

2 - Optical fibreChromatic Dispersion versus Wavelength

-10

-5

0

5

10

15

20

25

1200 1300 1400 1500 1600

Wavelength (nm)

Chromaticdispersion(ps/nm

.km)

G.652

(0.08

ps/nm

2.km)

G.653 E

DFA

bandwidth

G.655

G

.655


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2.11

2 - Optical fibrePolarization Mode Dispersion (PMD)

Rx

DGD

DGD: DIFFERENTIAL GROUP DELAY

PMD: POLARIZATION MODE DISPERSION

Isotropic optically transparent materials are those that have the same index of refraction, the same polarization and the samepropagation constant in every direction throughout the material. Materials that do not exhibit these properties are known asanisotropic.

Anisotropic materials have a different index of refraction in specific directions. Consequently when a beam of monochromaticunpolarized light travels through it in a given direction, it is refracted differently along the directions of different indices. So when anunpolarized beam enters the material, it is separated into two rays, each with different polarization and different propagation constant.This property of anisotropic crystals is known as birefringence.

PMD is the time averaged DGD at lambda (signal). This phenomenon was brought to the fore in long submarine links and is due tothe imperfection of the fibre core. This core is not strictly circular ( manufacturing imperfection, mechanical and thermal constraints onthe optical cable,). The consequence is a different propagation delay between the two propagation modes of light.

The result is once more a widening of the transmitted optical pulse.

The problem is complicated by the fact that the DGD is not constant. It varies depending on external constraints.

PMD is expressed in ps/ (km)1/2 and this parameter has been limited in 1995 by ITU-T to 0;5 ps/ (km)1/2 for new fibres.


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2.12

2 - Optical fibreImpact of GVD & PMD

Bit rate (Gbit/s)

Non-regeneratedspans

(km)

10

100

1000

10000

0.1 1 10 100

D=17ps/nm

.km

PMD=2

ps/km0.5

PMD=0.5

ps/km0.5

D=2ps/nm

.km

2.5

X 4

16

As shown in the diagram above, the throughput of a WDM system is divided by 16 when the bit rate is multiplied by 4.Consequently, optical fibres with a high PMD are not suited for transmission at 10 Gbit/s, since the length of non-regeneratedsections is too short.


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2- Optical fibre



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2.14

2 - Optical fibreNon-Linear Effects

Non linear dependency of refractive index on launched optical power

n=n0 + n2 x P(t)/Aeff

with n2 = 2.7 x 10-20 m/W

Aeff = effective area (80m for G652 and 50 m for G653)

MI : Modulation Instability

Modulation of refractive index by light intensity fluctuation

SPM : Self Phase Modulation

XPM : Cross-Phase Modulation

FWM : Four Wave Mixing

Scattering effects

SRS : Stimulated Raman Scattering

SBS : Stimulated Brillouin Scattering


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2.15

2 - Optical fibreModulation Instability

Pin = 8 dBm Pin = 16 dBm (BER floor)

0(10 dB /div, resolution 0.1 nm)

Transmission over2x100 km DSF

(GVD=0.1ps/nm.km)

When a single pulse of a monochromatic source has a wavelength above the zero dispersion wavelength of the fibre, anotherphenomenon occurs that degrades the pulse shape : two side lobes are symmetrically generated at either side of the originalpulse. This phenomenon is known as modulation instability and affects the SNR.

Modulation instability affects the signal to noise ratio as shown in the slide above and can be reduced by operating at low energylevels.


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2.16

2 - Optical fibreSelf Phase Modulation : SPM

0 50 100 150 200

Time (ps)

Power

Modulated optical power P(t)

Variable delaythrough the

nonlineardependency ofoptical length

Wavelength

Blue

Red

Time (ps)

0 50 100 150 200

positive impact upon pulse shape over G.652 fibres

Pulse shape

Falling

edge

Rising

edge

Optical signal phase modulatedproportionally to signal power


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2.17

2 - Optical fibreSPM Effects

2 dBm

17 dBm

18 dBm

20 dBm

Pin:

9 kmDCF

80 km

SMF

80 km

SMF

9 kmDCF

Tx RxPin Pin


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2.18

2 - Optical fibreCross-Phase Modulation : XPM

Time (ps)

Opticalpower

( ) ( )n n n

P t

A

P t

Ai

eff

j

effj i

= + +

0 2 2

Refraction index seen by channel # i :

XPM impact depends on

the per channel optical power the GVD the channel spacing the polarization states

(SPM) (XPM)

i

j

k

l

0 500 1000 1500 2000

400-ps pulses

XPM events(i.e. refractive index change

at the power transients ofthe copropagating channels)

Limited impact of XPM on highly dispersive fibre (G.652)


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2.19

2 - Optical fibreFour Wave Mixing

FWM waves created via 3rd-order intermodulation

process: ffwm = fk + fl - fm

FWM crosstalk is increased by:

large power per channel

low chromatic dispersion small channel spacing

Opticalfrequency

Opticalfrequency

In-bandcrosstalk

Out-of-band

crosstalk

Considering three wavelengths, fk, fl and fm, from the interaction between them, a fourth wavelength is created such that :

fwm = fk + fl - fmThe effect of four wave mixing on an optical link is an OSNR degradation and cross-talk. The FWM increases as the input power ofthe channels increase and decreases as the channel spacing increases. Consequently, the effect of FWM is larger at the near endthan at the far end.


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2.20

2 - Optical fibreImpact of FWM

High intensity of FWM inter-modulation productsover low local dispersion fibre

50 GHz spacing

Worst case configuration only

NZDSF

(GVD=3ps/nm.km)

SMF

(GVD=17ps/nm.km)

f1 + f2 = f3 + f4With only two channels :

f4 = 2f2 - f1or f4 = 2f1 - f2

2f1-f2 2f2-f1

f1 f2

Four wave mixing may also occur with two signals at different wavelength. In that case, the fiber refractive index is modulated at thebeat frequency of the two wavelengths.The phase modulation in that case create two sidebands, the intensity of these undesiredsignals being weaker than the mixing products of three signals.


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2.21

2 - Optical fibreStimulated Raman Scattering

Short wavelengthsource

Stimulatedemission

Residueemission

Excited atoms : high energy level

Low energy level atoms

SRS is described as the scattering of one incident photon by a molecule to a lower-frequency (i.e. higher-wavelength) photon (calledStokes wave) while at the same time the molecule makes a transition between vibrational states.

The incident photon acts as a pump to generate the down frequency-shifted photon.

Consider two light sources with two different wavelength propagating in the same optical fibre. The short wavelength source excitesatoms at a high energy level. These excited atoms can be triggered by other photons and drop to an intermediate energy levelreleasing optical energy at a longer wavelength.

The photon frequency that is emitted is determined by the following equation : E = hvSo, v = (Ehigh - Elow )/h

In WDM systems, SRS is undesirable since it may result in amplification of adjacent channels.


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2.22

2 - Optical fibreEffect of SRS

-10 dBm /channel(linear regime)

5.6 dBm /ch.

100 km#1

#32

Tx Rx10 Gbit/s

perchannel

0.7 dB

2.3 dB

In future dense WDM/large bandwidth

configurations, SRS is to be one of themain limiting effects

High energy channels (lower wavelengths) pump the low energy channels (higher wavelengths) in high optical power conditions.Consequently the multiplex will be unbalanced and noticeable SNR differences between channels will be observed at the end ofthe link. The longer the link, the larger the SNR gap between channels. The value of this gap depends on the number of channels(the occupied bandwidth) and the power per channel. It is possible to reduce this impairment by channel pre-emphasis tocompensate for the SRS effect. However, this solution is not satisfactory for very long links.


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2.23

2 - Optical fibreStimulated Brillouin Scattering

Electrostriction

French physicist Lon Brillouin studied first, around 1920, the diffusion of light by acoustic waves. One of the distinctive features heobserved was a frequency change of the scattered light. Its first "contribution" was indeed a negative one : it was demonstrated thatthe Brillouin effect is the most drastic limitation encountered when the light power within the fibre is increased. There is a powerthreshold above which any additional light is back-scattered due to its interaction with acoustic waves.

In a dielectric material such as the silica of an optical fibre, material density increases in the region of high electric field. Thus, when ahigh power optical signal travels through silica, the index of refraction of the fibre increases. This phenomenon called electrostrictionis one of the components of optical Kerr effect.

Let us consider an electrostrictive material where acoustic noise is due to the Bownian motion of its molecules (thermal noise). Part ofthe light traveling through this media, called here pump light, is backscattered by this acoustic noise : it's the spontaneous Brillouinscattering. This backscattered light, called Stokes light, propagates in the opposite direction and interferes with the pump light. Thus,due to the electrostriction, an acoustic wave is generated and stimulates the Brillouin scattering even more, which reinforces theacoustic wave, and so on. This loop process described here after is called stimulated Brillouin scattering (SBS).

The energy of the acoustic wave is negligible regarding the optical waves. This SBS process can be summarized as an energytransfer from the pump wave to the Stokes wave. Stimulated Brillouin Scattering is thus noting else than a optical gain experienced bythe stokes wave traveling through the electrostrictive material in presence of the pump wave.

The stimulated light is at a shorter wavelength. The part that is in the same direction as the original signal is scattered as acousticphonons, and the part that is in the opposite direction is guided by the fibre.

SBS limits the launched power per channel, thus reducing the span length. However, SBS is used in optical amplification : thebackward signal is used as a pump for the signal to be amplified.


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2- Optical fibre

2.4 Types of fibre


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2.25

2 - Optical fibreTypes of Optical fibre

SSMF :Standard Single Mode Fibre

G 652

PSCF : Pure Silica Core Fibre

G 654

NZDSF : Non Zero Dispersion Shifted Fibre

G 653

G 655

Tera Light

DCF : Dispersion Compensation Fibre

Standard single mode fibre has a step index profile.It is characterized by a large chromatic dispersion and a moderate insertion loss.The most common fibre, which complies with ITU-T recommendation G 652 has a typical chromatic dispersion of +17ps/nm.km at1550 nm and a loss around 0.2 dB/km.

The PSCF (Pure Silica Core fibre) which complies with ITU-T recommendation G 654 has a low loss (typically 0.18 ps/nm) and aslightly larger chromatic dispersion (20ps/nm.km). This fibre is well adapted to WDM transmission because its chromatic dispersionlimits the interaction between channels. Nevertheless, there is a drawback : high bit rate transmission on such a fibre requires a largeamount of DCF, which is somewhat expensive.

A new family of fibre has been developed with a complex index profile to achieve a small (but not null) and precise value for thechromatic dispersion. This fibre is known as non zero dispersion shifted fibre. It complies with ITU-T recommendation G 655. Notethat old G 653 DSF often causes problems when upgrading to WDM because the exact chromatic dispersion of this type of fibre wasuncontrolled and very small.

NZDSF usually has a loss similar to SSMF while the chromatic dispersion is usually in the range 4 to 8 ps/nm.km. It has an effectivearea in the range 50 to 72 m (similar to that of SSMF : 80 m)

Alcatel has developed an NZDSF called Teralight which is the result of optimizing these aspects. This fibre has a small chromaticdispersion of around 9 ps/nm.km and an effective area of 65 m.

The purpose of DCF is to compensate for chromatic dispersion of the line fibre. Typical chromatic dispersion is lower than -80ps/nm.km while the typical attenuation is less than 0.5 dB/km.


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2.26

Time allowed :

15 minutes

2 - Optical fibreExercise

1- Which kind of optical fibre is the best suited for WDM systems.

2- The distance between two optical transmission systems is 50

Km. The optical budget of the link is 25 dB. What is the

optical window used by the optical interfaces on both sides ofthe link ?

3- The length of an optical link is 5000 km. The optical interfaces

on both sides can accept 6400 ps/nm of chromaticdispersion. Is G652 optical fibre convenient for that link ?

4- What are the components of Kerr effect ?

5- Which parameters increase FWM?

1- The dispersion shifted fibre G653

2- The attenuation of the fibre is approximately 0.5 dB/Km (25/50). So the system uses the second window at 1300 nm.

3- The chromatic dispersion would be : 5000 x 17 = 85000ps/nm. It is far larger than 6400 ps/nm. So this fibre doesn't match theneeds.

4- The SBS and SBS

5- The increase of power per channel and the reduction of the distance between channels.



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2.27



2 - Optical fibreEvaluation

Objective: to be able to quote thelimiting factors in WDM.


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2.28

2 - Optical fibreNotes


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3 - Technical Solutions


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3.1 DCUs


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3.4

3 -Technical SolutionsDispersion Compensation Unit

SMF/DCF management, & simultaneous dispersion/slope compensation

DSF/DCF management & Dmux/Mux for individual channel adjustment

DSF

SMF

DCF

DCF

DSF : Dispersion Shifted Fibre

SMF : Single Mode Fibre

In terrestrial links, a few sets of DCF are defined. For instance : 20, 40, 60, 80 km. These sets are installed either in a terminal rack orin repeater rack (in this case, the DCF span is inserted in between the two stages of the optical amplifier).

Given that the chromatic dispersion of the fiber varies linearly with wavelength, the cumulated chromatic dispersion cannot be

simultaneously canceled for all channels.This linear variation called chromatic dispersion slope, is such that, if the cumulativedispersion is exactly compensated periodically for the channel at the center of the transmitted optical band, the dispersion for thechannels on both sides of the band will depend on the length of the link (typically +/- 6000 ps/nm for a 6000 km and 68 X 10 Gbits/slink).

To overcome this problem a new type of fiber called RDF (Reverse Dispersion Fiber) is under study. The main characteristic of thisfiber is that its chromatic dispersion and its chromatic dispersion slope is exactly the reverse of that of the normal fiber. The idea is tocombine RDF with PSCF in each section to cancel the cumulative chromatic dispersion at the end of each section for all wavelengths.In addition, this configuration is well suited since the chromatic dispersion is never canceled in the 1.5 m window, thus reducing theFWM contribution.


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3.2 PMD compensation devices


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3.6

3 -Technical SolutionsPMD Compensation Devices

polarisation

controller

OUT

(optical)

controlalgorithm

Counter-PMD

generator

feedbacksignal

1

2

3

Rx

4

IN

(optical)

A PMD compensator is necessary in practical situations where the fiber PMD is not low enough (old fibers and/or long links).Various architectures for optical and electronic compensation devices have been studied and several have been used in fieldtrials on existing 10 Gbit/s links. Recently, PMD compensation was also demonstrated at 40 Gbits/s.


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3.3 FECs


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3.8

3 -Technical SolutionsFEC Implementation

Transmitted bit sequence is encoded with specific algorithm

Reed-Solomon type (good performance/redundancy ratio)

7 % redundancy (255 bytes/239 bytes)

Received bit sequence is decoded with reverse algorithm

error correction is performed (up to 1024 consecutive errors) line error rate (before correction) is monitored in-service

RWA

1

SDHterminal

123

N SDHterminal

TWA

FECinsertion

FECextraction

FEC is a feature provided by 2.5 Gb/s and 10 Gb/s transponders . A FEC is a performance booster. It enables per-channel digitalmonitoring of DWDM performance. FEC uses advanced submarine digital signal processing technology compliant with submarinestandards (G.975)

Alcatel has introduced out-of-band FEC for superior performance over in-band FEC.

An in-band FEC uses the spare byte of an SDH frame to insert the extra traffic generated. Thus the bit rate is not modified. Whereasan out-of-band FEC adds extra traffic that depends on the type of FEC used.

FEC as performance improvement booster :- 8-9 dB improvement in OSNR tolerance

- 30 - 50 % increased distance between line amplifiers

- Instant upgrade to 10 Gb/s of 2.5 Gb/s systems designed without FEC

- Instant upgrade to 32 of 8 systems designed without FEC

- x 2 - x 5 distance between O/E regenerators

- x 2 tolerance to PMD

- Provides additional margin for links with limitations due to non-linear effects (e.g. FWM in DSF)

FEC as performance monitoring improvement :

- Quality monitoring of client channel at the WDM input interface by means of B1 non intrusive monitoring in SDH frame

Low_BER alarm

High_BER alarm

- Quality monitoring of optical channel at the WDM output interface by means of FEC:Corrected errors alarm

Uncorrected errors alarm

- Guarantees true monitoring of DWDM-related impairments


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3.9

3 -Technical SolutionsFEC Principle

2.5 Gb/s

signal

2.52.5 GbGb/s/s

signalsignal

FEC Parity Bits AddedFECFEC ParityParity Bits AddedBits Added

1 0 0 0 1 1 1 0 1 1 0 0 0 1 1 0 0

1 0 0 0 1 0 1 0 1 1 0 0 0 1 1 0 1

1 0 1 0 1 1 0 1 0 1 0 1 0 1 0 0 0

0 0 0 1 0 0 1 0 1 0 0 1 0 0 1 0 1

1 0 0 0 1 0 1 0 1 1 0 0 0 1 1 0 1

1 1 0 0 1 0 1 0 1 1 0 0 1 0 1 0 0

0 1 0 0 1 1 0 0 1 0 0 1 0 1 0 1 1

1 0 0 0 1 1 0 1 1 1 0 0 0 1 1 0 01 0 1 0 0 1 1 0 1 1 0 1 0 1 1 0 1

1 0 0 1 1 1 0 0 0 1 0 0 1 1 1 1

SERVICEbits

DATA

R

EDUNDANCY

24

64

239 255

FAW

1

In parallel with modulation techniques, there is a highly effective tool that can be used to considerably increase the transmissioncapacity : Forward Error Correcting code. The effectiveness of a correcting code is expressed in terms of coding gain, reflecting thedifference between the error ratio before and after correction in the receiver. The coding gain with a simple Reed Solomon code isaround 6 dB. However, for long 10 Gbit/s WDM links, it is necessary to introduce a more powerful code formed by concatenating twoReed Solomon codes giving a coding gain equivalent to 8 dB.

New types of FEC are currently being researched such as codes based on soft decision which sample the impulse on several levels;the code handles information which is no longer binary but multi-level, enabling a coding gain of 10 dB to be achieved.


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3.10

3 -Technical SolutionsWith or Without FEC ?

Received power (dBm)-49 -48 -47 -46 -45 -44 -43 -42 -41 -40 -39

BitErrorRa

tio

Without FEC

With FEC

-1210

-1010

-810

-610

-410

-210

5.7 dB5.7 dB


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3.4 TEQs and SEQs


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3.12

3 -Technical SolutionsShape EQualizer

-4

-3

-2

-1

0

1530 1535 1540 1545 1550 1555 1560 1565

Wavelength (nm)

Transmission(dB)

EDFA Filter band (12 mn)EDFA Filter ba nd (32 mn.)Equalizer for 15 EDFA's

27 nm

2

nm

Without gainflattening filter

With gain flattening filters

68 wavelengths with 0.4 nm spacing

-0,5

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

1525 1530 1535 1540 1545 1550 1555 1560 1565 1570

Wavelength (nm)

Gainprofile(db) 0,22 dB

36 nm

SEQ

(PASSIVE)

The natural optical bandwidth of an EDFA is around 25 nm and the spectral response approaching peak gain is approximatelygaussian. Consequently, the bandwidth of a link with, for example, 100 cascaded EDFAs is no more then 2.5 nm.

It is therefore necessary to introduce optical filters to increase the optical bandwidth. Alcatel has developed a technology which canbe used to etch an optical grating in few millimeters long in optical fiber. The resulting optical filter called Fiber Bragg Grating (FBG)behaves like an optical rejecter at a given wavelength. By optimizing the attenuation profile of this FBG, a spectral response that isthe inverse of the EDFA can be obtained creating a wide band EDFA. This technique has been used to achieve a 27 nm bandwidth.However, the gain response of an EDFA over a 27 nm band is uneven, a complex profile filter has to be created requiring several

cascaded FBGs. The ability to construct such filters is the key of success in deploying long and high capacity WDM links.


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3.13

3 -Technical SolutionsTilt EQualizer

17 dB

12 nm

Without cable ageing

With 0.8 dB loss increase

per section

TEQ

Active orPassive

In submarine networks, once the link is submerged, the gain profile gradually becomes distorted as the link cable ages, resulting in anadditional loss per section causing the peak gain of EDFAs to shift towards the low wavelengths. As the gain tilt is a linear function ofthe wavelength, these gain distortions can be compensated by inserting a few linear profile variable optical TEQs, which can beremotely controlled from the terminal.


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3.5 Modulation formats


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3.15

3 -Technical SolutionsModulation Formats

Formats

Non Return-to-Zero (NRZ)

Return-to-Zero (RZ), Solitons, Carrier-Suppressed RZ

Duobinary, Phase Shaped Binary Transmission (PSBT)

Criteria

Resistance to propagation effects (Att., GVD, PMD)

Resistance to amplification noise

Low spectral width

Compatibility with all optical regeneration

Cost


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3.16

3 -Technical SolutionsPSBT

0

0,25

0,5

0,75

1

150 350 550 750 950 1150 1350 1550

Time (ps)

1

0.75

0.5

0.25

0

1 0 1 10 0 0 0 0 01 1 1 1 1 1

0

30

60

90

120

150

180

150 350 550 750 950 1150 1350 1550

Time (ps)

1 0 1 10 0 0 0 0 01 1 1 1 1 1

Rebonds

I(t) (t)

phase shift in themiddle of 0

T=100ps

phase shift+ energy in 0 improved transmission !

(PSBT, Phase-Shaped Binary Transmissions)


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3.17

3 -Technical SolutionsSolitons

soliton power

low power

In 1850, John Scott-Russel , while riding along a channel, noticed that after a boat stopped suddenly, he created a wave thatpropagated downstream without being noticeably bent out of shape. But he didnt noticed that after colliding with other waves ofdifferent amplitudes those waves recover their original shape. They keep trace of this collision by only experiencing a phase shift.These very stable waves are called solitons.

A soliton is a wave that propagates without complying with the energy scattering laws. Generally speaking, this wave is intenseenough to excite a non-linear effect that compensates for the normal effect of energy dispersion.

The pulse preserves its shape in the optical fibre under specific conditions : very short pulses and specific power spectrum. Due to

the required pulse narrowness, RZ modulation is suitable.


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3.18

Time allowed :

10 minutes

3 -Technical SolutionsExercise

1- Where are generally located DCF spans in an optical link ?

2- What is the difference between an in-band FEC and an out-of-

band FEC ? Which one is more efficient ?

3- What is the bit rate at the output of an STM-16 transponderimplementing an in-band FEC with a Reed-Solomon code (7

% redundancy) ?

4- In which kinds of networks TEQs and SEQs must be used ?

5- What are the different modulation formats used in currentoptical networks ?

1- DCFs are generally located at both ends of a WDM link, in the terminal equipment.

2- An in-band FEC uses the free overhead bytes of an SDH frame whereas an out of band FEC uses extras bytes. Theredundancy factor is better in the latter case,hence improving the efficiency of the FEC.

3- Approximately 2.66 Gbit/s

4- Equalisers are only used in submarine networks where the length of the links requires such devices due to the accumulation ofrepeaters.

5- RZ, NRZ

Supprimer ce rectangle pour voir la solution


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3.19



3 -Technical SolutionsEvaluation

Objective: to be able to describe thetechnical solutions used to compensatefor optical fiber impairments


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3.20

3 -Technical SolutionsNotes


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4.2

4 - Optical ComponentsSession presentation

Objective: to be able to describe the function of opticalcomponents used in WDM systems.

Program:

4.1 Receivers

4.2 Lasers

4.3 Modulators






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4.1 Receivers


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4.4

4 - Optical ComponentsDetection Devices

PIN photodiodes

APD photodiodes

E=hc/

Photon

E


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4.2 Lasers


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4.6

4 - Optical ComponentsLasers

Fabry-Perot Lasers

Monolithic BRAGG Lasers (DBR)

DFB lasers

Semiconductor Quantum Well Lasers VCSEL Lasers

Optical components are key enablers of the communication revolution which started with the Internet at the end of the 20th century.Associated with WDM, they are the answer to the capacity dilemma for transport switching and routing.

At the transmit side, multicolor lasers now offer precise and stable wavelengths : typically +/- 0.1 nm over 15 years and within thetemperature variations that are standard for telecommunications (from -5 to +70C). These multicolor l asers require Multi-QuantumWell vertical structures for wavelength accuracy and adapted horizontal structures such as Buried Ridge Stripe (BRS) for highperformance. The deposition process is based on crystal growth techniques such as Molecular Beam Epitaxy which allow atom byatom deposition so that the nanometer layers are accurately controlled. Currently it is possible to to manufacture laser emitters with a

spacing of 0.4 nm between two adjacent wavelengths. This corresponds to a frequency spacing of 50 GHz.As wavelength density increases, reaching 25 GHz spacing in the future, the inherently low variation of BRS laser wavelengths as aresult of aging will nevertheless be too large compared with the required stability. Thus optical loops are mandatory to lock the laserto the desired channel. This locking function can be integrated in the same butterfly package allowing the final accuracy of 20picometers.

New devices like tunable lasers capable of being tuned over a range of wavelengths are now becoming available. They takeadvantage of the natural variation in the emission wavelength with temperature (0.1 nm/C).

Fabry Perot lasers : Fabry Perot lasers can generate several longitudinal frequencies at once. The semiconductor material, thefrequency spacing and the laser length determine the range of frequencies.

Monolithic BRAGG Laser (DBR) : cleaved edges of Fabry-Perot lasers result in laser light with insufficiently narrow line width.Narrower line widths may be accomplished by employing Bragg gratings as reflectors.

DFB lasers : DFB lasers are monolithic devices that have an internal structure based on InGaAsP waveguide technology and aninternal grating to provide feedback at a fixed wavelength determined by the grating pitch.

Semiconductor Quantum Well Lasers : these diode lasers have a very thin active region (50 to 100 Aor 7 to 10 atomic layers).

Then small currents produce large amounts of coherent light within a narrow line width.1 A(Angstrm) = 10 -10 m

VCSEL lasers : Fabry-Perot lasers, DFBs, DBRs, require substantial amounts of current to operate (tens of milliamperes). Moreover,their output beam has an elliptical cross section which doesnt match the cylindrical cross section of the core of an optical fiber. Astructure that produce a cylindrical beam is known as Vertical Cavity Surface Emitting Laser.


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4.3 Modulators


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4.8

4 - Optical ComponentsExternal Modulators

Phase control

Waveguide

P-type

MQW stack

N-type

N-type substrate

Fiber

VoltageWaveguide

Lm

Mach-Zehnder

MQW

Electrorefraction

Voltage

Optical modulators are integrated components designed to control the amount of optical power transmitted to an optical waveguide.So they can be positioned in line with CW lasers or monolithically integrated with the laser source.

The major advantage of external modulators is that they have negligible chirp (phase jitter) compared with direct modulation. Chirpalong with dispersion is another limiting factor for long distance transmission. In addition, external modulators can modulate highpower CW lasers.

Mach-Zehnder modulator : it consists of a Y-splitter junction, two phase modulators and a Y-combiner junction. The incoming signalis split into two parts. One of them is phase adjusted (by controlling the refractive index) and then the two parts recombine.

Depending on the phase delay, light destructively or constructively recombines and an on or off signal is obtained at the output.

MQW ( Multiple Quantum Well) directional couplers operate on light absorption properties. Light is absorbed when a voltage isapplied. Electro-absorption modulators are on and off devices. They display an almost logarithmic attenuation that depends on thevoltage applied. They can generate bit rates in excess of 40 Gbit/s with a modulation depth in excess of 45 dB.

The electrorefraction modulator directly controls the phase of an optical wave through it when a voltage is applied.


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4.9

4 - Optical ComponentsDirect versus External Modulation

Direct Modulation External Modulation

IinDC Iin

Electricalsignal IN

Electricalsignal OUT

Unmodulatedoptical Signal

Electricalsignal IN

Externalmodulator

Modulatedopticalsignal

Laser diodes bias current ismodulated with signal input to

produce modulated optical output

low cost, but

high chirp (spectral broadening) higher dispersion

Laser diodes bias current is

stable

more expensive, but

low chirp (spectral broadening)

better dispersion performance


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4.11

4 - Optical ComponentsOptical Filters

Dielectric filters

Bragg gratings

Arrayed Waveguide Gratings

Mach-Zehnder filter

Diffraction gratings

Transmitted beam

Reflected beams

Multi layer

structure

Incident beam

1

2

3

4 Optical Fiber

CoreCladding Grating

21 34

High index

Low index

L

L+L

1+ 2 2

1

Mach-Zehnder filter

Directional

coupler

Directional

coupler

Optical filters are key elements for the transmission and routing of WDM signals. Various solutions have been demonstrated toperform optical filtering, the most popular being :

- Dielectric thin film interference filters : they consist of alternating quarter-wavelength thick layers of high refractiveindex and low refractive index. 200 or more layers of material are deposited in a carefully controlled manner on a glasssubstrate in large deposition chambers. Chips are diced, polished, and precision mounted in metallic housings along withcollimators to yield a wavelength-specific, free-space device.

Thin-film dielectric devices are the most broadly deployed filters for low-channel-count DWDM systems in the 400 and 200 GHzchannel spacing regime. The mature technology offers good temperature stability, channel-to-channel isolation, and a broadpassband.

- Fibre Bragg gratings : it consist of a fibre segment of which index of refraction varies periodically along its length. Thesedevices perform better at narrower channel spacing and moderate channel counts (less than 16). Insertion loss anduniformity is very good for these components as they are fabricated from standard single mode fibre. Channel spacing asnarrow as 2.5 GHz (0.04 nm) have been demonstrated using the Mach-Zehnder approach

- Arrayed Wave Guide : they consist of a few layers of glass deposited on a silica or silicon substrate. The composition ofthe glass must be carefully controlled to present the correct index of refraction to the incident light. These layers arepatterned and etched using variants of standard semiconductor process techniques, photolithography and reactive ionetching. A strong advantage of AWG solution is that it can be manufactured using semiconductor technologies leading tohighly reproducible performance, high manufacturing yields and cost-effectiveness.

- The Mach-Zender filter is based on the interference of two coherent monochromatic sources that are based on the lengthdifference, and thus on the phase difference of two paths. In fibre optic systems, a phase difference between two opticalpaths can be artificially created.


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4.12

4 - Optical ComponentsOptical Mux-Demux

1+2+...4

1

2

3

4

PrismLens Lens

n2

n1

Diffraction gratings

1+2+...4

1

2

4

The main function of an optical demultiplexer is to receive from a fiber a beam consisting of several wavelengths and separate it intofrequency components which are coupled in as many individual fibers as there are frequencies. An optical multiplexer functionsexactly in the opposite manner.

There are two kinds of optical demultiplexer devices : passive and active. Passive demultiplexers are based on prisms, diffractiongratings, spectral filters. Active demultiplexers are based on a combination of passive components and tunable detectors.

A diffraction grating is a passive optical device that takes advantage of the diffraction property of light and reflects light in a directionthat depends on the angle of incidence, the wavelength and the grating constant (number of strips per unit length). Then a diffraction

grating reflects wavelengths in different directions when a mixed-wavelength beam impinges on it.


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4.13

4 - Optical ComponentsFilters Advantages and Disadvantages

Filter type Advantages DesavantagesThin-filmdielectricinterference

Mature technology Good temperature stability

Good wavelength selectivity

Difficult to produce narrow channelspaced filters (


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4.16

4 - Optical ComponentsBlack & White and Colored Interfaces

Terminal line

equipment rackTransmit

shelf

Optical transmitinterface unit

Output opticalsignal

Developed for single-channel applications (ITU-TG.957) Wide range of the output wavelength (1500-1580nm) Usually based on direct laser modulation

+ up to 150 km at 2.5 Gbit/s over standard fiber Moderate launched power (lower than +2 dBm) Large range for the launched power:

from -3 to +2 dBm Carrying only the SDH signal

Developed for multi-channel applications Selected and stable output wavelength with

respect to the system design (mux, amplifier) Based on external modulation

+ up to 600 km at 2.5 Gbit/s over standard fiber

Large launched power is desired Stable and adjustable launched power is wanted Can carry additional signal for OAM&P purposes


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4.17

4 - Optical ComponentsReceive Wavelength Adapter

W

DM

DEMUX

StandardReceiver

Opticalsignal

Electricalsignal

Standard optical interface

SDH network element

Electricaldemulti-plexingstages

StandardReceiver

Opticalsignal

Electricalsignal

Standard optical interface

SDH network element

Electricaldemulti-plexingstages

Electrical tributary signals

Receive Wavelength Adapter (RWA)

OptimizedReceiver

3R Rege-neration

Short-ReachTransmitter

Optical

signal

Electrical

signal

Optical

signal

Electrical

signal

Optical-electrical

conversion

Re-timing

Re-shapingRe-transmitting

Electrical-optical

conversion

In this implementation scheme, thestandard optical receive interface limits

the degradations that can experience theoptical channels before proper detection

Pulse peaking dueto self-phasemodulation

Thick upper rail due toamplifier noise/signalbeat noise

Optical-electrical

conversion


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4.19

4 - Optical ComponentsMulti-rate Clock Card

Multi-rate clock card

Multi-rate clock on board

Bi-directional transponder

100mbit/s 2.5gbit/s application range3R O/E/O regeneration

Transparent protocol support:

SDH/SONET (STM-n/OC-n), fast

Ethernet, gigabit Ethernet, ATM,FC, ESCON, FC, 2FC, FICON,

FDDI, digital video

Inventory Saving !!Inventory Saving !!

MCC

1 32 42 to 4 wavelengths

tunable laser

This Multi-Clock Card is a 3R-transponder that supports all bit rates from 100 Mb/s to 2.5 Gb/s: e.g. Fast Ethernet, FDDI,ESCON, Digital Video, STM-1/OC-3, STM-4/OC-12, Fiber Channel (FC), 2FC, Gigabit Ethernet, STM-16/OC-48. The bit rate isconfigured remotely by the Network Manager, enabling telecom operators to provide differentiated tarification according to thebandwidth made available.

This transponder covering all most common bit rates in the metro, including 2.5 Gb/s and tunable over 2 (soon 4) wavelengths isa real true universal transponder, ideally suited for Metro Access applications where operators require high flexibility and fastavailability for new service turnup.

The MCC transponder supports performance monitoring for SDH/SONET signals, based on the B1 byte.


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4.20

4 - Optical Components4 x ANY TDM Concentrator

Plug-in cartridge

STM-4

2.5

Gb/s

MCC

or

OCC

TDM concentrator card

FE

ESCON GbE

Optical b&w connection

The TDM concentrator allows to multiplex in the time domain up to 4 signals into one single 2.5 Gb/s optical channel, e.g. 4 xESCON into a STM-16/OC-48. Concentration provides cost-efficient transport for low bit rate traffic by reducing the number ofrequired optical channels, i.e. minimizing the number of transponders and the size of the mux/demux gear.

The TDM concentrator can be used to mix signals of different bit rates into one single 2.5 Gb/s optical channel: e.g. 2 x ESCON and 2x STM-4/OC-12 or 2 x GbE.

One of the distinctive features of the TDM concentrator is that it does not use a proprietary TDM multiplexing scheme but delivers a

fully compliant SDH/SONET frame. A concentrated 2.5 Gb/s signal can hence be directly and cost-efficiently connected to aSDH/SONET ADM/DXC without requiring prior de-concentration.

The TDM concentrator has a highly modular fabric, consisting of a housing card that can accommodate up to 4 cartridges, one foreach aggregated traffic. Cartridges come into three types:

- low bit rate cartridge: for Fast Ethernet, FDDI, ESCON and Digital Video

- SDH/SONET cartridge: for STM-1/OC-3 and STM-4/OC-12

- GbE/FC cartridge: for Gigabit Ethernet and Fiber Channel

4 x ANY is no WLA.

So it should get its ownchapter.


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4.22

4 - Optical ComponentsOptical Amplification Principle

Ground state

Excited state

Energy supply(pumping)

Spontaneousemission

Stimulatedemission

electron

photon

Noisegeneration

Opticalamplification

Signal in Signal out

Activefibre

fibre core doped with erbium ionsPumplaser diode

Pumpcoupler


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4.23

4 - Optical ComponentsEDFAs Structure

Double-stageamplifier:

1530 - 1560 nm

+14, +17 or +20 dBm

output power

Low-noise (NF < 6dB)

Allows insertion ofoptical devices withno impact on the

power budget:

OADM

DCU

Input

Preamp Postamp

980 nmpump

1480 nmpump

Output

Monitor Monitor MonitorLoss

Attenuator9 - 15 dB

OADM

Or...

DCU

Or/and...


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4.24

4 - Optical ComponentsOptical Amplifier Response

Gain-flattened EDFAs

16 channels, 200-GHz channel spacing, 440-kmtransmission,

100-dB amplifier chain (1 booster + 3 in-line EDFAs)

50dB

40 nm

Conventional EDFAs

Pch

Limited flat-gainbandwidth

LowSNR

Improvement of optical erbium-doped fiber amplifiers originally designed for single-channel applications was required for multi-channel purposes.

Flat gain/noise spectral responses are obtained using spectral filtering techniques in order to compensate for and to tailor the intrinsicnon-uniform spectral responses of erbium-doped fiber amplifiers.

With conventional EDFAs, designed for single wavelength applications, the spectral response was non-uniform :

- large per-channel power excursion

- least favored channels suffering from low signal to noise ratioWDM approach for line systems:

- with 2.5-Gbit/s channel bit rate, 16-fold increase in fiber distances with respect to 10-Gbit/s TDM

- allow effective fiber capacity increase by changing only the terminal equipment

- transparency to bit rates and standards, modularity, upgradability...

BUT at the expense of new technical challenges:

- transmitter wavelength selection and stabilisation, optical de-multiplexing

- optical amplification of WDM channels, noise accumulation

- non-linear effects along line fiber...


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4.25

4 - Optical ComponentsSignal to Noise Ratio

Opticalpower

Distance

Signal

Amplifier output power

Optical SNR

Amplifiernoise

Optical amplifiers work at constant output level determined by pumping power. Since some noise is added to the signal while it istraveling through the optical fiber, this noise is amplified in each repeater and consequently depends on the number of repeaters. Inparallel, some noise is generated in the optical amplifiers themselves and cumulated all along the link. The quality of the link dependson the OSNR (Optical Signal to Noise Ratio).


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4.26

4 - Optical ComponentsBER Versus SNR

-36 -34 -32 -30 -28 -26 -24 -22 -20

10 -5

10 -6

10 -7

10 -8

10 -9

10 -10

10 -11

10 -12

10 -13

10 -14

10 -15

10 -16

Biterr

orrate

Back-to-back (withoutEDFA)

SNRRx = 25 dB/0.1 nm






Received power

Erbium-doped fiber amplifiers intrinsically behaves in an analogous way. For instance, the gain/noise spectral responses depend on:

span loss

number of channels...

Typical Signal-to-Noise Ratio (SNR) requirements (for BER 21 dB/0.1 nm at 2.5 Gbit/s

SNR > 27 dB/0.1 nm at 10 Gbit/s

Signal-to-noise ratio figures are favoured by:

large per channel power (+ high amplified total output power required for large channel count)

small amplifier gain (+ the smaller the amplifier spacing, the higher the output end SNR)

small amplifier noise figure, pre-emphasis technique on the transmit side


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4.27

4 - Optical ComponentsRepeater Spacing in WDM Networks

30

40

50

60

70

80

90

0 2000 4000 6000 8000 10000 12000

Link length (km)

Repeaterspacing

(km)

16 x 10 Gbit/ s

42 x 10 Gbit/ s

68 x 10 Gbit/ s

105 x 10 Gbit/ s


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4.28

4 - Optical ComponentsQ Factor

Opticalpower

o

m1

m0

1Q =

m1 - m0

1 + 0

When noise only is considered20 log(Q) proportional to SNR (useful for margin evaluation)

Gaussian noise distribution

PDF

Decision threshold

BER erfcQ e

Q

Q

=

1

2 2 2

2

2

M0represents the average value of 0 and M1 the average value of 1.

0 and 1 represent the standard deviation of 0 and 1 respectively

Optical amplification is obtained at the expense of wide-band optical noise spectral density (Amplified Spontaneous Emission or ASEadded to the signals). The ratio of the per-channel power [dBm] to the ASE noise power [dBm/0.1nm] is the optical signal to noiseratio (SNR [dB/0.1nm])


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4.29

4 - Optical ComponentsRaman Amplification

Gain spectrum vspump wavelength Peak gain coefficient

versus fibre

dIsdz

= CR Ip Is

fibre type Peak CR coefficient (m-1 W-1)

PSCF 0.3 x10-3

typical NDSF 0.45 x10-3

typical DSF 0.6 x10-30,00

0,05

0,10

0,15

0,20

0,25

0,30

1450 1470 1490 1510 1530 1550 1570 1590 1610 1630 1650

wavelength (nm)

gR

(arb.unit)

@1455 nm

@1480 nmgRAeff

CR =related to fibre dopants

effective surface area

fibremolecularvibration

Energy transferPeak at 440 cm-1

Pump wave

Stokes wave

Pump wave

1455 nm

pump

Tx Rx

Raman effect System configuration

A Raman amplifier uses intrinsic properties of silica fibers to obtain signal amplification. This means that transmission fibers can beused as a medium for amplification, and hence that the intrinsic attenuation of data signals transmitted over the fiber can becombated within the fiber. An amplifier working on the basis of this principle is commonly known as a distributed Raman amplifier(DRA).

The physical property behind DRAs is called SRS (Stimulated Raman Scattering). This occurs when a sufficiently large pump wave isco-launched at a lower wavelength than the signal to be amplified. The Raman gain depends strongly on the pump power and thefrequency offset between pump and signal. Amplification occurs when the pump photon gives up its energy to create a new photon at

the signal wavelength, plus some residual energy, which is absorbed as phonons (vibrational energy).As there is a wide range of vibrational states above the ground state, a broad range of possible transitions are providing gain.Generally, Raman gain increases almost linearly with wavelength offset between signal and pump peaking at about 100 nm and thendropping rapidly with increased offset.

The position of the gain bandwidth within the wavelength domain can be adjusted simply by tuning the pump wavelength. Thus,Raman amplification potentially can be achieved in every region of the transmission window of the optical transmission fiber. It onlydepends on the availability of powerful pump sources at the required wavelengths. The disadvantage of Raman amplification is theneed for high pump powers to provide a reasonable gain.

This opens a new range of possible applications. It is possible, for instance, to partially compensate fiber attenuation using theRaman effect and, thus, to increase the EDFA spacing. The Raman pump wave can be conveniently placed at the EDFA locations.This saves costs as less EDFAs are needed on the link, and the number of sites to be maintained is reduced.

Raman amplifiers offer several advantages compared to EDFAs, including the following:

Low noise buildup

Simple design, as direct signal amplification is achieved in the optical fiber, and no special transmission medium is needed.

Flexible assignment of signal frequencies, as Raman gain depends on the pump wavelength and not on a wavelength-sensitivematerial parameter of the medium, such as the emission cross-section of dopant in the erbium-doped fiber (EDF).

Broad gain bandwidth is achievable by combining the Raman amplification effect of several pump waves that are placed carefullyin the wavelength domain.

However, despite the many advantages of Raman amplification, there can be some degradation effects. For example, not only thespecially launched pump waves but also some of the WDM channels may provide power to amplify the other channels. This wouldresult in power exchange between WDM channels and thus cross-talk leading to signal degradation.


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4.30

4 - Optical ComponentsRaman Amplification Experimental Results

1480 nm pumpedexperiment

About 1 dB improvement

expected with 1455 nmpumping

System guaranteed

improvement : 6 dB

1.5 dB repair possible in

50 last km

10-11

-58 -56 -54 -52 -50 -48 -46 -44 -42

0 mW

200 mW

400 mW

600 mW

800 mW

1 W

1.1 W10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

Experimental equivalent sensitivity (dBm)

Pump

Tx

Att.

TxSB

ER


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4.31

4 - Optical ComponentsRaman Amplifier Applications

Pump

Pump

Pump

Pump

TDM TDMTerminal Terminal

Pump Pump

PumpTDM TDMTerminal Terminal


Pump

+ Remotepostamplification

(1 or 2 fiber pumping)

TDM Terminal TDMTerminal

PumpTDM TDMTerminal Terminal+ Raman

preamplification


Pump

Pump

TDM TDMTerminal Terminal

+ Remotepreamplification(1 or 2 fiber pumping)

Erbiumdoped fiber

Unrepeatered submarine links


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4.32

Time allowed :

10 minutes

4 - Optical ComponentsExercise

1- Which kind of laser cannot be used in WDM transmissionsystems ?

2- Which type of photodiode is used in high sensitivity receivers ?

3- Why external modulators are used in high performance opticalsystems ?

4- Quote three types of optical filters.

5- What is the function of a wavelength adapter ?

6- Explain the reason why wide-band optical amplifiers have a

double-stage structure.

7- In which kind of links Raman amplification is used ?

1- Fabry Perot lasers cannot be used in WDM due to the width of the emitted spectrum incompatible with the channel spacing.

2- APD photodiodes are used in high sensitivity receivers.

3- External modulators are used in high performance systems due to their low chirp and thier ability to support high modulation bitrates.

4- Thin film dielectric filters, Bragg gratings, Mach-Zendher interferometer, diffraction gratings.

5- A wavelength adapter converts a black & white signal into a coloured signal.

6- An optical amplifier doesn't have a linear transfer function. Using two amplifiers allows to linearize the response in thetransmission band. Moreover, this double-stage structure is very convenient since it allows to insert devices such as variableattenuators or OADMs.

7- Raman amplification is mainly used in unrepeatered long haul links.



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4.34

4 - Optical ComponentsNotes


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5 - Optical Networks


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5.2

5 - WDM Optical NetworksSession presentation

Objective: to be able to identify the systems used in WDMnetworks and the architecture of these networks.

Program:


5.2 Optical network structure

5.3 Protections of optical networks

5.3 Supervision of WDM networks

5.5 WDM applications

5.6 Alcatel references in WDM


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5 - WDM Optical Networks



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5.5

5 - WDM Optical NetworksOptical Add & Drop Multiplexer

Add

ports

Addports

Dropports

Dropports

Transponder shelf or SDH equipmentTransponder shelf or SDH equipment

M

UX

DE

MUX

ALC

28 ch

32 chWDM

32 chWDM

32 chWDM

32 chWDM

Ch1

Ch2

Ch3

Ch4

As the number of channels transported by WDM networks increase, the number of added and dropped channels also increase.Initially it was considered desirable to be able to extract 25 % of the channels at each node. With increasing traffic, this value canrapidly grow to 100 %. The approaches and technologies available for developing add and drop modules fall into three maincategories :

- full de-multiplexing and multiplexing of channels

- cascaded single-channel add an drop devices : attractive for few channels (4 to 8) but limitations for high number ofchannels. Cross talk, high insertion losses and bandwidth filtering are the main issues.

- multi-channel add and drop devices : using the liquid crystal technology, this approach has the following advantages

. Low insertion loss

. Suitable for 50 GHz, 100 GHz and 200 Ghz spacing

. Transparency to the bit rate


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5.6

5 - WDM Optical NetworksAlcatel WDM NEs

32 channels

240 channels

40 channels

80 channels

1640 WMInternational and

national applications

1686 WMNational andregional/metro applications

16 channels

160 channels

1696 MSAccess/metro andenterprise applications

32 channels

Alcatel 1640 WM Alcatel 1686 WM : Alcatel's long-haul DWDM solution maximizes fiber bandwidth, relieves fiber bottlenecks,and offers seamless integration into existing and future optical networks. It increases flexibility with simplified turn-ups andexpansions, and uses a modular architecture that allows incremental growth to meet unpredictable demand.

As a key components of Alcatel's DWDM solution, the Alcatel 1640 OADM/WM and Alcatel 1686 WM, enables efficient migrationtoward all-optical networks while extending the life of legacy components. Their advanced error detection and correction method,pioneered by Alcatel, translates into significant performance and cost advantages as compared to industry-standard DWDM

systems.

Alcatel's solutions increases the distance between in-line amplifiers by 30 percent to 50 percent and quadruples the nu

Documents

Introduction to WDM