4_measurement in a dwdm system

8/17/2019 4_Measurement in a DWDM System

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Test & Measurements of a DWDM network

The optical and digital characteristics of all the different components must be

measured before use in DWDM systems. Components tested to meet the specificationsseparately, could interact unpredictably when installed in a system. A variety of tests must

therefore be performed during different phases of network implementation.

Component conformance tests

The critical optical parameters to be measured in each network component are

listed below

Transponder

– Center wavelength and spectral width of emitted channel

– !pectral stability over time and temperature

– "utput power #ma$imum %& d'm in accordance with laser protectionregulations( and output power stability

–!idemode suppression ratio #should be )* d'(

Multiplexer and demultiplexer

– Wavelengths of passbands of the different channels

– Channel crosstalk #pulse wavelength overlap(

– Channel insertion loss

– "ptical return loss #back+reflection ratio(

– olari-ation mode dispersion #MD(

– olari-ation dispersion loss #D(

Amplifier

– Channel center wavelength and channel spacing

– !pectral stability over time and temperature

–/ain and wavelength dependence of the gain

– 0oise figure

– "ptical signal+to+noise ratio #"!01(

– "utput power and output power stability

Dispersion compensating modules

– 2nsertion loss

– /roup velocity over wavelength

– Chromatic dispersion #CD(

Receiver

– 'ack+reflection

– "ptical and electrical bandwidth

–!ensitivity

There are additional digital parameter tests which must be performed before the

network elements are brought together into a network system.

These include

– Ma$imum tolerable 3itter and 3itter transfer function #4T5(

– ong term -ero bit+error rate #'61( stability


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Parameter tests on optical fibers

'efore fiber is laid, the CD value has to be measured especially for transmissionrates of %* /bps and above. The dispersion compensating modules can then be designed

for a controlled value of CD. The CD delay can be derived from optical time domain

reflectometer #"TD1( measurements at four wavelengths 7 %8%* nm, %)9* nm, %::* nmand %;9* nm. Corresponding group delays can then be calculated based on the

propagation time of the reflection of each wavelength. With these four group delay values

it is possible to appro$imate the CD delay versus wavelength from which the CDcoefficient can be calculated #figure %(. "nce the fiber is laid, the polari-ation mode

dispersion #MD( value must be measured given that it is strongly affected by

mechanical stress.

Chromatic dispersion coefficients for various fiber types (figure 1

This can be done via the fi$ed analy-er method 7 wavelength scanning 7#figure

9(. This method re


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The optical time domain reflectometer #"TD1( has become the standardinstrument for the testing of optical networks. They can also be used to locate defects and

mechanical stresses on the fiber itself such as micro and macro bending.

"ystem installation tests

During system installation, it is important to measure the optical parameters of thesystem. 2n a DWDM network, an optical spectrum analy-er #"!A( is used to measure

efficiently the power, wavelength and "!01 of each transmitted channel to ensure

transmission


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– !D% to !Dn are reference points directly at the output of the demultiple$er. 1% to

1n are at the input of the individual receiver modules

Testing with an "!A at reference test points in DWDM systems #figure )(

Typical OSA scan of a 16 channel DWDM system (figure 5)

"ystem acceptance tests

'efore a network system is handed over to the customer, it must undergo anacceptance test in which all critical optical parameters are measured once more for

• Center wavelength of each channel

• !pectral stability over time and temperature

• Channel spacing and channel crosstalk

• eak power of each channel and overall@total power

• "!01


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• /ain and noise figures

The measurements of the digital parameters are performed either end+to+end #before

the transponder and behind the receiver( or via a loop through the entire DWDM system.The following parameters can be tested with a signal analy-er

•

4itter transfer function #4T5(• Wander analysis

• ointer analysis

• ath trace

• Alarm tests

• Transmission clock transparency

• erformance monitoring #'% byte(

• 1ound trip delay #needs to be done in loop(

• 'it+error rate with pseudo random bit se


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system growth steps. 0ew technologies will soon reach users in local areas, small businesses or even at home causing the


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Recommendations

2n order to plan and implement fle$ible, future+proof DWDM systems andcomponents, basic standards must be defined to ensure correct interaction of components

and modules from different manufacturers. The 2nternational Telecommunication =nion

Telecommunication !ector #2T=+T( is responsible for defining international standards@recommendations. Whilst the standard responsibilities of the 2T=+T lie at application

level, the 2nternational 6lectrotechnical Commission #26C( is responsible for those taking

effect at product level. A variety of wavelengths used in telecommunication systems can be selected by individual manufacturers. According to 2T=+T /.;>9, all data channels in

DWDM systems should fall into a specified %** /E-@*. nm or :* /E-@about *.) nm

channel grid based on a center fre8.% TE- #see table ;(. This corresponds to

an optical wavelength of %::9.:9 nm. Current DWDM deployments tend to use the :*/E- channel spacing, although spacings of 9: /E- have also been successfully tested.

Table ; gives an overview of 2T=+T recommendations specific to DWDM systems and

associated system components.

*T+,T recommendations for D-DM systems

Recommendation

number

Title contents

/.;:* Definition and test methods for the relevant parameters of

singlemode fibers

/.;:9 Characteristics of a singlemode optical fiber cable #standard

fiber(

/.;:8 Characteristics of a dispersion shifted singlemode optical fiber

cable dispersion shifted fiber #D!5((

/.;:) Characteristics of a cut+off shifted singlemode optical fiber cable

/.;:: Characteristics of a non+-ero dispersion shifted singlemodeoptical fiber cable non+-ero dispersion shifted fiber #0BD!5((

/.;;% Definitions and test methods for the relevant generic parameters

of optical amplifier devices and subsystems

/.;;9 /eneric characteristics of optical amplifier devices and

subsystems

/.;;8 Application related aspects of optical amplifier devices and

subsystemsdescribes nonlinear effects(

/.;;) "ptical safety procedures and re9 "ptical interfaces for multichannel systems with opticalamplifiers #DWDM systems, channel spacing grids and referencetest points(

/.&*> 2nterfaces for the optical transport network #"T0( #9.& /bps,%*.& /bps, )8 /bps, 56C and digital wrapper(

/.>:& "ptical interfaces for e


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/.>:>.% "ptical transport networks with physical layer interfaces

The electromagnetic spectrum can be divided into different bands for the use in

telecommunication transmissions. Though not standardi-ed, figure 8* illustrates thespectral bands and the names by which they are commonly known.

%ptical bands

DWDM fre9

Abbreviations

Abbreviation Description

A"0

A!

A!6ATM

'61

CDd'

DC5

DCM

D6M=D!5

DW

DWDM6@"

6D5A

56C5WM

/bps

/ig626C

2

All optical network

Automatic protection switching

Amplified spontaneous emissionAsynchronous transfer mode

'it+error ratio

Chromatic dispersionDecibel

Dispersion compensating fiber

Dispersion compensating module

Demultiple$er Dispersion shifted fiber

Digital wrapper

Dense wavelength division multiple$ing6lectrical+to+optical converter

6rbium doped fiber amplifier

5orward error correction5our wave mi$ing

/igabit per second

/igabit ethernet2nternational electrotechnical commission

2nsertion loss


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2 2nternet protocol

2T=2T=+T

4T5

M2M=

mW

nm

0M! 0BD!5

"@6

"ADM

"CC"5A

"56"M

"1

"!A"!C

"!01

"TD1

"T0"C

2nternational Telecommunication =nion2T= Telecommunication !ector

4itter transfer function

Main point of interestMultiple$er

Milliwatt

0anometer

0etwork management system 0on+-ero dispersion shifted fiber

"ptical+to+electrical converter

"ptical add@drop multiple$er

"ptical connection controller "ptical fiber amplifier

"ptical front end"ptical +factor meter

"ptical return loss

"ptical spectrum analy-er "ptical supervisory channel

"ptical signal+to+noise ratio

"ptical time domain reflectometer

"ptical transport networks"ptical cross connect

DMD

o!

1'!o!

15A

1

!'!

olari-ation dependent lossolari-ation mode dispersion

acket over !"06T@!DE

seudo random binary se


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!DE

!"A

!"06T!M

!1!

TbpsTDM

T

WDMM

!ynchronous digital hierarchy

!emiconductor optical amplifier

!ynchronous optical network !elf phase modulation

!timulated 1aman scattering

Terrabit per secondTime division multiple$ing

Transponder

Wavelength division multiple$ingCross phase modulation

4_measurement in a dwdm system

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