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Neven Dragašević, dipl. ing.CEOneven.dragasevic@mteh.hrwww.mteh.hr+385 99 222 1844

ROADM Testing and Characterization inNext-Gen Optical Networks

Impacts of ROADMs and advancedmodulation formats (40/100 Gbps)

Neven Dragašević, dipl. ing.April 2010

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Outline

History of DWDM Systems Power Measurement vs. Spectrum Analysis – Why is OSNR so

Important? Network Arhitecture of Next-Gen Optical Networks ROADM Network Elements 40/100 Gbit/s Challenges Testing of DWDM Network Elements Standard (IEC 61280-2-9) vs. In-Band OSNR Measurement

Method Question and Answer Session

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1310 nm 1550 nm

1310 nm 1550 nm

1310 nm 1550 nm

1310 nm 1550 nm

Early WDM2 channels1310nm & 1550nm

Second generation WDM2 to 4 channels400 GHz spacing1550 nm window (C band)

Dense WDM8 to 16 channels100 to 200 GHz spacing1550 nm windowC Band

Second generation DWDM40 to 100 channels25 to 50 GHz spacing1550 & 1625 nm windowsC & L bands

The WDM Evolution

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The ITU band and grid

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The ITU Grid

Transmission channels and ITU bands G.694.1

S-band1460 to 1530

C-band1530 to 1556

L-band1565 to 1625

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Power measurement

For a system to operate correctly, power transmitted needs to be sufficient toovercome all optical impairments along the path so that the receiver « sees »enough of the signal

In a DWDM system, you need to compensate attenuation per wavelength with theappropriate use of amplifiers, making sure that at the end, signal power will bedominant over noise

OSA is the tool of choise to precisely measure power and noise in each peak, withthe ratio of both giving an excellent indication of the expected Quality of Service

RRRRR

Power

NoiseOSNR OSNR

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Sensitivity curve: BER vs Power

Power

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What is spectral testing?

Measuring power as a function of wavelength

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Type of OSAsDiffraction Grating Simulation

White LightSource

Detector

Grating

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Network Service Provider Architecture

ROADMNetwork

STM-64

Edge Router

AccessRouters

STM-64

Access Network

ENIU

MSPP

MSPP

IP DSLAM

Residentialaccessnetwork

ONT

CMTS

FTTTower(IEEE802.16 WiMAX)

FibreAccess(xPON)

CATVAccess

EnterpriseCustomer

MetroNetwork

SuperHead-End

Video Service OfficeLocal Content

CopperAccess

GigE

GigE

GigE

GigE

10GigE

10GigE

10/100M Eth.

Long HaulInterconnection(STM-16/STM-64 or 10GigE over λ)

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Acronyms (Add new set of acronyms for Protocol)

DCM Dispersion compensation module LGX/OSX Light guide/optical system cross-connect LIAM Line input amplifier (receiver) LOAM Line output amplifier (transmitter) LRAM Long-reach amplifier (transmitter) OLA Optical line amplifier RCMM Reconfigurable channel multiplexer module ROADM Reconfigurable optical add/drop multiplexer SRTM Sub-rate multiplexer transponder module MRTM Multirate transponder module MSPP Multiservice provisioning platform MSTP Multiservice transport platform SMTM Singlemode transponder module TTM Tunable transponder module WSS Wavelength selective switch WXC Wavelength cross-connect

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Next-Generation WDM Evolution

1995 2000 2005 2010

10 Gbit/s

ManagedROADM

Sing

le te

chno

logy

Mul

tiple

tech

nolo

gies

PtP 2.5 Gbit/s

OADM ring

SwitchedOTN

Multiservice

40 Gbit/sGMPLS

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ROADM Features

Service interfaces: SDH (STM-1 to STM-64) Ethernet (100 M, GigE, 10 GigE LAN/WAN) Fibre Channel

Number of wavelengths available: Up to 32, 44 or 72

Features used: Legacy SDH Mux (ADM) Ethernet-over-SDH (Next-Gen SDH) Optical transport network (OTN) forward error correction (FEC)

Network configurations: Ring, multiring interconnect, mesh, multidegree and drop and insert

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Optical switchand line amplifier shelf

Transponder cards(service cards)

100 M/GigE10 GigE

FCSDH

10.7G OTN(G.709)

DWDMand OLA

Anatomy of an ROADM Network Element

Packet Transport NG-SDH

ROADM GM

PLS

/AS

ON

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Add and Drop Architectures: MUX/DEMUX

40-DM

UX

40-DM

UX

Pre-ampPre-amp

40-M

UX

40-M

UX

BoosterBooster

4040 λλ

This offers an expensive solution.

4040 λλSONETOr

SDH40 λ

SONETOr

SDH40 λ

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Add and Drop Architectures: OADM

8 λ-DMUX8 λ-DMUX

WDMWDM

8 λ-MUX8 λ-MUX

WDMWDM9 to 409 to 40 λλ

1 to 81 to 8 λλ 1 to 81 to 8 λλ

8 λSONET /SDH

8 λSONET /SDH

Pre-ampPre-amp BoosterBooster

4040 λλ 4040 λλ

This offers a less expensive solution, but not much flexibility!

OADM

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Add and Drop Architectures: ROADM

With this solution, wavelengths can be added and dropped atany time.

WavelengthBlocker

TransmittersReceivers

Drop Module Add Module

DEMUX MUX

VOAs andTap Monitors

Booster

DCM

Pre-Amp

DCM

Ref CRC

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Add and Drop Architectures: ROADM Bidirectional

DMUXDMUX

WSSWSS

MUXMUX

4040 λλ 4040 λλ

8 λSONET /SDH

8 λSONET /SDH

Pre-ampPre-amp BoosterBooster

4040 λλ 4040 λλ

4040 λλ 4040 λλBoosterBooster Pre-ampPre-amp

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Add and Drop Architectures: ROADM Multidegree

AB

C

DE

F

G

H

PP-MESH-8 WXCWXC

MUXMUX

DMXDMX

BB

PP

WXC

WXC

MU

XM

UX

DM

XD

MX

BB

PP

WXCWXC

MUXMUX

DMXDMX

PP

BB

WXC

WXCMU

XM

UX

DM

XD

MX

PP

BB

Ref: Cisco

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Add and Drop Architectures

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ROADM Definition

ROADM is defined as: Demultiplexing and multiplexing Any-to-any, hitless switching Wavelength blocking Per-wavelength or per-port attenuation

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ROADM Definition: Microelectronic MechanicalSystem (MEMS)

Its mirrors re-direct the signal inthe proper direction(s).

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DWDM with 40/100Gbit/sDWDM with 40/100Gbit/s

Faster modulation = larger spectral response

- FWHM means nothing- OSNR must be measured differently- Same in-channel Power = less Peak Power

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DWDM with 40/100 Gbit/sDWDM with 40/100 Gbit/s

Faster modulation = larger spectral response

- Spectral Shape must be monitored- High risks of Cross-Talk

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DWDM with 40 Gbit/sDWDM with 40 Gbit/s

Impact on testing:

Optical Spectrum Analyzers need New Feature for thisapplication

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Modulation schemes

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Advanced modulation formats (40/100 Gbps)Advanced modulation formats (40/100 Gbps)

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Advanced modulation formats (40/100 Gbps)Advanced modulation formats (40/100 Gbps)

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A typical DWDM system

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Commissionning a link

OSA testing:• transmitter and receiver performance, optical

amplifier performance, error behavior, and noiseaccumulation

• Center wavelength, signal power are measured withthe OSA but the most important parameter is theOSNR.

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Power

OSNR

Channel spacing

CentralCentral

OSA testing – Critical parameters

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System Testing - Transmitter

1•Main parameters :

• Center wavelength• Optical power for every channel

• Carriers must remain within specifications• Wavelengths must remain stable• Power must not fluctuate beyond systemtolerance

•The center frequency should be inaccordance with the ITU-T grid

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System Testing - Transmitter

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System Testing – MUX/DEMUX

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• Main parameters :•insertion loss• power flatness• SNR•Crosstalk

•In ROADM networks :•center wavelength•Bandwidth

• MUX have upgrade ports toaccommodate future capacitygrowth; similar tests should beperformed on the upgradepassband.

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System Testing - MUX

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System Testing - EDFA

3• Main parameters :

•gain• power flatness• peak power• OSNR

• Adding or dropping a wavelength will have a significantinfluence on the amplification performance.

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Gain

Gain Flatness

Noise Figure

Critical System Parameters

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System Testing - OADM

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• Main parameters :• passband insertion loss and power flatness•OSNR – Measured In-Band!• X-talk

• There are 3 types of paths that have to beverified: the pass-through (express) path, the drop

paths, and the add path.• The OADM is tested like mux/demux.

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System Testing - Receiver

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•Main parameters :• center wavelength• optical power for every channel

• The OSNR at the receiver end shouldbe higher than 18dB for a good BER.

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Standar vs. In-Band OSNR Measurement Method

2 cases where OSNR measurement cannot be done with IECinterpolation method – Filtered signals (ROADMs) and large oroverlapping spectra (40 Gb, UDWDM)

EXFO’s method explained: Polarization diversity

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Standard OSNR measurement method

IEC 61280-2-9

Recommended Method• OSNR is generally measured using interpolation method, as recommended in IEC

(shown above)

Alternate method• Another option, when interpolation is not possible, is to turn off a portion of the peaks

and directly measure the noise at that point• changing the EDFA load affects the noise level and leads to errors• very time consuming.

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OSNR measurement methodSpecial case of Large or ultra dense signals

Interpolation method

For very large (80 GHz) signals or UDWDM (33 GHz, 25 GHz)Interpolation method over-estimates the noise level

Noise level with IECmethod

True Noise level

• Using the interpolation method in the case of very large ordense signals leads to faulty results

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Standard OSNR measurement methodSpecial case of filtered signals

• For filtered signals (ROADMs), Interpolation method under-estimatesthe noise level as noise is « carved » with the signal by the filter

• Different paths have different Noise Contribution

• Using the interpolation method in the case of filtered signalsdropped or passing through ROADMs leads to faulty results

Real noise(shoulder)Interpolated noise

Filter shape

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Closer look to the filtered signal

The use of ROADMsdoes not allow directutilisation of IEC 61280-2-9 OSNR measurementprocedure.

actual channelnoise is higher thanInterchannel noise

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Advanced modulation formats

Special challengesfor an OSA:

Correctly identifypeak (multipeak)

Correctly measurethe total signalpower in thechannel

Correctly measurenoise when X-talk isdominant

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Channel definition with EXFO OSA

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EXFO Innovative In-band OSNR methodPolarization diversity method

• Theory: Signal is polarized and noise is depolarized

• EXFO OSA have a polarization controller and polarizationbeam splitter at the input , allowing automated In-bandOSNR measurement

NoiseIn-bandNoise

In-band

SignalSignal OSNR

Power vspolarization

Ppeak

PNoise

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In-band OSNR – Polarization diversity method

•Relies on the polarization to discriminate the signal from the noise

• Uses internal polarization diversity detection• Does not require high extinction of peak to evaluate noise -> (It only requires that

both polarizations peak differ by a few dBs, not on minimizing one or the other likeothers do)

•Polarization diversity vs Polarization nulling

• Requires a few dBs discrimination only• Testing time 15 s to <1 min• No need to null polarization in each channel• PMD effects can be minimized when averaging large number of scans

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EXFO OSA Design

Monochromator, Grating-basedOSA

Double-pass

ADC

PMF 9µm

MMF 15-22 µm

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Double-pass Monochromator

Single-passmonochromator

Double-passmonochromator Improve dynamic

range Improve response

shape

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EXFO’s In-band Approach IllustratedStart with PolA, PolB and PolA + PolB

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Produce PolA-PolB (Cancels noise)

Difference isproportional tosignal

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Raise difference to « match » peak ...And reconstruct signal

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... subtract « reconstructed signal » from PolA+PolB

OSNR

you get the NOISE!Compare with PolA + PolB OSNR

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Pol A

Pol B

In-band OSNR illustrated

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Conclusion

The trends we see today are changing the way we need to test thenetwork during qualification and commissioning of the link

Knowing what is coming and how to address those changes is key toBeing future proof

EXFO’s automated In-band method provides fast, repeatable andreliable measurements for field upgrades from 10G to 40G/ROADM

Questions?

Visit our booth or www.mteh.hr for details