Optical and DWDM technology - DFN · PDF fileOptical and DWDM technology. Lübeck, 8 June 2015. Transport Network Architect, GÉANT. Networks ∙ Services ∙ People . . Objectives

Networks ∙ Services ∙ People www.geant.org

Guy Roberts

DFN-Forum Kommunikationstechnologien

State of the art and application in R&E networksOptical and DWDM technology

Lübeck, 8 June 2015

Transport Network Architect, GÉANT

Networks ∙ Services ∙ People www.geant.orgObjectives

• Today’s optical technology• The pre-coherent era

• Super-channels and photonic integration

• GÉANT network update in 2012• Alien Wave field trial update

• The future – the next generation of optical technology

Contents


Optical Technology


Wavelength Division MultiplexingScaling Fiber Capacity

DWDM Multiplexing -

Combining wavelengths

DWDM De-multiplexing -

Separating wavelengths

PreAmplifier

Networks ∙ Services ∙ People www.geant.org 5

Dense wavelength division multiplexingC band and ITU grid – before coherent technology

• Dense wavelength division multiplexing (DWDM)

• First used the 1525–1565 nm band (C band) to make use of erbium doped fibre amplifiers (EDFAs)

• Each wave carries a 10Gbps NRZ modulated signal

• Approx. 80 waves per fibre on C band 50GHz ITU-T grid

• Early systems used only fixed multiplexers• Reconfigurable Add Drop Multiplexers (ROADMs) allow

wavelengths to be switched and add/dropped under network management control


Fiber impairmentsCompensating for fibre effects

AttenuationThe power of the signal is absorbed

as it passes along the fiber

DispersionThe signal pulse is “smeared out”

as it passes along the fiber

Chromatic Dispersion

Modal Dispersion

Polarization Mode Dispersion

Advances in optical transmission are driven by new techniques to remove

impairments

Non-LinearEffects


A History of DWDM Capacity x Distance InnovationsEvolution up to 2006

Laser technology evolves to fiber/EDFA

“sweet spot”

EDFA innovation enables WDM –multiple waves

in a fiber

Chromatic Dispersion Management enables 2.5G to 10G evolution Forward Error

Correction directly increases Cap*Dist

Addition of L-Band could double fiber capacity*

Raman amplification gives excellent distance boost

Emmanuel B. Desurvire. “Capacity Demand and Technology Challenges for Lightwave Systems in the Next Two Decades”JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 12, DECEMBER 2006

1


• From intensity modulation to phase modulation• Polarization muxing

• Roughly 2x increase in Capacity * Reach• Increasing modulation order (BPSK, 3QAM, QPSK)

• Increasing the “bits per symbol”

• Coherent detection• Increased detector sensitivity• Linear detector, enables electronic CD and PMD compensation• >10x increase in Capacity * Reach

• Soft Decision Forward Error Correction (SD-FEC)• Roughly 2x increase in Capacity * Reach vs HD-FEC

The First Coherent EraInnovations are in production today

More bits/symbol allows the bit rate to be increased without increasing the symbol rate

2


• Widespread adoption of photonic integrated circuits (PICs)*1• Introduction of transmitter-based signal processing• The move to higher order modulation (eg. 8QAM, 16QAM)• The move away from fixed DWDM grids to flexible grids• The ubiquitous use of super-channels

• An acceptance that we may be approaching capacity*reach product limits for optical transmission over existing fiber types

3

The Second Coherent EraWhat does the future hold?

*1 Infinera is the market leader in PIC technology today


The Pre-Coherent Era


Optical ModulationThe First 30 Years

Laser Modulator

Detector

Tx

Rx

Intensity Modulation

Simple detector, simple receiver(note: square law detector – linearity is lost)

• For most of the history of optical communication an On/Off Keying modulation was perfectly adequate

On/Off keying works well up to 10 Gbps

Faster transmission rates made possible by improvements to

the laser and modulator


What can we do about it?In the pre-coherent era

Attenuation

Modal dispersion

Chromatic Dispersion

PMD

Non-linear effects

Amplify the signal

Single mode fiber

Narrow line width lasers and dispersion Compensating Fiber

Too small to worry about up to 10Gb/s

Limit the power of the signal at any point in the fiber

Huge compute power of DSP can be harnessed to remove fibre impairments, but it requires a linear receiver….

Laser Modulator

Detector

Tx

Rx

Coherent detection


The First Coherent EraApprox. 2010 – 2015

*These dates are open to interpretation, but I am using commercially available product as a benchmark


…waves have a speed, wavelength and frequency

What does “coherent” mean?

Wavelength

Tx• More bits per symbol (eg. QPSK)• More tolerant to fiber impairments

Rx• Mix received signal with a local oscillator• Linear detector (allows use of DSP)

Use phase changes to encode data

Adapting radio modulation techniques


Pol-Mux QPSK ModulationQPSK - 2 bits per symbol

Im{Ex}

Re{Ex}This structure called a “Super Mach Zehnder”

Laser

QuadraturePhaseShiftKeying

Q

I

90

S1

S2 Signalinputs

Shows one of the two polarizations


QPSK Encoding + polarizationPM-QPSK, 4 bits per symbol

Q

I

X-Pol

Y-Pol

Q

I

X Polarization

Y Polarization

PM-QPSK carries 4 bits per “symbol”(A symbol is now the X and Y polarizations)


Coherent WDM Detection

LO

The incoming signal that uses a phase-basedmodulation technique is sent into the mixer

This means there’s a “reference laser” in the receiver

Coherent Detector

PD Photodetector

ADC Analog to Digital Converter

DSP Digital Signal Processor

LOLocal Oscillator

It’s mixed with a reference signal from the local oscillator that is tuned to the

wavelength we would like to detect

ADC DSPPDMixer

Green SignalDetectedRed SignalDetected


• Very low noise amplifier• Phase-based technique

• More robust in the face of fiber impairments• Allows us to use more complex phase constellations to cram more bits into a modulation symbol

• Polarization can be used as a modulation variable• Highly compatible with a parallel processing approach• Linear detector

• Enables us to use sophisticated signal processing for linear impairments

Key Advantages of Coherent Detection


Coherent Detector In More DetailOne wavelength

Incoming carrier(2 polarizations, each with 4 phase states)

ADC A/D ConverterAMZ Adjustable Mach ZehnderDSP Digital Signal ProcessorLO Local OscillatorPD Photo DetectorPBS Polarization Beam Splitter LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ

Optical Circuit Electronic Circuit

PBS

PBS


LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS

1

Step 1: Separate the X and Y polarizations


ADC A/D ConverterAMZ Adjustable Mach ZehnderDSP Digital Signal ProcessorLO Local OscillatorPD Photo DetectorPBS Polarization Beam Splitter



LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS

2

Step 2: Mix the two optical sources – signal and local oscillator





LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS

3

Step 3: Extract interference patterns in the AMZ array





LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS

4

Step 4: Convert optical signals to analog electronic signals

Coherent Detector In More Detail: One wavelength




LO

PD

PD

PD

PD

ADC

ADC

ADC

ADC

DSP

AMZ

AMZ

AMZ

AMZ


PBS

PBS

5

Step 5: ADC converts analog to digital and DSP will process





Digital Signal ProcessingAt the core of coherent technology

CD C

omp.

CD C

omp.

Clock Recovery

PMD

Com

pens

atio

n

SD-F

EC

Cloc

k Re

cove

ryCl

ock

Reco

very

Deci

sion

and

Soft

Met

ricDe

cisio

n an

d So

ft M

etric

ADC1Ix

ADC2Qx

ADC3Iy

ADC4Qy

X-Pol Signal

Y-Pol Signal

I

Q

I

Q

21 3 4

1. CD compensation2. PMD compensation3. Intradyne clock recovery4. Soft Decision FEC


Digital Signal ProcessingCompensating for chromatic dispersion

Tx Rx

1 or 0?If an optical pulse had an

ideal infinitely narrow

spectrum

But in practice real lasers have a

finite linewidth

This decision is much harder!

One pulse may actually merge

into the preceding and

following pulses

Reversal of this “smearing” of the signal is done with equalization using digital filters in the time and/or

frequency domain


• Rapid changes in polarization need to be tracked• 2 polarizations are split into 4 linear equalizers• Peak PMD tolerance to 500ps• Tolerant to PMD transients of 10,000 rad/s

Digital Signal Processing, PMD CompensationInfinera PIC-Based, FlexCoherent Super-Channel Solution

Linear transversal equalizer. Taps are summed and weighted using algorithms:- adaptive least mean square (LMS) - constant modulus algorithms (CMA) equalization.

Rahn, J.; Sun, H.; Wu, K.T.; Basch, B.E. IEEE Journal of Lightwave Technology, 2012, Issue 99: Real-Time PMD Tolerance Measurements of a PIC Based 500Gb/s Coherent Optical Modem


Hard and Soft Decision FEC Evolution

log (BER)(Lower is better)

Optical Signal to Noise Ratio (OSNR)

Impact of better FEC is to drive down OSNR for a given BER

No FEC

1

Without FEC, a high OSNR is needed to ensure a low Bit Error Rate (BER)1

HD-FEC

2

HD-FEC e.g Reed Solomon code, delivers >9dB of NCG 2

1st GenSD-FEC

3

1st Gen SD-FEC e.g convolution codes such as Viterbi, delivers >11dB of NCG

3

2nd GenSD-FEC

4

2nd Gen SD-FEC targeting 12dB of NCG4

DWDM vendor’s proprietary FEC solutions differentiate their

products from other vendors

Further FEC gains are getting harder and harder to achieve… “saturation” at about 12dB NCG


Super-Channels andPhotonic Integration


Can single wavelengths scale to higher data rates?Serial Electronics Performance Limitations

Transistors(000)

Clock Speed(MHz)

Performance

10M

100K

1K

10

1970 2010

Moore’s Law is still valid (# transistors on chip still scaling), but serial

processing rates have flat-lined

QPSK Data Rate per wavelength

Serial processing rate

100Gb/s 32GBaud

200Gb/s 64GBaud

400Gb/s 128GBaud

1.2Tb/s 384GBaud

For single wavelength transponders

Higher order modulation can help – but only with significant

reductions in reach


Scaling the DWDM Layer to >100G

A Super-channel implements multiple carriers in a single line cardSingle operational cycle, Single unit of capacity

600 Optical Functions Integrated250 Fiber Connectors Eliminated

Power/Space ReducedReliability Enhanced

PIC makes it practical

500Gbps Line Card (1.2Tbps+ Future)

Super-Channel


500Gb/s in a single line card

AOFX-1000-T8-1-C8

FC 1-4/1-6/1-8

Large Scale PICs Optimal for Super-Channels

500G RX PIC

500G TX PIC

10 channels of Tx

10 channels of Rx


• Coherent has made it possible to break the “10Gb/s Barrier”

• The state of the market today (PM-QPSK) is:

A Review of First Generation Coherent Transmission

100Gb/s per wavelength

4,500kmreach

9.5Tb/sper fiber

Coherent super-channels add operational scale to these capabilities


GÉANT network upgrade


• DTN-X solution from Infinera

Why Infinera?

• Photonic integration• Large pools of “virtualised” bandwidth

500G super channels

• Ease of use

• Excellent service wrap

For the optical layer GÉANT selected…


Convergence

ZONE

Backbone Network ArchitectureA major upgrade & rationalisation

Fibre Leased Circuits

GRBE TR ILEE LV LTSK

HR SI

UK

NL

DE

FR

ES DK

CZ

AT

IT

HU

CH

RO BG

PL

IE

MT CY

“Fully featured POPs” Off fibre netPOPs

IP/MPLS only POPs NREN POPs

(RouterlessPOPs)

Circuitsover GÉANT Leased

circuits

MK RS

ME

DWDM

TDM (SDH)

IP/MPLS

Packet Transport (IP/MPLS)PT LU

RU

Off fibre POPs

FRITDE

Hamburg AAP Marseille AAP

Milan (GARR) AAP

Converged Packet Transport Platform Leasedcircuits


• In GN3 the GÉANT dark fibre footprint has been re-procured.

• On most routes DANTE was able to negotiate a temporary second fibre pair to support migration.

• The fibre footprint update provide the opportunity to remove some shared-risk points in the network.

• A new flexibility point has been added to the network in Marseilles

• An AAP has been added in Hamburg and Milan.

• Significant improvements have been made to diversity around Geneva.

The shared risks have now been removed from both the east and west routes out of Geneva.

Re-procurement in GÉANT3Dark Fibre and Additional Access Points (AAPs)

New GÉANT day-1DF footprint

Out fromGeneva

New diversity east of Geneva

Improved diversitytowards NL & DKImproved Frankfurt diversity


Alien Waves


Field Trial

• GÉANT has made use of SURFnet’s dark fibre to carry GÉANT DWDM trunks.• Field trial is now complete and showed excellent results

• Production grade service is now planned

39

Alien WavesDo aliens and lasers mix?

Alien Waves

• ‘Alien’ refers to data transmission laser light from 3rd party equipment• An alien wave system multiplexes alien light together with local signals

using Dense Wavelength Division Multiplex (DWDM) technology.


Alien WavesEconomic theory

• Economics textbook theory shows that a system with fixed and variable costs has a break-even point.

• At a fixed price, low volumes result in a loss.


• Alien waves allow multiple providers to share one common fibre infrastructure

• This means that there will be more services on the fibre, and the fixed cost are shared across many services, reducing the overall cost.

• The collaboration model agreed between Surfnet and GÉANT is for the fixed costs to be shared equally between each of the providers using the common fibre infrastructure.

• In the case of Amsterdam to Hamburg this is Surfnet, Nordunet and GÉANT. In future this may also include PSNC.

• Common costs are reduced by 2/3s compared to GÉANT only fibre.

• Solution also retains GMPLS control plane and ability to rapidly turn up new services.

• TeleGeography: “huge demand (for alien waves) from nontraditional operators”

Alien WavesEconomics in practice


Alien WavesAmsterdam-Copenhagen-Hamburg-Frankfurt fibre

• Two stretches of TeliaSonera fibre

• Ams-Cop• Fra-Cop

• Telia Sonera fibre expires 2015.

• Decision not to renew the contract.

• Copenhagen PoP will close

• Need are new solution for Amsterdam to Hamburg


Alien WavesEliminating redundant fibre infrastructure

• R&E community has two parallel fibre systems between Amsterdam and Hamburg.

• Yellow cable is Surfnet.• Green cable is GÉANT.

• Duplication of infrastructure dilutes utilization.

• Objective 1: remove one fibre system and share remaining fibre

• Objective 2: retain GMPLS control plane

• Solution: alien waves


Phase I

• Will test the technology and operational procedures.• Objective is to understand if GÉANT can make use of SURFnet’s dark fibre to carry our DWDM trunks.

Fixed Filters

• Phase I has used fixed filters to insert alien waves• Convenient for fast trial, but does not scale well

• After successful competition of trail, a production solution will use ROADMs

• ROADMs allow waves to be remotely turned up and reduces inter-site fibres

44

Alien WavesField trial setup


AOLX-500

AOLM-500

AOLX-500

OTC OTC

DANTE Amsterdam

DANTEHamburg

DTN-X

AOLX-500

AOLX-500

SURFnetAmsterdam

6500 CPL6500 CPL

SURFnetHamburg

AOLM-500

Pair of loan AOLMs in the blue spectrum

(OCG 2)

Existing DANTE fibre

Switch Matrix

ATC ATC

N x amps

N x amps

SURFnet/Cienaline system

Pair of loan ATCs with OLA and OFM-4-D

Existing AOLX on OCG 7

AOLX-500

Switch Matrix

DTN-X

Alien WavesField Trial Connectivity


1,00E-07

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

-30 -25 -20 -15 -10

pre-

FEC

BER

channel Rx power (dBm)

AOLM-500-T4-1-C5 Hamburg Pre-FEC sensitiviy

Ham 1556.56nm

Ham 1554.94nm

• Receive sensitivity measured on two wavelengths in each direction

• Better than 10 dB receive margin in both directions – i.e power into Rx can fall by 10dB before errors are seen

1,00E-07

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

-30 -25 -20 -15 -10pr

e-FE

C BE

RChannel Rx power (dBm)

AOLM-500-T4-1-C5 Amsterdam re-FEC sensitiviy

Ams 1556.56nm

Ams 1554.94nm

Alien WavesTest results – excellent receive margin


Infinera pre-FEC Q out of range alarm

Infinera Loss of Frame alarm

Alarms and notification aid debugging: four types signal deterioration can be distinguished1. If there is a loss of fidelity of the signal, this can be detected using pre-FEC threshold crossing notification2. If the alien wave becomes so degraded that the Q-factor drops below 10, Infinera will raise a pre-FEC Q

out of range alarm3. A cut in the SURFnet fibre results in a Loss of Frame (LOF) alarm, some signal from the optical amplifiers

leaks through, so there will not be a Loss of Signal (LOS) alarm.4. If the local patching between GÉANT and SURFnet sites is broken, then a Loss of Signal (LOS) alarm is

raised.

Alien Waves Test results – Infinera alarms allow debugging of alien wave


Alien WavesIn production - flexible solution uses ROADM technology

• GÉANT has signed a contract with SURFnet for alien waves from Amsterdam to Hamburg.

• SURFnet ROADMs have been upgraded to colourless add-drop.

• Now multiplexed set of 10 x 50Gbps waves from the Infinera PIC can be injected straight into the Ciena ROADM

• Reduces the number of optical patch cords between GÉANT and SURFnet racks.


The future


• First generation coherent focused the Digital Signal Processing on the Receiver

• Next goals for coherent technology:1. Is there a way to achieve higher spectral efficiency?

Including more flexible use of fiber spectrum2. Can we manage Chromatic Dispersion even more than today?3. Non-linear effects are now the dominant impairment

• Is there a way to combat these effects?

• Fundamental difference is the addition of Digital Signal Processing in the Transmitter

What are the problems we’re trying to solve?


Evolving Transmitter Technology

S1

S2

90

• Today: 100Gb/s PM-QPSK• 16-35GBaud symbol rate• 4,500km reach

Soon: 200Gb/s PM-16QAM 16-35GBaud symbol rate 700km reach (real world)

DAC

DSP

LaserNote that serial symbol rates are not increasing

Showing one wavelength for simplicity


What Functions do the DSP and DAC perform?

S1

S2

90

DAC

DSP

Laser

4 Important functions1: High order modulation

2: Pulse Shaping

3: Pre-dispersion

4: Non-linear compensation


Adding Even More Bits Per Symbol

How wide is the peak?

50GHz

PM-QPSK: 100Gb/sPM-16QAM: 200Gb/s

QPSK 16QAM2 bits 4 bits

Double this up for Pol-MuxingPM-QPSK

4 bitsPM-16QAM

8 bits


QPSK 16QAM2 bits 4 bits

Modulation order

PMBPSK

PMQPSK

PM8QAM

PM16QAM

C-Band Capacity (Tb/s)

5

10

15

20

Typi

cal R

each

( ,0

00 k

m)

5

10

Adding Even More Bits Per Symbol

You don’t get something for nothing!

Double this up for Pol-MuxingPM-QPSK

4 bitsPM-16QAM

8 bits


BPSK 1 bit per symbol

FlexCoherent™ Line CardsNote: All modulations are polarization muxed (PM)

QPSK 2 bits per symbol

16QAM 4 bit per symbol 16QAM

QPSK

BPSK

Fiber CapacityReach

3QAM 8QAM

3QAM

8QAM

“Intermediate modulations” can have unusually high efficiency value

over the installed base of fiber


Without DSP and DAC

Intelligent Pulse Shaping

With DSP and DAC: signal from the side lobes has been intelligently incorporated

into the main pulse

We can decrease channel spacing, and increase total fiber capacity, without a reach penaltyICI

Too close a channel spacing will result in a reach penalty


Pulse-ShapedQPSK Spectrum

• Using a DAC, driven by a DSP in the transmitter it’s possible to “shape” the pulse intelligently

• Pulses can be spaced “at the Baud rate”• Eg: 32GBaud signal could be spaced at just over 32GHz• The additional spacing is known as the alpha, and the

practical limit for alpha is 3-4% before OSNR penalties are incurred50GHz

Nyquist DWDM

Laser Mod

DSP DAC

Transmitter

Alpha


• Increase fiber capacity by removing guard bands

• Removal of channel filters is made possible by coherent receivers.

• Creates a trade off between fiber capacity and routable elements

FlexGrid Line SystemsFewer guard bands

1.2 Tb/s462.5 GHz

Multi-CarrierSuper-Channel

12 x 100G Waves

Conventional Per-Channel DWDM Filtering

1.2 Tb/s600 GHz

12 x 100G Waves


FlexGrid Line SystemsHow many carriers?

• Closer packing of wavelengths possible today

• Modulation schemes with higher bit rates will require advances in silicon


Pre-dispersion

Tx Rx

1 or 0?We know a signal will be dispersed as it travels along the fiber

Using DSP and DAC we can apply negative direction of

dispersion before transmission

Dispersion behavior can be tightly

controlled

In reality we would divide up the job of dispersion compensation

between Tx and Rx


• Promising demonstrations of NL Compensation gains of >1dB OSNR improvements• An array of complex algorithms:

• NPCC: Nonlinear Polarization Crosstalk Correction• AFCPR: Adaptive Fee-Forward Carrier Phase Recovery• RF Pilot waves• DBP: Digital Back Propagation• Tx Pulse Pre-dispersion (aka pre-compensation, pre-emphasis)

Non-Linear Compensation

Expect to see commercial implementation over the next

few years


Next Generation CoherentThe End Result

Extended C-Band capacity increases from 9.5Tb/s to 12Tb/s using PM-QPSK. PM-8QAM delivers 18Tb/s, and PM-16QAM delivers

24Tb/s.Tx Pulse Shaping

Most applicable on submarine links, but allows for a 5X increase in Chromatic Dispersion compensation.

Dispersion management

Algorithms are still evolving. Longer term expectation is to deliver around a 1dB OSNR improvement.

Non-linear compensation


Thank You!


63

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Documents

Optical and DWDM technology - DFN · PDF fileOptical and DWDM technology. Lübeck, 8 June 2015. Transport Network Architect, GÉANT. Networks ∙ Services ∙ People . . Objectives