bell labs transmission cohérentesur fibre optique

1 © Nokia 2016

Bell Labs

Transmission cohérente sur fibreoptiqueCFOR: Technologies des réseaux à fibres optiques multi-terabit/s

• Gabriel CHARLET

• 1er Dec. 2016

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2 © Nokia 2016

Introduction

• Long haul optical systems transport almost all data traffic (>99%).

• Undersea optical systems are the successor of the first telegraphic undersea cables !

• Transport data, video, internet, voice (fix & mobile) over a single network.

• Initial deployment of optical networks in the 1980s.

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• Direct detection

• Basics of coherent detection

• Implementation of coherent transponder

• System performances obtained thanks to coherent

• Conclusion

OUTLINE

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• Benefits from fiber optics:

- Low attenuation ~0.2dB/km

- Wide bandwidth with low attenuation (>>10THz)

- Efficient optical amplification (Erbium doped fiber amplifier over 2 x 5THz)


Optical data transmission

Encoding bits on an optical signal, fiber transmission and reverse operation

Fiber optics

RX

0100101110110101 0100101110110101 electronic

optic

TX

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Point to point WDM system

TX1 λ1

TX2 λ2

TX λ80

RX1 λ1

RX2 λ2

RX λ80

Mux

Fiber

Demux

Optical amplifier

• Typically 80 to 100 channels (wavelengths) with 50GHz spacing

• 100-200Gb/s per channels typical in 2016

• Span length : 50 to 100km (10 to 30dB span loss typically)

• 10-20 amplifiers for terrestrial systems, up to 200 for undersea systems !

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Electrical field and spectrum of laser

f

Constellation diagram SpectrumEvolution of optical field

Ex

0 1 Re

~5fs (@1.55µm)

Im

t

• 1.55µm used in long distance telecom industry to benefit from the lowest attenuation of fiber and of optical amplification from EDFA.

• This wavelength corresponds to a frequency of ~200THz

Laser


Modulation format, constellation diagram, spectrum

2 x B


Ex 10 011

0 1

OOK

Re

25ps (@40Gb/s) ~5fs (@1.55µm)

Im

t

• Constellation diagram represents the field of the electrical signal at the center of eachsymbol.

• Spectrum width (of first lobe) is twice the modulation speed

Laser Modulator

B

8 © Nokia 2016

Direct detection

Direct detection is the conventional way to detect optical signal.

= QUADRATIC detection of electric field,

Optical signalElectrical signal~|As(t)|²

Ex 10 011

25ps (@40Gb/s) ~5fs (@1.55µm)

t

Decision element

1 0 1 1 0

9 © Nokia 2016





• Conclusion

OUTLINE


Modulation format and constellation diagramPhase shift keying

2 x B


Ex 10 011

0 1

OOK

Re

25ps (@40Gb/s) ~5fs (@1.55µm)

2 x B

Ex 0 π 0 0 π BPSK0π

Re

Im

Im

QPSK5π/4 3π/4π/4Ex

5π/4

3π/4

−π/4

π/4

2 x B/2Re

Im

t

OOK : on off keyingBPSK : binary phase shift keyingQPSK : quadrature phase shift keying

11 © Nokia 2016

Differential detection

Direct detection is the conventional way to detect optical signal.

= QUADRATIC detection of electric field,

Differential detection= Optical demodulator + direct detection

- Suited for detection of phase modulated signal

Optical signalElectrical signal

Optical signalElectrical signal

~|As(t)|²

~|As(t)+As(t+T)|²

As(t)

As(t+T)

12 © Nokia 2016

Coherent detection= LINEAR detection of the electric field, by beating with local oscillator (LO)

- LO frequency = Signal frequency � homodyne detection

- LO frequency ≠ Signal frequency � heterodyne detection

- LO frequency ≈ Signal frequency � intradyne detection


Coherent detection

Optical signal

Local oscillator (LO)

Electrical signal

~As.ALO

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• No optical amplifier available at that time.

• Transmission distance was mostly limited by fiber attenuation.

• « Amplification » provided by strong local oscillator.

• Amplitude shift Keying (ASK) vs Phase shift Keying (PSK) ?


Motivation for coherent detection in the 1980’s

150km fiber 30dB

150km fiber 75km 45dB

As.ALO vs As.As


Coherent detection in the 1980’s

Homodyne or heterodyne ?

f

Modulatedsignal

BW

LO


Coherent detection in the 1980’s

Homodyne or heterodyne ?

But EDFA was invented…

f

Modulatedsignal

BW

LO

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• Technological evolutions:

- Availability of high speed ADC (high bandwidth, high sampling rate)

- Strong progress of CMOS digital signal processing capabilities

• Optical transmission landscape

- Phase shift keying modulation used again! (with differential detection) DPSK, DQPSK

- Electronic Signal processing (FFE, DFE,… for chromatic dispersion, PMD… mitigation)

- Willingness to increase spectral efficiency


Renewal interest for coherent detection ~2005

First “modern” coherent detection experiment in 2004-2005

17 © Nokia 2016

Coherent detection= LINEAR detection of the electric field, by beating with local oscillator (LO)

- LO frequency = Signal frequency � homodyne detection

- LO frequency ≠ Signal frequency � heterodyne detection

- LO frequency ≈ Signal frequency � intradyne detection


Intradyne coherent detection

Optical signal

Local oscillator (LO)

Electrical signal

~As.ALO


Interference at coherent mixer outputCoherent mixer or 90° hybrid

))(( ttis

seA ϕω +

Coherent Mixer

)]()sin[(4)(2 ttAAtI loslos ϕωω +−=

)]()cos[(4)(1 ttAAtI loslos ϕωω +−= In phase “I”

Quadrature “Q”

tilo

loeA ω(1)

(2)

(1)+(2)

(1)-(2)

(1)+j(2)

(1)-j(2)Local oscillator

Signal

)()()( 432 tPDtPDtI −=

)()()( 211 tPDtPDtI −=+-

+-

π

0

π/2

−π/2

)]()cos[(2)).((22*

1 ttAAAAEEEEPD olsolsolsolsols ϕωω +−++=++=

)]()cos[(2)).((22*

2 ttAAAAEEEEPD olsolsolsloslos ϕωω +−−+=−−=

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Polarization diversity receiversignal polarization fluctuate over time => polarization diversity RX required

0100011…

Q//Pol. splitter

Local oscillator

ADC

ADC

ADC

ADC

DS

P

I

Q

I//signal

Coherentmixer

3dB

//

Coherentmixer


Polarization division multiplexing: PDM

Double the bit rate transported

QPSK5π/4 3π/4π/4Ex

5π/4

3π/4

-π/4

π/4

2 x B/2Re

Im

QPSK5π/4 3π/43π/4Ey

5π/4

3π/4

-π/4

π/4

2 x B/2Re

Im

PDM QPSK

5π/4

3π/4

-π/4

π/4

Re

Im

5π/4

3π/4

-π/4

π/4

Re

21 © Nokia 2016





• Conclusion

OUTLINE

22 © Nokia 2016

Digital coherent receiver versus direct-detected systems…

10 Gb/s NRZ receiver(direct-detection)

Coherent receiver

signal

Singly-polarized

signal

Local oscillator

Coherentmixer

signal

ADC

Polarization-multiplexed signal

X

Y

arbitrary state of polarisation

(SOP)

Analog-to-Digital

Converters

Digital Signal

Processing

BER

Photodiode

Decisiongate

Amplitude, phase and

polarization!!

just intensity…

X

Y

DSP


Coherent receiver

Digital signal processing

CoherentMixer

Local oscillator

Sampling(scope)

DSP(PC)

Fiber out

Storage(scope)

ADC

ADC

ADC

ADC

j

j

(I+jQ)//

(I+jQ)

Pol

ariz

atio

n D

emul

tiple

xing

and

Equ

aliz

atio

n

Car

rier

freq

uenc

yan

dP

hase

Est

imat

ion

Sym

bol

iden

tific

atio

n

BE

R &

Q²-

fact

or

stor

age

CD

com

pens

atio

n

+

+

scope

100Gb/s PDM-QPSK (experimental data)


Coherent receiver


CoherentMixer

Local oscillator

Sampling(scope)

DSP(PC)

Fiber out

Storage(scope)

ADC

ADC

ADC

ADC

j

j

(I+jQ)//

(I+jQ)

Pol

ariz

atio

n D

emul

tiple

xing

and

Equ

aliz

atio

n

Car

rier

freq

uenc

yan

dP

hase

Est

imat

ion

Sym

bol

iden

tific

atio

n

BE

R &

Q²-

fact

or

stor

age

CD

com

pens

atio

n

+

+

scope



Coherent receiver


CoherentMixer

Local oscillator

Sampling(scope)

DSP(PC)

Fiber out

Storage(scope)

ADC

ADC

ADC

ADC

j

j

(I+jQ)//

(I+jQ)

Pol

ariz

atio

n D

emul

tiple

xing

and

Equ

aliz

atio

n

Car

rier

freq

uenc

yan

dP

hase

Est

imat

ion

Sym

bol

iden

tific

atio

n

FE

C

stor

age

CD

com

pens

atio

n

+

+

scope


28 © Nokia 2016

• Each symbol has a given spectral extension (~Baudrate).


Chromatic dispersion compensation block

t

f

S1 S2 S3 S4 S5 S6 S7 S8

29 © Nokia 2016

• Each symbol has a given spectral extension.

• After transmission and chromatic dispersion impact, symbols overlap each others

• Frequency domain implementation to delay some frequencies and resynchronize all frequency components from transmitter symbol.

• Each symbol can overlap > 1000 symbols for transoceanic links.


Chromatic dispersion compensation block

t

f

30 © Nokia 2016

• 2010: first apparition of high speed CMOS ADC

• 56GS/s sampling rate, 8 bit resolution and ~15GHz analog bandwidth

• Total amount of data to be processed for 100Gb/s PDM-QPSK: 4x56x8= 1.8Tb/s!

• Unpractical to exchange 1.8Tb/s between 2 separate chips => single chip required for ADC and DSP. CMOS required for low power DSP.

• In 2010, first ADC-DSP chip with 65nm CMOS technology including

• 4 ADCs (56GS/s, 8bits)

• CD compensation (~2000km)

• Clock recovery

• MIMO 2x2 (polarization deMux, equalization)

• Optical carrier frequency estimation

• Optical carrier phase estimation

• FEC (product code BCHxBCH)


ADC and DSP in a single CMOS chip

31 © Nokia 2016

MIMO processing (polarization demultiplex. & equalization)

• Equalization is NOT done thanks to channel estimation (through training sequence or pilot), NOR by using decision on the symbol after full DSP processing.

• Blind equalization using only the intensity information (before phase processing). => no overhead associated with training sequence or pilot.

+

+

feedback

xout

FIR filter (N taps)hxx

hyy

hyx

hxy

Butterfly structure

yout

xin

yin


Polarization Mode Dispersion (PMD) and coherent detection

• PMD caused by fiber optics imperfections (stress, non perfect circularsymmetry…).

• PMD is a stochastic effect. Differential group delay (DGD) can be ~3 times larger than average DGD value.

• PMD has been a major limitation for the deployment of 40Gb/s systems.

• But entirely solved with MIMO equalizer of coherent receiver !

z

x

y

z0

zL

t

x

y

à z0

t

x

y

à zL

DGD (ps)

33 © Nokia 2016

• Phase estimation is a key part allowing to avoid the use of « optical phase locked laser » of the 1980’s.

• Preceded by coarse frequency estimation to allow for several GHz of frequencyoffset between TX and RX laser.

• Various algorithms available, here for QPSK.


Carrier phase estimation (CPE)Required because intradyne detection is used

∑( · )N(.)4 arg( · )/4 unwrap

Ca

rte

sia

n=

> P

ola

r

delay

Po

lar=

>

Ca

rtesia

n

Removedata

Average noise

φ+κπ/2+Ν4φ+4Ν 4φ φ

+π/4

34 © Nokia 2016

• Optical communication requested BER is 10-13 (or even 10-16).

• Maximization of transmission reach and/or capacity is obtained through the use of forward error correction similarly to any other digital systems.

• But processing of up to 500Gb/s in a single chip !

• Code rate (r) vs overhead (OH):

• Code rate r=0.8 => overhead 25%

• Code rate r=0.5 => overhead 100%

• 2000: 7% overhead and code RS 255-239: FEC threshold 10-4 (Q~11.5dB)

• 2003: 7% overhead and code BCHxBCH: FEC threshold 4 10-3 (Q~8.5dB)

• 2012: 25% overhead and “soft decision” LDPC (low density parity check): FEC threshold ~2 10-2 (Q~5.5dB)


Bit error rate and forward error correction (FEC)

)1/1( −= rOH

K bitsFEC encoding

K bits Overhead

N bits

NKr /=Code rate

35 © Nokia 2016

100

1000

10000

2010 2014 2018 2022 2026

0

20

40

60

80

100

120

2010 2014 2018 2022 2026

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Maximizing bit rate per device to minimize cost per bitMaximizing baudrate, format level & carrier count per ASIC

Ba

ud

rate

(Gb

au

d)

Bit

rate

(Gb

it/s

)

200G

1T

65

nm

40

nm 28

nm

Tremendous progress of high speed optical coherent interfaces

100G

Dual λ400G

X2 for each generation

36 © Nokia 2016

multi-channel photonic integration:

• Dual PDM I/Q modulator

• dual tunable laser

• dual coherent receiver

• dual ADC/DSP/DAC chip…


Opto-electronic components

PDM I/Q modulator (LiNbO3, InP, siPho)

Quad driver (InP, GaAs)

Coherent receiver (InP, SiPho, free space…)

ADC, DSP, DAC (CMOS)

Tunable laser (InP)

37 © Nokia 2016

Spectrum shaping thanks to DAC (Digital to Analog Convertor) at TX

Spectrum shaping thanks to digital filter in transmitter to reduce spectrum occupancy and increase spectrum efficiency

Time domain(impulse response)

Frequency domain(spectrum)

t

Sinc

T

f

B=1/T

Rect

f t

T2B

Log

scal

eAmplitude

Wavelength (0.1nm/div)

Pow

er (

5dB

/div

)

“NRZ”RRC 0.01

2B

1.01B

39 © Nokia 2016

1

2

4

8

16

32

64

2000 2005 2010 2015


Progress in submarine capacity per fiber(transmission distance > 6000km)

Ca

pa

city

(Tb

/s)

10G

40G

100G

200G+

400G

Strong capacity increase thanks to coherent detection

Direct/diffdetection

coherentdetection


Shannon limitFor dual polarization signal, AWGN channel

0

2

4

6

8

10

12

14

-2 2 6 10 14 18 22

Info

rma

tio

n b

its

pe

r sy

mb

ol

SNR [dB]

QPSK

16QAM

64QAM

C=2. log2(1+SNR)


Example of recent record experiments

0

10

20

30

40

50

60

70

2008 2010 2012 2014 2016 2018

Ca

pa

city

(Tb

/s)

15.5Tb/s155x100Gb/sHD-FEC

38.7Tb/s155x250Gb/s16QAMSpectrum shapingSD-FEC

65Tb/sProbabilistic shaped64QAMNonlinear mitigation

42 © Nokia 2016

• Highest capacity over submarine distance.

- Probabilisticconstellation shaping

- Nonlinear mitigation

- Adaptive rate FEC


65Tb/s transmission over 6,600kmThanks to Probabilistic Constellation Shaping and NL mitigation

Highest capacity, close to expected limits

1

2

3

4

5

6

7

0 5 10 15 20 25

GM

I [b

its/

sym

bo

l/po

lar.

]

PS64QAM

SNR [dB]

Shannon

5.4

64QAM

Probabilistic constellation shaping: PCS 64QAM

43 © Nokia 2016

• Optical channel is not an arbitratry white Gaussian noise (AWGN) channel.

• Kerr nonlinearities because of propagation through optical fiber.


Nonlinear mitigation

TX CDComp

MIMO2x2

Phase Est.Fiber optic

channel

062

.2

2

3

3

32

22 =+

∂∂−

∂∂−+

∂∂

AAT

Ai

T

AA

i

z

Ai γββα

Kerr nonlinearities

Attenuation Dispersion

FEC

44 © Nokia 2016

• Optical channel is not an arbitratry white Gaussian noise (AWGN) channel.

• Kerr nonlinearities because of propagation through optical fiber.


Nonlinear mitigation

TX MIMO2x2

Phase Est.

FECFiber opticchannel

062

.2

2

3

3

32

22 =+

∂∂−

∂∂−+

∂∂

AAT

Ai

T

AA

i

z

Ai γββα

Kerr nonlinearities

Attenuation Dispersion

CD

NL

CD

NL

CD

NL

46 © Nokia 2016

• Coherent detection has revolutionized the world of optical communication.

- Removed the need for optical dispersion compensation

- Solved the PMD issue

- Practical way to use the 2 polarization of the light

- Drastically Increased the spectral efficiency

• We are now approaching the spectral efficiency limits of fiber optics…


Conclusion

400Gb/s on a single wavelength !

bell labs transmission cohérentesur fibre optique

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