nonlinear impairments in high - ieeegrouper.ieee.org/groups/802/3/10gepon_study/public/... ·...


Line Rate Evolution

• Explosion of interest to fiber nonlinearities in the fiber optic system started with introduction of 10 Gb/s systems due to necessity to use higher optical power for higher bit rates

0.01

0.1

1

10

100

1980 1985 1990 1995 2000 2005 2010Year

Bit

Rat

e (G

bit/s

)SONET/SDHEthernetFiber Channel


Where Fiber Nonlinearities Were Studied?Long Haul WDM Transmission System

• Attributes of the long haul (LH) WDM transmission system– Link length 500-1500 km with span lengths of 80-100 km– Bit Rate 10 Gb/s, 100 or 50 GHz channel spacing on ITU grid– C band (1530 – 1565 nm) and L band (1570-1625 nm) – Erbium Doped Fiber Amplifier (EDFA) with 2 stages for DCM

RXOptical

Amplifier

Fiber/Cable

Terminal EquipmentTerminal Equipment

Dem

uxM

ux

Span 80-100 km

Link 500-1500 km

TX

DCM

2nd stage1st stage


Passive Optical NetworksLower Nonlinearity due to Shorter Length

EPO

N

CENTRAL OFFICECENTRAL OFFICE

B-G

PON

1:32 (64)

EthernetPON

B or G-PON ITU-T G.983/984

PassiveSplitter

PassiveSplitter

1:32

IEEE 802.3ah

• Attributes of PONs– Link length up to 20 km, split ratio up to 64– Bit rates up to 2.5 Gb/s (10 Gb/s is considered)


Nonlinearities Fiber Impairments

Nonlinearity System impacted How it impact the system

Stimulated Brillouin Scattering PON

Limits launched optical power, increases noise.Impairs video signal through CNR, CSO and

CTB degradation

Stimulated Raman scattering

LH (WDM), PON

Transfers power from blue channel to red channels i.e. effectively increasing attenuation

for blue channels. Raman Xtalk in PONs.

Cross-phase modulation LH (WDM)

Distortion of the phase of adjacent transmitted channel that is converted into amplitude

distortion.

Self-phase modulation LH Distortion of the phase of transmitted optical pulse that is converted into amplitude distortion

Four wave Mixing LH (WDM) Generation of the FWM tones that coincide with other channels. Appears like nonlinear noise

Stimulated Brillouin Scattering

(SBS)


Experimental Observation of SBS

backscattered light in forward direction

optical fiber with given index profile

Reflected power

Input (signal) power

νB~10 GHz

ΔνB

γB

Freq

Pow

er

νreflectedνsignal

-40

-30

-20

-10

0

10

20

0 2 4 6 8 10 12 14 16 18 20

input power, dBm

refle

cted

pow

er, d

Bm

Fiber 2

Fiber 1

Transmitted power


• Brillouin scattering occurs due to interaction of light with acoustic waves

• How it originates: thermal motion of molecules

pressure fluctuations (acoustic waves)

refractive index variations

light scattering

• Most efficient scattering occurs in a backward direction and induces a Doppler frequency shift of the scattered light

Spontaneous Brillouin Scattering


Stimulated Brillouin Scattering

• Backscattered light (Stokes wave) from acoustic noise interferes with the input optical wave

• An acoustic wave is generated due to electrostriction (increase in the material density in regions of high optical intensity)

• The acoustic wave further stimulates Brillouin scattering• Brillouin scattering reinforces the acoustic wave and so on...

input optical wave

backscattered (Stokes) wave

input + reflected interference

acoustic wave

electrostriction⇓


SBS limits maximum launch power to the fiber• To maximize the number of network users or/and transmission

distance or bit rate high input optical power is required

Analog Transmitter

SiSiHead End

SiSiSiSiSplitter

End User Optical Receivers

●●●

Fiber lossSplitter loss

• Optical power launched in the fiber is limited by SBS

Pin

Pref

Pout

Pin

P out

P ref

Fiber 2Fiber 1


SBS Impact on Digital Transmission• Creates additional noise from

forward scattered Stocks component

Misc MarginRX Sensitivity

(e.g. -28 dBm for 622 Mb/s)

Launched Power (e.g. 6 dBm)

FiberLoss

SplitterLoss

Connectorsand splices

• Limits power budget and reflects signal in Tx

34 d

B


SBS Threshold Enhancement: Dithering

SBS threshold is ~7 dBm

Threshold is ~17dBm

Phase ModulationAmplitude

Phase ModulationFrequency

AM

PLI

TUD

EA

MP

LITU

DE

FREQUENCYFREQUENCY

SBS THRESHOLDSBS THRESHOLD

No suppressionNo suppression

With Phasemodulation

With Phasemodulation

∏=

φ+π=N

1nnnn0 )tf2cos(iAexp(E)t(E

• Broadening linewidth spreads optical power across a wider spectrum, so peak power is below threshold but total power is increased

• “State of the art” externally-modulated transmitters typically have maximum SBS-limited output power of ~17 dBm.

• Broadening linewidth spreads optical power across a wider spectrum, so peak power is below threshold but total power is increased

• “State of the art” externally-modulated transmitters typically have maximum SBS-limited output power of ~17 dBm.


SBS Threshold Enhancement: Fiber Design

-35

-30

-25

-20

-15

-10

-5

0

5

10

10 11 12 13 14 15 16 17 18 19 20 21 22 23

Input Power (dBm)*

Bac

ksca

ttere

d Po

wer

(dB

m) Standard Single-

Mode FiberSBS Optimizedfiber

3 dB

SBS optimized fibers allow doubleoptical power with no degradation of

video quality

SBS optimized fibers allow doubleoptical power with no degradation of

video quality

Picture quickly degradesabove standard SBS threshold

SBS optimization allows the same optical powerwith no degradation

in picture quality

*Impact of SBS will vary with distance, sohigher powers may be supported at shorter distances. Values here are measured at 50 km.

*Impact of SBS will vary with distance, sohigher powers may be supported at shorter distances. Values here are measured at 50 km.


5 6 7 8 9 10 11 12 13 14 1533

36

39

42

45

48

51

CN

R (

dBc)

O ptical Input Pow er (dBm )

CH02 (55.25 MHz) Carrier-to-Noise Ratio

5 6 7 8 9 10 11 12 13 14 1535

40

45

50

55

60

65

- C

SO (

dBc)

O ptical Input Pow er (dBm )

CH02 (55.25 MHz) Com posite Second Order D istortion

Analog Distortion Measurements :Fiber 1, Fiber 2 and SBS Optimized Fiber

Fiber 1Fiber 2SBS Optimized Fiber

*No Dithering is used in these measurements


Summary: Stimulated Brillouin Scattering• SBS limits power of the digital signal launched in the fiber

and thus limits max transmission distance or split ratio– Major degradation is due to increased insertion loss of fiber

and lower optical power at the Rx

• SBS can be mitigated by – Dithering (spectral broadening of the launched signal)– Using fibers with increased SBS threshold

• Analog signal is much more sensitive to SBS because – Requires high power at the Rx and launched power– CNR and CSO degrade with the onset of SBS

Stimulated Raman Scattering

(SRS)


Experimental Observation of SRSRaman pump (contra-)optical fiber

Amplified Signal PS

Raman pump (co-)

Signal (PS)

0

0.1

0.2

0.3

0.4

1450 1470 1490 1510 1530 1550 1570 1590 1610 1630 1650

Wavelength (nm)

Ram

an G

ain

(1/k

m-W

)

energy transfer(phonon emission)

pppsRp

sp

sspsRs

PPPCdzdP

PPPCdzdP

α−λλ

−=

α−=

αs

αp

( )⎥⎥⎦

⎤

⎢⎢⎣

⎡−

α= α− z

p

p

Raman,eff

RR

pe1)0(P

AgexpG


Raman Amplification

GFF

TRANSM. BANDS C, L, ...1430-1500 nm

13 THz (≅100 nm @ 1550nm)

RAMAN PUMPS

AMPLIF.WINDOW

EDFA

RAMANPUMPS

HYBRID RAMAN-EDFA

Pow

er o

r Gai

n

λ

Virtual level

IL>Amplification

Pump photon

IV>

IU>

Phonon emission

• Amplification by stimulated Raman scattering became feasible due to progress in semiconductor pumps


RxEDFA

Pow

er (d

B)

Distance

Signals without Raman Raman Pump

Raman Amplification

Signals with Raman

Higher signal power at EDFA –

Higher OSNR

Raman Amplification OSNR ImprovementRaman Amplification OSNR Improvement

RAMANPUMPS

HYBRID RAMAN-EDFA

Tx

• Distributed Raman amplification improves OSNR by 3-5 dB


SRS Induced Impairments in WDM Systems Signal to Signal Interaction (Raman Tilt)

• SRS transfers energy from the signals in the blue part of the WDM spectrum to the signal in the red part as channel

• Appears as a increased “tilt” in fiber attenuation, blue channelexperience excessive OSNR loss, can be mitigated by pre-emphasis

EDFA Fiber EDFA

λ

1565 nm1530 nm

energytransfer

λ

1565 nm1530 nm

34567 GAIN RIPPLE = 1.79

AMP 12

-17

-16

-15

RIPPLE = 1.51Pout (dBm)Pin (dBm)


Raman Crosstalk in PONs (ITU-T G.983/984) Wavelength Assignment

Video1550 nm

ONTONTWDM

Video

Data

POTS

EDFA(Erbium Doped Fiber Amplifier)

OLT(Optical Line Terminal)

1 x 32Splitter Data and

Voice1310 nm

Data and Voice

1490 nm

1310 nm 1490 nmDownstreamUpstream

Voice and Data

1550 nm

Video

Digital TV550 MHz 860 MHz42 MHz

Voice and Data

Wavelength Assignment

Analog TV HDTV/VOD


•Low power digital signal acts as Raman pump for high power analog signal

•Raman interaction causes two penalties

•Power depletion of digital signal

•CNR degradation of analog signal

Raman Interaction

Digital signalat 1490 nm

Analog signals at 1550nm

Raman GainSpectrum

CNR Degradation

Pump depletion


Pump depletion: theoretical model

• Use undepleted signal approximation

( )⎥⎥⎦

⎤

⎢⎢⎣

⎡α−−

αλλ

−= α− zePCPzP pz

s

sR

p

spp s1)0(exp)( 0

pppsRp

sp

sspsRs

PPPCdzdP

PPPCdzdP

α−λλ

−=

α−=

Extra loss due to Raman effect

• Average power penalty

effsR LPCκ=ΔdB

pse λλ=κ /)log10( 10

sL

eff seL α−= α− /)1(


0

0.5

1

1.5

0 3 6 9 12 15 18 21

Analog Signal Power (dBm)

Dig

ital S

igna

l Pow

er L

oss(

dB)

Digital Signal Depletion

• Digital signal suffers from loss of power as the analog signal power is increased

• Effect occurs at current launched analog power levels (~18 dBm) and gets worse at stronger power levels

• Overcome by launching higher power digital signal

Signal Depletion


CNR degradation

• CNR penalty decreases with subcarrier frequency and bit rate• Mitigated by pre-emphasis of subcarriers and higher bit rates

SMF NRZ, Pdigital = 0 dBm, L=10.6km

ΔCN

R (d

B)

Bit rate (Mb/s)

ΔCN

R (d

B)

Frequency (MHz)

TheoryExperiment

622Mb/s

CNR Degradation


Summary: Raman Cross-Talk Penalties• Raman amplifier based on SRS is employed in LH system

to boost OSNR and extend the system reach

• SRS between individual channels in WDM systems results in “Raman tilt” that results in higher effective attenuation forshort wavelength channel

• SRS can be impairment in PON systems– Depletion of 1490 nm digital signal in the presence of strong

analog 1550 nm signal

– CNR degradation of analog signal due to transfer of power variations from digital signal

Self Phase Modulation(SPM)

Cross Phase Modulation(XPM)


Self-phase modulation (SPM)Experimental observation

RXOptical

Amplifier

Fiber/Cable

Span 80-100 km

TX

DCM

2nd stage1st stage

Higher channel power


Theoretical DescriptionNonlinear Phase Shift

• Rigorous pulse propagation and SPM penalty calculation must be done by solving nonlinear Shrödinger equation

Dc2

2

2 πλ

−=β0EEiE

2iE

21

zE 2

2

2

2 =γ−τ∂

∂β+α+

∂∂ eff

2

An2

λπ

=γ

∫∫ ==L

ineff

L

NLNL dzzPzAzndzzPz

0

2

0

)()()(2)()(

λπγφ

• SPM penalty becomes significant when nonlinear phase φNLbecomes comparable to π


Self Phase Modulation (SPM) as combinedeffect of nonlinearity and dispersion

Dispersion convertsphase modulation in amplitude modulation

Time (ps)

Inte

nsity

(a.u

.)δω

+D-D

• SPM and linear dispersion can pull pulse apart (in –D fibers) or compensate linear broadening (in +D fibers) leading to a formation ofclassical soliton

from

G.P

. Agr

awal

, N

onlin

ear F

iber

Opt

ics

PAnL2)t(

eff

2effλ

π=ΔΦ

Nonlinearity createsphase modulation

-D+D

Step 1: Phase distortion

Step 2: Conversion to AM


Cross Phase Modulation (XPM)

• XPM is similar to SPM except the fact that phase distortion is created by adjacent channels

t

I,ω “pump” channelStep 2: Conversion to AMStep 1: Phase distortion

t

I,

t)t(P2

AnL2

t)t()t(

eff

2eff ∂

∂λπ

−=∂

ΔΦ∂−=ω

)t(P2AnL2)t(

eff

2effλ

π=ΔΦ

+D

Four Wave Mixing

(FWM)


-12

-8

-4

0

4

8

12

1510 1520 1530 1540 1550 1560 1570 1580 1590

Wavelength [nm]

Dis

pers

ion

[ps/

nm-k

m]

G.653 FiberDispersion

EDFA Gain

G.653 and G.655 Fiber Types

G.653

G.655

G.655


Four-Wave-Mixing (FWM)Experimental Observation

1550 1551 1550 1551

1549

FWM-generatedwavelengths

1552

λ(nm)λ(nm)

Nonlinear propagation

-50-45-40-35-30-25-20-15-10-50

1525

1526

1527

1528

1529

1530

1531

1532

1533

1534

1535

1536

1537

1538

1539

1540

1541

1542

Wavelength (nm)

OSA

Out

put (

dBm

) FWM crosstalk (XFWM)

( ) )(Dfc

PAn2X

22

22

2

FWM

FWM

2

eff

2FWM

λΔλ

∝βΔ

βΔ+αα

=η

η⎟⎟⎠

⎞⎜⎜⎝

⎛λπ

∝

FWM


Four-Wave-Mixing (FWM)Dependencies on Fiber/System Parameters

-50-45-40-35-30-25-20-15-10-50

1525

1526

1527

1528

1529

1530

1531

1532

1533

1534

1535

1536

1537

1538

1539

1540

1541

1542

Wavelength (nm)

OSA

Out

put (

dBm

)

1550 1551 1550 1551

1549

FWM-generatedwavelengths

1552

λ(nm)λ(nm)

Nonlinear propagation

• XFWM increases Pch2

• XFWM decreases as 4th

power of channel spacing• XFWM decreases

quadratically with dispersion and Aeff

Larger Aeff Fiber

( ) )(Dfc

PAn2X

22

22

2

FWM

FWM

2

eff

2FWM

λΔλ

∝βΔ

βΔ+αα

=η

η⎟⎟⎠

⎞⎜⎜⎝

⎛λπ

∝


FWM Efficiency as a function of Dispersion and Channel Spacing


Summary

Nonlinearity System impacted How it impact the system

Stimulated Brillouin Scattering PON

Limits launched optical power, increases noise.Impairs video signal through CNR, CSO and

CTB degradation

Stimulated Raman scattering

LH (WDM), PON

Transfers power from blue channel to red channels i.e. effectively increasing attenuation

for blue channels. Raman Xtalk in PONs.

Cross-phase modulation LH (WDM)

Distortion of the phase of adjacent transmitted channel that is converted into amplitude

distortion.

Self-phase modulation LH Distortion of the phase of transmitted optical pulse that is converted into amplitude distortion

Four wave Mixing LH (WDM) Generation of the FWM tones that coincide with other channels. Appears like nonlinear noise

Thank you!

Questions?

nonlinear impairments in high - ieeegrouper.ieee.org/groups/802/3/10gepon_study/public/... ·...

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