Alan E. WillnerUniversity of Southern California

March 16, 2006

Combating Degrading Effects inNon-Static and ReconfigurableWDM Systems and Networks

IPKUC Berkeley: EE233

Special “Thank You”to

IPKand

CCH

19851975 1981

200019961990

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

1968

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Through the Years with the CEO of KLT

Kaminows & Yarivs

How did I get into optical comm?

IPK(Father of Photonics)

AEW(Son of Photonics)

The Kaminow’s Grandchildren at USC

Outline

1. Brief Discussion on Starting a Compnay

2. A Few Suggestions on Giving a Presentation

3. Introduction to Reconfigurable Networks

4. Degrading Effects in Systems

5. Dispersion Compensation

6. Modulation Formats

1. Perspective on Optical Communications2. Phaethon Phacts3. First Steps: Get Prepared4. Write a Business Plan5. Venture Capitalists: Your Partners6. The “Pitch”7. Cultural Differences8. Building the Core Team9. Founder’s Dilemma: Step Aside10. Be Honest With Yourself: Failure IS an Option

Phaethon: Brief Discussion on Starting a Company

The Right Message for the Right Audience

•Every audience is different. •Therefore, the presentation must be different.

Speaker’s Bits/sEff

ectiv

e Pr

esen

tatio

n

100%

The optimum slides depend critically on the specific audience.

Too many speakers are here!

Audience #1

Audience #2

“I DON’T KNOW”IPK, 1988

You don’t need to know everything.An audience can smell if you are “snowing” them.

The Three Most Important Words

Outline

1. Brief Discussion on Starting a Compnay

2. A Few Suggestions on Giving a Presentation

3. Introduction to Reconfigurable Networks

4. Degrading Effects in Systems

5. Dispersion Compensation

6. Modulation Formats

Skiing Optical Fiber Trails

Alan Willner & James Harris (Stanford)

Beginner Intermediate AdvancedBunny Extreme

OC-482.5 Gb/s

OC-19210 Gb/s

OC-76840 Gb/s

OC-3072160 Gb/s

OC-12622 Mb/s

“Just like the super-highway builders of the 1950’s and 1960’s, you are putting yourselves out of business by building so much infrastructure.”

The Capacity vs. Demand Cycle

Time

Demand

Quote from Bob Lucky at AT&T Bell Labs in ‘89

Capacity

Gro

wth

Growth of automobile traffic is a few %/year, but growth of

internet traffic is >100 %/year.

The capacity-vs.-demand cycle will change much more rapidly

for comm. than for roads.

Overcapacity Under capacity4 years

Optical Amplifiers

Year

Syst

em C

ompl

exity

1990 1995 2000

Next?(OPA,

ultrawide, smart)

RamanEDFA

TransientsMulti-λEDFA

Single-λEDFA

SOA

2005

“Is amplifier research over?”

True for MANY technical issues as systems become more complex!!

“You can transmit∞ bandwidth

over0 distance !!”

L. Mollenauer, 1990

Limitations of Optics

80 82 84 86 88 90 92 94 96 98 00Year

0.01

0.1

1

10

100

1000

10000

100000C

apac

ity (G

b/s)

25 Tb/s

Amplifiers PMD Crosstalk Nonlinearities

Experimental

Commercial

Dispersion

Sustained Growth in Capacity

Capacity Toward 25 Tbit/s

• Chromatic Dispersion

• Fiber Nonlinearity

• Polarization Mode Dispersion

• Channel Xtalk

• L-band EDFAs

• Raman Amplifiers

• Novel Modulation Format

• Polarization or bidirectional interleaving

Higher Data Rate

Closer ChannelSpacing

Wider Optical Bandwidth

Higher Spectral Efficiency

0.05 Bits/Hz >1 Bits/Hz

• Fiber Nonlinearity

OC-48 OC-768 100 GHz 12.5 GHz 10 nm 300 nm

• Available Components

Source: Tingye Li and Herwig Kogelnik

Bit-Rate Distance ProductB

it R

ate

-Dis

tanc

e ( G

b/s

km)

1970 1975 1980 1985 1990 1995 2000 2005 Year

1101

102

103

104

105

106

107 WHAT’S NEXT ??WDM + Optical AmplifiersOptical AmplifiersCoherent Detection

1.5μm Single-Frequency Laser1.3μm SM Fiber0.8μm MM Fiber

Origin of Dispersion and Nonlinearities

The refractive index of the fiber depends upon both the frequency and the power of the signal

n(ω , P)

ChromaticDispersion

Nonlinearities (1)Self-phase Modulation (SPM)(2)Cross-phase Modulation (XPM)(3)Four-wave Mixing (FWM)(4)Stimulated scattering

Self-phase Modulation( ) ( )IRefractionofIndex

VacuuminLightofSpeedIVelocityPhoton =

The intensity variation in the pulse causes phase modulation since different parts of the pulse see a different refractive

index, which leads to pulse broadening.

time time

Transmission through fiber

n1

n2

n3

λ2

timeλ3

Fiber

Cross-phase Modulation

The glass that a photon in the λ3 pulse “sees” changes as other channels (with potentially varying power) move to coincide

with the λ3 pulse.

timeλ3

λ2

λ1

Fiber

λ1

λ2

timeλ3

λ1

n1

n1’ n1’’

v3

v2

v1

Phase-matching and NonlinearitiesNonlinear effects are most prominent when signals are phase-matched

Dispersion disrupts phase matching

Tx

Nonlinear effects

Nonlinear effects

DD

No phase-matching – fewer nonlinearities generated

Chromatic dispersion is necessary!

Ch. 1

Ch. 2

Ch. 1

TxCh. 2

Cross-phase Modulation and DispersionPhase shift due to cross phase modulation:

ΔΒ = 2γLe (dP/dt) ∝1/(DΔλ)ΔB : Pulse broadening in frequency domain, γ : Nonlinear coefficientLe : Effective nonlinear length, D: Dispersion, Δλ : Channel spacing

Performance vs. Dispersion Performance vs. Channel Spacing

02468

101214161820

SNR

(dB

)λ

Δλ Δλ

Central Channel

0 0.2 0.4 0.6 1.210.8 1.4 1.6 1.8 2Δλ(nm)

Single Channel

3 Channels 360 km span P = 15 mW/ch. 0.08 dB/km loss D = 16ps/nm/km

D. Marcuse et al, JLT, 1994

02468

101214161820

SNR

(dB

)

0 2 4 6 12108 14 16 18 20Dispersion (ps/nm/km)

3 Channels 360 km span P = 15 mW/ch. 0.08 dB/km loss Δλ = 1 nm

Dispersion-compensating fiber

Transmission fiber

0

100

50 100 150 200Distance, km

Tot

al a

ccum

ulat

eddi

sper

sion

, ps/

nm

TX RX

Dispersion Management Techniques

Accumulated dispersion at the receiver is close to zero

Outline

1. Brief Discussion on Starting a Compnay

2. A Few Suggestions on Giving a Presentation

3. Introduction to Reconfigurable Networks

4. Degrading Effects in Systems

5. Dispersion Compensation

6. Modulation Formats

Management of Chromatic Dispersion

Distance (km)

Dis

pers

ion

(ps/

nm)

+D -D +D -D +D -D Dtotal= 0

Positive DispersionTransmission Fiber Negative Dispersion Element

-D -D -D

0

Rx

Accumulated dispersion at the receiver is close to zero

0

1

2

3

4

0 20 40 60 80 100

10 Gb/s NRZ20 Gb/s NRZ20 Gb/s RZ40 Gb/s NRZ

10 Gb

40 Gb

20 Gb

NRZRZ

* w/o any compensation

Distance (km)

Pena

lty (d

B)

Power Penalties Due To Uncompensated Dispersion in SMF

L.D. Garrett, Invited Short Course, OFC, 2001

Various Dispersion Maps for SMF-DCF and NZDSF-SMF

Dispersion Values (in ps/nm/km) :SMF : ~ +17, DCF: ~ -85, Non-Zero DSF: ~ ± 0.2

+DSMF + DCF

0Distance (km)

100 200 300

1. High local dispersion: (D ≥ 17) High SPM, Low XPM, Low FWM

2. Short compensation distance

1. Low Local Dispersion: (D ~ ±0.2) Low SPM, Suppressed XPM, Suppressed FWM

2. Long compensation distance

Non-Zero DSF + SMF

-D

0

Distance (km)

1000 2000 3000

Fixeddispersion

compensation

Dispersion compensating fiber (DCF): reliable dispersion

characteristics

Higher-order-mode DCF: low loss, lownonlinearity

Linearly-chirpedFBG: low loss, low

nonlinearity: can be customized

Photonics crystal fiber: high negative dispersionvalue

: low nonlinearity

Methods for Fixed Dispersion Compensation(1) Dispersion compensating fiber (DCF)

-100

-80

-60

-40

-20

0

20

40

1300 1350 1400 1450 1500 1550 1600 1650

Wavelength (nm)

Dis

pers

ion

(ps/

nm-k

m) 17 ps/(nm.km)

-90 ps/(nm.km)

SMF

DCF

Dispersion Compensating FiberDCF Design Performance

1520 1540 1560 1580 16001500

1.0

0.9

0.8

0.7

0.6

0.5

0.4

Los

s (dB

/km

)

Wavelength (nm)

-80

-85

-90

-95

-100

-105

-110

Dis

pers

ion

(ps/

nm-k

m)

DCF

SMF

0 200 400-200-400

0

0.5

1.0

1.5

Radius (A.U.)

Δn

(%)

Vary the waveguide dispersion

Vary the waveguide structure

Experimental Setup System Performance

System Demonstration of Dispersion Compensation by DCF

Y.K. Park et al, PTL, 1995

1.5μm Transmitters

1

3

2

Standard Fiber LinesL = 0.275dB/kmD = +17.5 ps/nm.km

DCF

5

4DCF

DCF 67Rx

60 km

Total Loss : 100 dB including connectorsTotal dispersion : ~6300 ps/nm

Bit

Err

or R

ate

Received Power (dBm)-14 -13 -12 -11 -9 -8-10 -7

Error-Freefor 5 days

-4

-5

-6

-7

-8

-9

-10-11-12-13-14

2×10Gb/s, 223-1 PRBS NRZ

× Back-to-backSNR > 40 dB

• 1558 nm ChannelSNR > 25.75 dB1552 nm ChannelSNR > 25.25 dB

Problems of DCF1. Loss

The loss of DCF is about 0.5 dB/km, while a conventional SMF's loss is less than 0.25 dB/km

2. Nonlinearity Typically, nonlinearities are proportional to

N.L. ∝ n2*P*L/AeffWhere

n2 is the nonlinear index coefficient, P is input power,L is the overall effective length, andAeff is the fiber’s effective area.

The Aeff of DCF is about 20 μm2, but SMF’s Aeff is about 80 μm2

3. Figure of Merit The figure of Merit (FOM) of a DCF is defined as

FOM = Dispersion / Attenuation

4. Dispersion Slope Mismatch

θ

Photosensitive fiber

λ1 λ2 λ3

Chirped Pitch

Incident

Reflected

Linearly-Chirped FBGUniform FBG

Fiber Bragg Grating: Concept and Configuration

λRef

lect

ion

λTran

smis

sion

Uniform Pitch

λ

Ref

lect

ion

λ

Tran

smis

sion

UV Light

Interference Pattern

λ2λ3λ1

Wavelength (nm)

Nor

mal

ized

Ref

lect

ivity

(dB

)

Tim

e D

elay

(ps)

A Typical Linearly Chirped Fiber Bragg Grating

Slope (ps/nm)= dispersion

Ref

ract

ive

Inde

x C

hang

e

Grating axis (mm) Wavelength ( nm)

Frequency Space

0 1 2-1-2

FourierTransform

Real Space

Sinc Sampling

Square Sampling

Filte

r R

espo

nse

Squa

re

Sinc

Sinc

Squa

re

Sampled Fiber Bragg Grating

Characteristics of Sampled-Chirped Fiber Bragg Grating

• Channel Spacing: ~200 GHz (1.6 nm)• Dispersion: ~ -1400 ps/nm (~80 km of SMF) • Time Deviation from Linearity: < 30 ps

M. Ibsen et al, PTL, 1998

1551 1552 1553 1554 1555 1556 15570

800

1600

Wavelength (nm)

Tim

e D

elay

(p

s)0

-8

-16

Ref

lect

ion

(dB

) Ch. 1 Ch. 2 Ch. 3 Ch. 4

D1=-1410 ps/nm D2=-1406 ps/nm D3=-1392 ps/nm D4=-1392 ps/nm

Ch. 1 Ch. 2 Ch. 3 Ch. 4

Photonic Crystal FiberA large index

contrast between core and cladding

High dispersion

• Conventional fiberLimited by material incompatibilities(e.g. loss)

ncore

ncladding

??

Δn

ncore≥ β/k ≥ ncladding

Single-material coreDesign effective index

of near cladding

• Photonic Crystal Fiber: Large range of index

between silica and airwithout loss problem

: enlarging the core areaby modification of near cladding

Core Diameter (μm)D

ispe

rsio

n (p

s/nm

/km

)

0.4 0.6 0.8 1 1.2 1.4

500

-500

-1500

-2500

Second mode cut-off

@ 1550 nm

T.A. Birks et al, PTL, 1999

• Dispersion value up to 2000 ps/nm/km.• Bandwidth up to 100 nm.

Dispersion Compensation Using Photonic Crystal Fiber

Distance (km)

Accumulated chromaticdispersion

OC-192 limit

OC-768 limit

Partial compensation

SMF DCF

• The tolerance of OC-768 systems to chromatic dispersion is 16 times lower than that of OC-192 systems.

Perfect compensation

The Need for Tunability

Because of temperature changes, the zero-dispersion wavelength shifts, and dispersion itself changes at a fixed wavelength due to the dispersion slope.

Chromatic Dispersion Variation due to Temperature

-10

0

10

20

30

1250 1350 1450 1550 1650

Δλo(T)

ΔDDisp. Slopedλo/dT ~ 0.03nm/C

Wavelength (nm)

Dis

pers

ion

(ps/

nm)

Accumulated Dispersion Changes as a Function of the Link Length and Temperature Fluctuation

-100

-50

0

50

100

-40 -30 -20 -10 0 10 20 30 40

Acc

umul

ated

Dis

pers

ion

Cha

nge,

ΔD

(ps/

nm)

Temperature Change, ΔΤ (ºC)

NRZ 40 Gb/s Limit

L=1000 km

L=500 km

L=200 km

Dispersion Slope ~ 0.08 ps/nm2.kmdλO/dT ~ 0.03 nm/ºC

NRZ 40 Gb/s Limit

-100

-50

0

50

100

-40 -30 -20 -10 0 10 20 30 40

Acc

umul

ated

Dis

pers

ion

Cha

nge,

ΔD

(ps/

nm)

Temperature Change, ΔΤ (ºC)

NRZ 40 Gb/s Limit

L=1000 km

L=500 km

L=200 km

Dispersion Slope ~ 0.08 ps/nm2.kmdλO/dT ~ 0.03 nm/ºC

NRZ 40 Gb/s Limit

-100

-50

0

50

100

-100

-50

0

50

100

-40 -30 -20 -10 0 10 20 30 40-40 -30 -20 -10 0 10 20 30 40

Acc

umul

ated

Dis

pers

ion

Cha

nge,

ΔD

(ps/

nm)

Temperature Change, ΔΤ (ºC)

NRZ 40 Gb/s Limit

L=1000 km

L=500 km

L=200 km

Dispersion Slope ~ 0.08 ps/nm2.kmdλO/dT ~ 0.03 nm/ºC

NRZ 40 Gb/s Limit

Filter-induced Chirp

• Zero amplitude change produces zero phase change across channel

• Chirp is induced across channel• Chromatic dispersion will result• Attenuation is of secondary

importance

λ λ

Tra

nsm

issi

on

(a) Ideal Flat TapNo phase change

(ΔΦ = 0)

(b) Sloping Tails

Phase change

(ΔΦ ≠ 0)

filter

channel

0

1

2

3

4

5

0 20 40 60 80 100 120 140 160Distance (km)

OC-768

No Compensation

Tunable Compensator (500-2100 ps/nm)

Fixed 80 km Compensator

Value of Tunable Dispersion Compensation(OC-768)

Dispersion Compensation Using a Non-linearly Chirped Fiber Bragg Grating

λ1 λ2 λ3

Incident

ReflectedExternal

Mechanical Stretcher

Chirped FBGt

t

λ3λ1

K.-M. Feng et al, PTL, 1999

RelativeTime

Delay (ps)

Wavelength (nm)

Nonlinearly Chirpedλ0

Slope Change at λ0StretchRelative

TimeDelay (ps)

Wavelength (nm)

Linearly Chirpedλ0

No Slope Change at λ0Stretch

Dispersion Tuning of the Compensator

Tim

e D

elay

(ps)

Wavelength (nm)

-100

0

100

200

300

400

500

1550.5 1551 1551.5 1552 1552.5 1553 1553.5

1000 ps/nm

300 ps/nm

0 V300 V500 V700 V

1000 V

500ps/nm

λ0

K.-M. Feng et al, PTL, 1999

40 Gbit/s Dispersion CompensationSingle Grating Approach

Use a FBG w/ a small nonlinear chirp that provides:• negligible intra-channel 3rd-order dispersion• small dispersion tuning range results

Output

Sampled NC-FBG

Input 12

3

Configuration:D

ispe

rsio

n(p

s/nm

)

λ

tuning

Ch1 Ch2 Ch3 Ch4 λ

10

20

30

4050

60

00 2 4 6 8

Position Along Fiber (cm)

0.0

0.2

0.4

0.6

0.8

1.0

Coa

ting

Thi

ckne

ss (u

m)

Tem

pera

ture

(a.u

.)

50 μm

B.J. Eggleton et al, PTL, 1999

Temperature profilewith thickness of a metal film plated onto the 7.5-cm FBG

Cross sectional optical micrographs of the plated fiber at the ends of the FBG

Electrical Tunable Power Efficient Dispersion Compensating FBG

1552.5 1553.5 1554.5 1555.5-20

-15

-10

Wavelength (nm)

dB

-5

0 A B C D E

Voltage0.4 0.6 0.8 1.0 1.2

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

Dis

pers

ion

(ns/

nm)

100200300400500600

01552.8 1553.8 1554.8

Wavelength (nm)

Del

ay (p

s)

A B C DE

B.J. Eggleton et al, PTL, 1999

Characteristics of the Dispersion Tunable FBG

Reflectivity spectra for FBG device with (A) 0 V–unchirped (B) 0.5 V (C) 0.81 V (D) 1.0 V (E) 1.1 V

Dispersion versus voltage and theoretical fit

Electronic Chromatic Dispersion Equalizer

• Optimization of the transmission channel in terms of simplicity, robustness and transparency.

• Possibility to produce small size, low cost, adaptive structures that can be fully integrated with remaining receiver circuitry.

T T

××× w1 w2 wn

• • •

• • •

∑

Sin

Sout

Advantages

Typical architecture of feed forward equalizer (FFE)

Determine theamplitude of the different delayed signals

Decision feedback equalizer (DFE)

Electronic Equalization of Transmission Impairments

H. Bülow, invited, OFC, 2002

Fiber length (km)0 20 40 60 80 100 120 140

Pow

er p

enal

ty (d

B) 7

6543210

-1

w/o FFE

w/ FFE(LMS)

Q-optimization

• Calculated penalty at 10 Gbit/s• Least-mean-square (LMS) algorithm is used• In FFE, # of taps = 7, Tc=T/2• Q-factor optimization leads to an improvement of 0.8 dB

compared to LMS adaptation

TotalDispersion

SMFDCF

Distance

λ1

λ2

λ3

Dispersion Slope Mismatch Caused by The Different Slopes of Transmission Fiber

(SMF or NZDSF) and DCF

Tunable effectivecoupling

Phase Shifter

In Out Channel Spacing

Gro

up D

elay

(T)

frequency

Channels (channel spacing = filter FSR)

C.K. Madsen et al, PTL, 1999

All-pass Filter Structure for Chromatic Dispersion and Dispersion Slope Compensation

• Dispersion range : -378 ps/nm (BW=7.5 GHz) ~ -3026 ps/nm (BW=3.4 GHz)

• Multiple stage guarantees wider passband (14 GHz) with dispersion of 1800 ps/nm.

Polarization-mode Dispersion (PMD)

• PMD is a result of a slight asymmetry in the fiber core.

• Different polarizations can propagate at different speeds.

Current fiber: PMD < 0.2 ps/km1/2

Many installed spans: PMD > 1 ps/km1/2

Discrete in-line components: PMD ~ 1 ps

PMD is a random, stochastic, time-varying, temperature-dependent, and frequency-dependent effect!

0

2

4

6

8

Rel

. OSN

R P

enal

ty (d

B)

-2

RXE-PMDC

(FFE)

O-PMDC(1-stage)

O-PMDC(2-stage)

OPMDC(Multi-stage)

PMD/Tb

0.0 0.1 0.2 0.3 0.4 0.5

E-PMDC (FFE+DFE)

Optical vs. Electronic PMD Mitigation

(Bulow and Lanne, OFC ‘03)

•Good enough?•“If you find yourself fighting Si, don’t.” (T. Li quoting A. Penzias)•Reed-Solomon FEC, tapped-delay-line equalizers, etc.

FEC has been shown to be effective in correcting errors due to optical noise, fiber chromatic dispersion and nonlinearity

Forward-Error-Correction (FEC) Coding

Data

FECOverhead

Encoder

Errors

RecoveredData

Transmission

Decoder

M. Tomizawa et al, ECOC, 2000

Poisson error correctingcapability of RS(255,239)

Without FEC

Fluctuating DGD

-2-3-4-5-6-7-8-9-10-20

-18

-16

-14

-12

-10

-8

-6

-4

Log(Input average BER)

Log(

out p

ut a

vera

geB

ER)

10.7Gbit/s transmission,RS(255,239) code

<Δτ>=33ps

<Δτ>=38ps

<Δτ>=43ps

Static DGDBut FEC method can’t reach the code capability for Poisson errors because due to DGD fluctuations, error distribution is not Poisson.

Using FEC Coding for High PMD Systems

FEC method can correct a long term BER of 10-6 to near error free for PMD

degradation

Outline

1. Brief Discussion on Starting a Compnay

2. A Few Suggestions on Giving a Presentation

3. Introduction to Reconfigurable Networks

4. Degrading Effects in Systems

5. Dispersion Compensation

6. Modulation Formats

Some important modulation formats

Chirp-free

Amplitude modulation

NRZ RZ VSB/SSB

Binary (OOK)

Chirped

C-NRZ CRZ ACRZ

Pseudo-multilevel Partial response Multilevel

CSRZ DB AMI M-ASK

OOK … on/off keyingNRZ … non return-to-zeroRZ … return-to-zeroVSB … vestigial sidebandSSB … single sideband

AMI … alternate-mark inversionM-ASK … multi-level amplitude shift keyingDPSK … Differential phase shift keyingDQPSK … Differential quadrature PSK

C-NRZ … chirped NRZCRZ … chirped RZACRZ … alternate-chirp RZCSRZ … carrier-suppressed RZDB … duobinary

Phase modulation

Binary (DPSK) Multilevel (DQPSK)

Common Digital Modulation FormatsNRZ (non-return-to-zero) modulation format

0 T 2T 3T 4T 5T 6T 7T Time

0 1 0 1 1 1 0

Inte

nsity

1

0

TBit time

RZ (return-to-zero) modulation format

0 T 2T 3T 4T 5T 6T 7T Time

Inte

nsity

2

1

0

Bit time

Envelope

Optical Carrier Wave

T Envelope

Optical Carrier Wave

Q-Factor vs. BER

Eye Diagram

Sampling Time IntervalO

utpu

t Vol

tage

Decision Level

μ1, σ1

μ0, σ0

BER(V) = 12 [erfc( μ1 − V

σ1) + erfc( V −μ 0

σ0)]

Q ≡μ1 − μ0

σ1 + σ0

USCOptical

LaboratoryCommunications

jxc

Modulation Formats

• To achieve higher spectral efficiency(decrease the cost/bit)

• To make transmission more robust to:chromatic dispersionpolarization mode dispersionfiber nonlinearitieschannel crosstalk

• To support more low speed end userssecure transmission

Spectrally Efficient Data Formats

f

BW=1/2 Δ fNRZDuobinary

NRZ

RZCRZ/DM Soliton

ΔfNRZ

f0

BW= Δ fNRZ

BW= 2Δ fNRZBW > 2Δ fNRZ

f

BW=1/2 Δ fNRZDuobinary

NRZ

RZCRZ/DM Soliton

ΔfNRZ

f0

BW= Δ fNRZ

BW= 2Δ fNRZBW > 2Δ fNRZ

• Higher spectral efficiency for bandwidth allocation• Enhance robustness of fiber transmission

Input power density(mW GHz-1)

10-3 10-2 10-1 100 101

Spectral Efficiency

•P.P. Mitra, et al., Nature, 2001•J. Kahn, et al., JSTQE, 2004

Spec

tral

eff

icie

ncy

(bits

/s/H

z)

6

5

4

3

2

1

0

Spectral efficiency = Channel capacity (b/s) per unit bandwidth (Hz)

Max channel capacity = Bandwidth * (1 + log(S / N))(for linear channels with additive white Gaussian noise)

γ= 0Linear Channels

γ = 1 W-1 km-1

γ = 0.1 W-1 km-1

Limitations• Fiber nonlinearities• Optical amplifier noise• Quantum limits• PMD and CD

On-Off Keying (OOK)

(b) Return-to-Zero (RZ)

(a) Non-Return-to-Zero (NRZ)

Optical Frequency Offset (GHz)

Pow

er (d

Bm

)

-40 -20 0 20 40-60-40-20

200

-40 -20 0 20 40-60

-40

-20

20

0

Optical Frequency Offset (GHz)

Pow

er (d

Bm

)

Time (t)

Am

plitu

de

Time (t)

Am

plitu

de

2

4

6

8

10

Transmission Distance (km)

0100 200 300 400 500 600 7000E

ye C

losu

re P

enal

ty (d

B)

RZNRZ16 channels

NRZ vs. RZ Modulation FormatWorst WDM Channel @ 40 Gbit/s

RZ increases transmission distance

RZ has less phase matching for long strings of “1” bits

M. I. Hayee et al, PTL, 1999

Duobinary Coding

Benefits:• Minimum-bandwidth signal• High dispersion tolerance• Suppresses nonlinear effects

b k = a k + b k- 1Bibary

T

XORT

+

Duobinarya k

c k = b k + b k- 1

0

1Binary Input a k

Duobinary Output c kCoding

Generation of Duobinary Signal

0

1

2

Received Optical Power (dBm)

Bit-

Erro

r-R

ate

10-11

10-9

-34 -30 -26 -22

10-7

10-5

10-3

X. Gu, IEE Proceedings-Optoelectronics, 1996

Duobinary vs. Binary

10 Gb/s NRZ

Binary Back-to-BackDuobinary Back-to-BackDuobinary 100 km SMFBinary 100 km SMF

Comparison of spectrum of various modulation formats

• Doubinary shows the narrowest spectrum, hence the most spectrally efficient

• DPSK and NRZ have almost the same spectrum, while DPSK has 3-dB better sensitivity

Differential Phase-Shift-Keying (DPSK)

DPSK

t

1 1 0 1 00

RZ-DPSK

t

1 1 0 1 00

Pulse appears in every bit

Constant optical power

Phase-Shift Keying in Optical Systems

DPSK Encoder

DFB

NRZ Data

Pulse Carver

Phase Encoder

MZ delay inter-ferometer with 1 bit delay

Data

Balanced receiver

Fiber link

1 0 1 1

Conventional OOK

RZ-DPSK

1 0 1

+ + - -

Advantages over OOK:• 3-dB receiver sensitivity• Tolerant of power fluctuation• Tolerant of optical filtering• Less sensitive to nonlinearity

DPSK Transmitter and Receiver

Laser

T

DataXOR

IntensityModulator

Clock

DPSK

Fiber Link

RxMZI

T

Decoding DirectDetection

RZ-DPSK

PhaseModulator

Benefits:• Improve receiver sensitivity• Suppresses nonlinear effects

Phase-Shift Keying – Selected Results

-5

-6

-7

-8

-9

-10

-11-12

-40 -38 -36 -34 -32 -30 -28 -26

On/Off KeyingDPSK, Single DetectorDPSK, Balanced Detectors

Received Power (dBm)

Lo g(E

rro

rPro

babi

lity)

45 photons/bit, 3.2 dB away from the quantumlimit of 21.8 photons/bit (1 dB due to NF)

Balanced Detection: 1 × 10-9 OSNR requirement of 17.9 dB

A. Gnauck et al., PTL, Jan. 2003

Note: 42.7-Gb/s record results:J. Sinsky et al., OFC’03 PD39: 39 photons/bit, 17.0-dB OSNRW. Idler et al., ECOC’03 Th2.6.3: 16.8-dB OSNR

42.7-Gb/s RZ-DPSK BER Curves33% RZ Formats, EDFA Preamplifier NF ≈4.0 dB

DPSK vs. IM

K. Yonenaga, OFC, 1997

Wavelength (nm)

Q F

acto

r (dB

)

18 1551

20

22

24

26

28

1553 1555

Benefit versus ComplexityModulation format Req. OSNRNRZ-OOK 16.5 dB

Duobinary 17.4 dB

33% RZ-OOK 14.9 dB67% CSRZ 15.1 dB33% RZ-DPSK 11.0 dB67% RZ-DPSK 11.1 dB

50% RZ-DQPSK 12.2 dB

Data

ClockData

Precoded Data

LP

ClockPrecoded Data

Hardware complexity

Controlp/2

ClockPrecoded Data

Precoded Data

Mach-Zehnder modulator

Delay interferometer

Spectra of modulation formats

Long Haul Transmission – Recent Result

• 64*42.7 Gb/sover 8200 km (50 GHz Spaced)I. Morita et al. ECOC ’03, PD

• 70 GHz Spaced 40*42.7 GHz over 9400 kmT. Tsuritani, et al. JLT Jan. 2004

• 64*42.7 Gb/sover 8200 km (50 GHz Spaced)I. Morita et al. ECOC ’03, PD

• 70 GHz Spaced 40*42.7 GHz over 9400 kmT. Tsuritani, et al. JLT Jan. 2004

•128*10G over installed non-slope matched undersea cable

J.-X Cai, et al. OAA ’04 PD

•128*10G over installed non-slope matched undersea cable

J.-X Cai, et al. OAA ’04 PD

•149*42.7 Gb/s over 6120 km (50 GHz spaced)

G. Charlet, et al. OFC ’04 PD

•160*42.7 Gb/sover 3200 km (50 GHz spaced)

B. Zhu, et al.JLT, Jan. 2004

KDDI (Japan)

Tyco(USA)

Alcatel(France)

Bell-LabsUSA

Modulation Formats• PMD impairments depend on the data formats

and transmitter/receiver designs

• Short pulse may be more robust to PMD- need more DGD to cause outages- RZ works better than NRZ without PMD compensation

• Wide spectrum is more susceptible to higher-orders of PMD- higher-order PMD decreases the tolerance of RZ systems- NRZ works better than RZ with simple PMD compensation

Is there any modulation format good for PMD?

•After compensation, higher orders of PMD become important!• Spectral efficiency concerns will drive new modulation formats!

(C. Xie et. al., OFC, 2003)

<DGD2>1/2/Tb

Out

age

prob

abili

ty

NRZ NRZ

DPSK

-NRZ

50% RZ

DPSK-NRZ

50%

RZ

w/o Comp.w/ 1-stage Comp.

0.1 0.2 0.4 0.5 0.60.310-5

10-4

10-3

10-2

10-1

•W/o PMD compensation, shorter pulse-widths formats perform better• W/ 1-stage PMD compensation, narrower bandwidth formats perform better

Modulation Format