optical fiber communication: revolutions before our · pdf fileoptical fiber communication:...

Optical Fiber Communication: Revolutions before Our Eyes

Guifang LiCREOL, The College of Optics & Photonics

University of Central Florida

CREOL Industrial Affiliates DayMarch 12, 2015

1

2

2009 Nobel Prize in Physics

Conclusions

How, Why?

Space-Division Multiplexing

3

Introduction

Digital Coherent Optical Communication

Outline

Capacity Limits of SMF

Critical Challenges & Solutions

Future Challenge & Prospects

The Optics Happy Era

4

• Shannon Capacity Formula:

Bandwidth Signal-to

Noise

Spectral Efficiencybits/s/Hz

b

Ratio

its/s Hz

2 = W log (1 / )

C S N

Channel Capacity Why Optical

Optical Communications

1011001

1011001

Free‐Space Propagation Loss‐ 300 dB (Diffraction)

D=35 cm

Antenna Gain+120 dB

Antenna Gain+120 dB

Link Loss=60 dB

80 km

Wavelength (nm)

wavelength (nm)E

0

0.3

0.6

0.9

1.2

1300 1400 1500 1600

Fibe

r Los

s (d

B/k

m) O LCS

AllWave fiber

SMF

80 km

Link Loss=16 dB

What have we been doing all those years?

Why Fiber

Internet

Internet

6

Optical Communication

CladdingCore

Optic Communication System

TX RX

Economy

E-Commerce

Tele-conf

Intelli-Transp.

Tele-Med

Dist. learn

Everyday Life

Introduction

7

Traffic DemandInternet traffic will increase in the foreseeable future

10x

10 years~100x！

HDMI Cable

@HomeOptical Fiber Everywhere

Active Optical Cables 8

Introduction

Around the globe

Optical Fiber Everywhere

9

Introduction

Tingye Li, Herwig Kogelnik, Alan Willner

Introduction Tech Evolution

EDFA

11/2 delayOptical Hybrid

Data, Ed(t)

LO, ELO(t)

*Re ( ) ( )LO dE t E t

*Im ( ) ( )LO dE t E t

RL

RL

0[ ( )]( ) ( ) cj t tE t A t e

02 2[ ( )]( ) ( ) ( )cj t tI t A t e A t

Photodetector

Introduction Noises & Distortions

Direct Detection

Pre‐Amplified Direct Detection

Coherent Detection

2 =W log (1 / )C S N

Photodetector

Optical Pre‐Amplifier

12

Introduction Noises & Distortions

Noises/Modulation FormatsBefore 1980 Intensity Modulation Direct Detection (IMDD) Thermal Noise Limited: Sensitivity N=1000s Photons/bit

1980‐1990 Phase‐Shift Keying (PSK) with Coherent (Homodyne) Detection

Shot‐Noise Limited w/o amplifiers: Sensitivity N=9 Photons/bit for BPSK ASE‐LO Beat Noise Limited w/ amplifiers: Sensitivity N=18 Photons/bit for BPSK

1990‐2005 Intensity/Phase Modulation using Direct Detection with Optical Preamplification Signal‐ASE Beat Noise limited:

Sensitivity N= 39 photons/bit (IM); N= 20 photons/bit (DPSK)

Since 2005 Digital Coherent Optical communication

TX RX

2 =W log (1 / )C S N

Reason 1 for Demise of CoherentPolarization and Phase Management Polarization after a few meters of fiber propagation is

uncorrelated with the input polarization. Phase locking is very challenging!

00( ) exp ( ) ( ) j t

s TxE t A j t t e sBinary Phase-Shift Keying 0, for bit 1's and 0's

00( ) exp ( ) j t

TxE t A j t e :Tx Laser PhaseTx

LaserDiode

PhaseModulator

Conventional Phase Locking Techniques:1. Injection Locking2. Phase Locked Loops

Reason 2: Erbium-Doped Fiber Amplifier (EDFA)

PumpLaser

Er‐DopedFiber

WDMn = 0.7

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

1510 1530 1550 1570 1590

Wavelength (nm)

Gai

n (d

B/m

)

gmin

gmax

Ease and Robustness ofPre‐amplified IMDD to achieve 39 photons/bit

The Wavelength-DivisionMultiplexing (WDM) Revolution

120 kmOA

OA

120 km 120 km

Optical Amplifiers and WDM - 20 Gb/s

OC-48OC-

48OC-48OC-

48OC-48OC-

48OC-48OC-

48

OC-48OC-

48OC-48OC-

48OC-48OC-

48OC-48OC-

48

DS3OC3/12

DS3OC3/12

WDM: Wavelength Division Multiplexing

DS3

Conventional Transmission - 20 Gb/s1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE

40km 40km 40km 40km 40km 40km 40km 40km 40km

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTEDS3

DS3

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

R

1310RPT

RLTE

LTE

In Each Direction:12 fibers 1 fiber; 36 regenerators 1 optical amplifier

Circa 1994

fiber

mux demux

6 x 10 Gb/s

...

STM-64

..

.

STM-64

16

Linear Distortions:• Chromatic Dispersion: different wavelengths (colors, frequency) of

light travel at different speeds in the fiber

• Polarization-Mode Dispersion

Nonlinear Distortions:• Kerr Nonlinearity

kmnmpsDispersion

Increases with the distance and bit rate

'0 2 0 2

eff

Pn n n I n nA

0 2 where NLeff

k nPLA

Increases w/ distance and power

Decrease w/ Aeff

Noises and DistortionsIntroduction

101 Bit Pattern AfterTraversing A LengthOf Optical Fiber

Bit Period

1 0? 1

Statistical Nature of PMD

Freq

uenc

y of

occ

urre

nce

Instantaneous DGD (ps)

Maxwellian distribution of the instantaneous DGD

PMD 3.5PMD

Prob.(DGD>3.5xPMD)=10-6 = 32 sec/year

Prob.(DGD>3xPMD)= 4x10-5 = 21 min/year

• PMD vector goes through a random walk.

• PMD is statistical due to environmental fluctuations.

• In 3D space, PDF of a random variable through random walk is Maxwellian.

• The mean square value of DGD scales with fiber length.

• Mean DGD scales w/ square root of fiber length, in units of .

• PMD is also frequency depend. Frequency-dependence of PMD is called high-order PMD.

1 42 3n

Mode-coupling at random locations with random strength

…

/ps km

Polarization-Mode Dispersion (PMD)

Optical Fiber PMD

Conclusions

How, Why?


18


Digital Coherent Optical Communication

Outline

Capacity Limits of SMF



Introduction

Is there an Easy Solution to Fiber Dispersion?

E

1300 1400 1500 1600Wavelength (nm)

O LCS

-10

0

10

20

Dis

pers

ion

(ps/

nm.k

m)

SMF

How About Dispersion-

Shifted Fiber?

DSF

Fiber Is Tricky: Nonlinearity

effjj A

Pnnn 2 Wcmn /102.3 2162

effAnkP 20where

NL

LLdzzPLdz

00

M

jmmjeffjNL PPL 2,

Time Frequency Time

SPM Dispersion

The index of optical fiber depends on the intensity of the light inside

The phase of optical signal after fiber propagation depends on its own intensity:Self‐Phase Modulation (SPM)

TimingJitter

By the same token, in a WDM system, the phase of optical signal after fiber propagation depends on the intensity of all other channels:Cross‐Phase Modulation (XPM)

Increases with distance and power

Four Wave Mixing in Fiber

kjiijk

Lkjieff

ijkijk ePPPL

DP

2

3

2

2

22

2

12/sin41

L

L

eLe

ddD

cD

c kji

kjkiijkkji 2)())((

2

22

Three waves at different frequenciescan mix to create a fourth wave:

The power of the fourth wave:

Is maximized when fiber dispersion is zero

Dispersion & Nonlinearity Management

100 km20 dB

NZDSF

1 dB

DCF

5 km

DCM

DCF

E

1300 1400 1500 1600Wavelength (nm)

O LCS

-10

0

10

20

Dis

pers

ion

(ps/

nm.k

m)

SMF

Non-Zero Dispersion-

Shifted Fiber(NZDSF)

~4 times lower dispersion than SMF

~4 times lower dispersion than SMF

100 km20 dB

NZDSF

1 dB

DCF

5 km

DCF

0DCF

Dispersion

Distance

• Net zero dispersion for the span• Non-zero local dispersion to

suppress nonlinear effects

Dispersion-Compensation

Module

23

Non-Zero Dispersion Shifted/ Medium Dispersion Fibers

23

+NZDSF -NZDSF +NZDSF -NZDSF

+NZDSF -NZDSF

25 km0.245 dB/km

115 m2

25 km0.208 dB/km

40 m2

Mukasa et. al, J. Opt. Fiber. Commun. Rep. 3, 292–339 (2006)

Nonlinearity Tolerance and Intensity Waveforms: NRZ vs RZ

Non Return‐to‐Zero NRZ

AM

fc

LD AM

NRZ

After transmission over 960 km SSMF

NRZ (2 dBm)

RZ (2 dBm)

LD AM

NRZ

Return‐to‐Zero

RZ

Nonlinearity Tolerance and Phase Waveforms: RZ vs CSRZ

RZ

Carrier‐Suppressed RZ

CSRZ π π π0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0fc/2

LD AM

NRZ

AM

fc

LD AM

NRZ

After transmission over 960 km SSMF

RZ (2 dBm)

CSRZ (5 dBm)

Manipulating phase can improve nonlinear tolerance !

Differential Phase-Shift Keying (DPSK)

Sensitivity

DPSK: 20 photons/bitOOK: 39 photons/bitLess power needed for DPSK reduced NL.

• Constant AmplitudeEvery bit experiences the same

deterministic SPM.

• LO Free 1 Bit Delay

+-

DPSK Encoded Data (Optical) Demodulated Data

)cos(

Binary phase shift keying (BPSK)On-off keying (OOK)

ERe

EIm

ERe

EIm

Introduction Coherent: 2nd Coming


2015

D=0N=1000s

N=9

+/-D=0N=39

+D, -DN=20

Coherent 2+D

N=18

EDFA

Conclusions

How, Why?


28

Introduction

Coherent Optical Communication

Outline

Capacity Limits of SMF Capacity Limits of SMF




29

Coherent Communication• Encoding: information is encoded on

the electric field of the lightwave.

• Decoding: measure the electric field.

• The local oscillator has to be matched in phase and polarizationwith the incoming data.

• Difficulties in optical phase locking and polarization tracking were the main obstacles for coherent, the 1st coming.

• Recent successes in coherent transmission relied on DSP-basedphase and polarization management

ERe

EIm

(1,1)

(1,0)

(0,0)

(0,1)

/2 delay

Data, Ed(t)LO, ELO(t)

*Re ( ) ( )LO dE t E t

*Im ( ) ( )LO dE t E t

900 Optical Hybrid

QPSK 16 QAM

30

1. DSP-Based Phase Management:

/2 delay

Data, Ed(t)LO, ELO(t)

*Re ( ) ( )d LOE t E t

*Im ( ) ( )d LOE t E t ADC

ADC

DSP

Output

( ) exp ( ) ( )d nE t A j t t

4

4

exp 4 ( ) 4 ( )

with 4 ( ) 2

exp 4 ( )

d n

d

n

A j t t

t m

A j t

Example: DSP Algorithm for QPSKarg(.)

(.)4 PhaseEstimation (.)/4

++- Output

( )d t

4 ( )n t

( ) ( )d nt t

( )n t30, , , :Data Phase2 4

: Phase Noise of Received Signal

d

n

Free-running

How?Digital Coherent Comm.

R. Noe, J. LightwaveTechnol. 23, 802 (2005).

31

y

x

y

x

EE

JJJJ

EE

2221

1211'

'

Tx Rx

Wireless Communication

Antenna

Tx Rx

Optical Communication

PBSPBC

xE

yE

'xE

'yE

Random polarization rotation fiber

Han & Li, Optics Express 2005

''

1 1' , : A Set of Training Symbolsxx x xi i i

yy y yi i i

EE E EJ J J

EE E E

2. DSP-Based Polarization DemultiplexingHow?Digital Coherent Comm.

Multiple-Input-Multiple-Output (MIMO)

32

Why?Digital Coherent Comm.


2015

D=0N=1000s

N=9

D=0N=39

+D, -DN=20

(8) Coherent 2+D

N=18

EDFA

DSP can perform a number of other

functionalities better than or impossible for optics in WDM

systems

Dispersion governed by the linear Schrodinger equation:

Time Domain: 061

2 3

3

32

2

2

tA

tAj

zA ,

Frequency Domain: 322 3

1 ,2 6

A j j A zz

2 3

2 32 6, 0,j j z

A z A e

Why?Digital Coherent Comm.1. Digital coherent communication enables

electronic dispersion compensation

Dispersion is an all-pass complex filter, on the E-field of light, with a transfer function given by

2 32 32 6 ( )

j j zH e

To reverse the effect of dispersion, 1. Detect E-field of received signal

(thus coherent detection) *H H

34

Transfer function of dispersion :

-10 -5 0 5 10-200

-150

-100

-50

0

50

100

150

200

-10 -5 0 5 10-200

-150

-100

-50

0

50

100

150

200

-10 -5 0 5 10-200

-150

-100

-50

0

50

100

150

200

2 32 32 6 ( )

j j zH e

H

2. Apply a filter with

1( )h t H

Why?Digital Coherent Comm.1. Digital coherent communication enables

electronic dispersion compensation (EDC)

Tap-Delay Line Filter

M. G. Taylor, IEEE Photon. Technol. Lett. 16, 674 (2004).

35

Electronic Dispersion Compensation:

• Eliminate the need for DCFs (small effective area)Less nonlinearity improved performance

• Fewer amplifiers reduced noiseimproved performance

• No DCF and fewer amplifiers reduced cost

+D

SMF DCF

‐D

DCF SMF DCFDCF

+D ‐D

Why?Digital Coherent Comm.Benefits of electronic dispersion compensation (EDC)

EDC

36

• PMD compensation using a matrix of tap-delay line filters, instead of simple Jones Matrix.

• Non-ideal frequency responses of all components in the transmitter/receiver

y

hxx

hxy

hyx

hyy

+

+

Carrier recovery

Frequency offset

Decision circuitry

Real

Imag

Real

Imag

CD comp

CD comp

x

Carrier recovery

Frequency offset

Decision circuitry

Courtesy: Seb Savory, Optics Express Feb 2007.

Why?Digital Coherent Comm.2. Enables electronic compensation of

other linear distortions/impairments

Nortel Coherent DSP Chip

10 Gsymbol/s QPSK with Pol. Multiplexing => 40 Gb/s

Real-Time Receiver

• 193 nm CMOS • 10 Million Gates• 12 trillion operations per second

• 100 Engineers; 3 year effort

Courtesy: Kim Roberts, Optics Express January 2008.

A MilestoneDigital Coherent Comm.

38

0,A t

ˆstepN z ˆ

stepN z

stepz stepz

ˆstepN z

stepz

,A z t…..ˆ2stepz

D

ˆ2stepz

D

ˆ2stepz

D

ˆ2stepz

D

ˆ2stepz

D

ˆ2stepz

D

ˆ ˆA N D A

z

2 3

2 32 31ˆ

2 6 2j

Dt t

2ˆNLN j A

Propagationin Real Fiber

ˆ2stepz

D

ˆ2stepz

D

ˆ2stepz

D

ˆstepN z ˆ

stepN z

stepz stepz

ˆ2stepz

D

ˆstepN z

stepz

,A z t

ˆ2stepz

D

ˆ2stepz

D…..

ˆ ˆ ˆ ˆA N D A A N D A

z zPropagation inVirtual Fiber

0,A t

Why?Digital Coherent Comm.3. Electronic nonlinearity compensation:

Digital Back Propagation (DBP)

Transmitter

CoherentReceiver 39

DBP Experimental Details

• 3 WDM Channels

• Loop Length: 160 km

• Modulation Format: BPSK

• Symbol Rate: 6 Gsymbols/s

• Channel Spacing: ~6.5 GHz

• All 3 channels can fit into the 13 GHz analog bandwidth of the realtimescope

• 3 channels are orthogonal, i.e., minimal linear cross talk

40

DBP Experimental Results:

760km NZ-DSF, PL=6dBm

Goldfarb& Li, PTL 2008

= 280nlL km

= 750woL km

= 100fwmL km

One-StepDBP

Conclusions

How, Why?


41

Introduction


Outline





Capacity=BWSE=500Gb/s

SpectralEfficiency

(b/s/Hz)

42

Bandwidth

10

1 2 3 4 N5 6 7 ……WDM

X-Pol

Y-Pol

100 GHzElectronic Bottleneck

Non

linea

rlim

it5

b/s/

Hz

10 THzEDFA Bandwidth

LimitMuxDoF

/f

SMF Capacity Limit:~10THz 10b/s/Hz =100Tb/s(Nonlinear Shannon Limit)

PolMux

Pol

Multiplexing Methods Capacity Limits

5

SMF Capacity LimitsCoherent Comm.

D. Richardson, et. al. “Filling the Light Pipe” SCIENCE 330(15):327,2010

Demand Increase: 10x/4Yr

Single Fiber CapacityRequirement: 10x/7yr

SMF Capacity Saturation

NL Shannon Limit 100Tb/s

43

SMF Optical SMF Optical Communication will experience a

Capacity Crunch ～2020

Coherent Comm. Tech. vs Demand

44

• Shannon Capacity Formula:

– Coherent increase S/N: logarithmic growth

• Increasing signal power in a single channel by a factor of M

• Transmitting same total power (M S) in Mchannels

2 log ( / )C S N

2 2 2 log ( / ) = log ( ) log ( / )C MS N M S N

2 M log ( / )C S N

2 =W log (1 / )C S N

Price of SECoherent Comm.

Conclusions

How, Why?

Division MultiplexingBeyond: Space-Division Multiplexing

45

Introduction


Outline





46

2. FMF: Few-Mode Fiber1.SMF

Spatial DoF: Mode…

Searching for a New DoF for Multiplexed Transmission.

MuxDoF Core

3. Multicore FiberCladding Core 1

Core N

Core 2

A New FrontierBeyond Coherent

Space-Division Multiplexing (SDM) =MDM+Core Mux

Capacity # Modes

47

Mode-Division Multiplexing: MDM

FMF

MC-FMF1. MDM： # of Channel

D=# of Modes

2. Core Mux： # of ChannelsD=# of Cores

3. SDM： # of Channel

D=#of Mode Core

SDM Goal: 100x single-fiber capacity increase

SDMBeyond Coherent

Conclusions

How, Why?


48

Introduction


Outline





49

Core Mux: increasing multiplexed channelsDue to mechanical properties of SiO2，fiber

cladding diameter is limited to ～250um; any larger will limit flexibility/deployability.

Within the limited fiber cross-section, increasing # of cores leads to• Small core-to-core distance Increased crosstalk• Small core diameter Increased Nonlinearity

Therefore # of cores is limited to ~20.

Core-multiplexing cannot increase capacity 100x

Core-MuxSDM Critical Challenge

50

Mode Crosstalk in MDM

011011

010110

110010

011011011011011011010110110010011011

010110110010011011

010110110010011011

CouplingPoint

010110110010011011

010110110010011011

010110110010011011

010110110010011011

010110110010011011

010110110010011011

Delay∝Distance

DSP computational complexity∝Delay/Distance∝# of Channel2

Digital Signal Processing(DSP)

010110110010011011

010110110010011011

010110110010011011

3 9Operations

011011

010110

110010

Mode-MuxSDM Critical Challenge

51

DSP complexity is a critical challenge，which determines the feasibility of MDM.

TX RX2000km

30 modes

Compared with SMF system：

Capacity increased: 30x DSP complexity increased:1,000,000x compared to EDC, unrealistic power consumption

Mode-MuxSDM Critical ChallengeMode Mux: DSP Complexity Example

Fast Mode

Slow Mode

Slow Mode

Fast Mode

LP01

LP21

LP01

LP21

52

High Crosstalk： DSP Complexity∝ Distance

Low Crosstalk：DSP Complexity∝ Distance

Increase Mode Crosstalk

=2000

=45

Solution 1SDM Critical Challenge

K.‐P. Ho and J. M. Kahn, J. Lightwave Technol., 2012

53

Frequency-domain equalization can achieve orders of magnitude savings in computation

Time-Domain：

Crosstalk• at different

times• Between

different modes

2D computation

Frequency Domain：

• No crosstalk between different frequencies

• Crosstalk only between modes

1D computation

FourierTransform

Solution 2SDM Critical ChallengeFrequency-Domain Equalization

N. Bai and G. Li, Photonics Techology Letters Vol.24 Issue 21. 1918‐1921(2012).

For a 10-core fiber，DSP complexity can reduce by a factor of 10.

54

FMF:1 core x6 modes

DSP Complexity∝# of Channel2

MC-FMF: 3 cores x 2 Modes

DSP Complexity∝Channel2/# of Cores

Cladding

Divide-and-Conquer

Solution 3SDM Critical Challenge

Optics + electronics

h h h h h hh h h h h hh h h h h hh h h h h hh h h h h hh h h h h h

h hh h

h hh h

h hh h

1 core x 6 modes 3 core x 2 modes

1. Strong coupling among MDM dimensions :

2. Multi‐core few‐mode fibers

wc scL L

Technologies to overcome mode crosstalk: combination of optics and electronics .

3. Frequency‐Domain Equalization

SolutionsSDM Critical Challenge

Conclusions

56

Outline

Digital coherent technology has brought the SMF capacity to the nonlinearity Shannon limit.

Single‐mode fiber capacity crunch is coming. Space‐division multiplexing (for which coherent and DSP provides the foundation) can potentially be the disruptive technology.

Fundamental research opportunities in SDM abound, both in terms of optics and electronics.

Future applications in Fundamental‐mode transmission and SDM will make the next few years very exciting for optical communications research.

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