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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
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!
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?
Space-Division Multiplexing
18
The Optics Happy Era
Digital Coherent Optical Communication
Outline
Capacity Limits of SMF
Critical Challenges & Solutions
Future Challenge & Prospects
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
Tingye Li, Herwig Kogelnik, Alan Willner
2015
D=0N=1000s
N=9
+/-D=0N=39
+D, -DN=20
Coherent 2+D
N=18
EDFA
Conclusions
How, Why?
Space-Division Multiplexing
28
Introduction
Coherent Optical Communication
Outline
Capacity Limits of SMF Capacity Limits of SMF
Critical Challenges & Solutions
Future Challenge & Prospects
The Optics Happy Era
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.
Tingye Li, Herwig Kogelnik, Alan Willner
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?
Space-Division Multiplexing
41
Introduction
Coherent Optical Communication
Outline
Capacity Limits of SMF Capacity Limits of SMF
Critical Challenges & Solutions
Future Challenge & Prospects
The Optics Happy Era
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
Coherent Optical Communication
Outline
Capacity Limits of SMF Capacity Limits of SMF
Critical Challenges & Solutions
Future Challenge & Prospects
The Optics Happy Era
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?
Space-Division Multiplexing
48
Introduction
Coherent Optical Communication
Outline
Capacity Limits of SMF Capacity Limits of SMF
Critical Challenges & Solutions
Future Challenge & Prospects
The Optics Happy Era
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
Page 55
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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