Optical Fiber Communications: Even More Fun in the Post-Bubble Era
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Transcript of Optical Fiber Communications: Even More Fun in the Post-Bubble Era
Optical Fiber Communications:Even More Fun in the Post-Bubble
EraJoseph M. Kahn
Department of Electrical
Engineering
Stanford University
www-ee.stanford.edu/~jmk
Clean Slate Seminar, February 6, 2006
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AcknowledgmentsAcknowledgments
Adaptive Signal Processingin Multimode Networks
Elad Alon Shanhui Fan Mark A. Horowitz Wei Mao Rahul A. Panicker Mahdieh B. Shemirani Xiling Shen Vladimir Stojanovic
StrataLight Communications Keang-Po Ho
Modulation and Detection in Single-Mode Networks
Ezra Ip Alan P. T. Lau Dany-Sebastien Ly-Gagnon Jin Wang
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Optical Networks: Meter to Megameter ScaleOptical Networks: Meter to Megameter Scale
Sensors
Local- and campus-area
Access Metropolitan Long-haul Submarine
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Optical Networks: Meter to Megameter ScaleOptical Networks: Meter to Megameter Scale
Sensors
Local- and campus-area
Access Metropolitan Long-haul Submarine
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Local- and Campus-Area NetworksLocal- and Campus-Area Networks
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Optical Fiber TypesOptical Fiber Types
50mn
Graded-index multi-mode
8-10 mn
Single-mode
50mn
Step-index multi-mode
Wide-area, metro-area networks
Limitations: amplifier noise, fiber nonlinearity
Throughput (with WDM): 80 channels 40 Gb/s 4000 km
Local-area networks
Limitation: large modal dispersion
Throughput (without WDM): 100 Mb/s few km
Local-area, campus-area networks
Limitation: moderate modal dispersion
Throughput (without WDM): 1 Gb/s few km
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Modes in Optical FibersModes in Optical Fibers
Modes
Mutually orthogonal solutions of wave equation having well-defined propagation constants.
Propagate without cross-coupling in ideal fiber.
Typical multimode fiber supports of order 100 modes.
Modal coupling
Bends and imperfections couple modes over distances of the order of meters.
Coupling varies on time scale of seconds.
t
Transmitted
t
Received
Modal dispersion
Different modes have different group delays, causing pulse spreading.
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MotivationsMotivations
New techniques needed to:
Extend 10 Gb/s Ethernet over multimode fiber (currently limited to 300 m)
Enable 100 and 1000 Gb/s Ethernet over multimode fiber
Optical signal processing scales better than electronic signal processing to high bit rates, long fibers, multiple WDM channels.
Lesson from digital subscriber lines: ubiquitous, bandwidth-constrained media should be exploited to the limit.
Modal dispersion is analogous to multipath fading: should it beeliminated or exploited?
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Principal Modes in Multimode FibersPrincipal Modes in Multimode Fibers
Multimode fiber Supports 2N ideal modes (including 2 polarizations).
Ideal modes are strongly coupled by bends and imperfections.
Principal modes PMs are linear combinations of ideal modes.
Input PMs: a set of 2N orthogonal modes at fiber input.Output PMs: a set of 2N orthogonal modes at fiber output.
Each input PM propagates to the corresponding output PM:Without cross-coupling to other PMs.With a well-defined group delay.
In a given fiber, the PMs change slowly over time.Adaptive signal processing can identify and track PMs.
S. Fan and J. M. Kahn, Optics Letters, January 15, 2005.
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Adaptive Single-Input, Single-Output TransmissionAdaptive Single-Input, Single-Output Transmission
Spatial LightModulator
Multimode Fiber
OOKModulator
AdaptiveAlgorithm
Fourier Lens
Iin(t)
Trans.Data
Transmitter
Low-Rate Feedback Channel
Photo-Detector
Clock & DataRecovery
ISIEstimation
Rec.Data
ISI ObjectiveFunction
Receiver
Iout(t)
Spatial LightModulator
Multimode Fiber
OOKModulator
OOKModulator
AdaptiveAlgorithmAdaptiveAlgorithm
Fourier Lens
Iin(t)
Trans.Data
Transmitter
Low-Rate Feedback Channel
Photo-DetectorPhoto-
DetectorClock & Data
RecoveryClock & Data
Recovery
ISIEstimation
ISIEstimation
Rec.Data
ISI ObjectiveFunction
Receiver
Iout(t)
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Controlling MMF Impulse Response via SLM Controlling MMF Impulse Response via SLM
Input principal modes:
Mode incident on SLM:
SLM reflectance:
MMF impulse response:
Iin(t) Iout(t)h(t)Iin(t) Iout(t)h(t)
t
h(t)
2N pulses
t
h(t)
2N pulses
h(t)
2N pulses
2N-Mode Fiber(2N 2N)
… …
Spatial LightModulator(1 2N)
Photodetector(2N 1)
| |2
| |2
| |2
| |2 …Iin(t) R Iout(t)
InputIntensity(Scalar)
OutputPhotocurrent
(Scalar)
2N
2
1
2N-Mode Fiber(2N 2N)
… …
Spatial LightModulator(1 2N)
Photodetector(2N 1)
| |2
| |2
| |2
| |2 …Iin(t) R Iout(t)
InputIntensity(Scalar)
OutputPhotocurrent
(Scalar)
2N
2
1
),(),( ,in,in yxnn kkyx Ee
n
N
n SLMyxyxyxyx
L
tdkdkkkkkkkVP
eth
n
2
2
1
*0
0
ˆ,,,Re,in
zHE
),( yx kkV
),(),( 00 yx kkyx Ee
),(),( ,in,in yxnn kkyx Hh
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System Model and Adaptive AlgorithmSystem Model and Adaptive Algorithm
Continuous-time impulse response:
Discrete-time impulse response:
Objective function quantifying ISI:
Note that F(g(nT; t0)) > 0 when eye open and F(g(nT; t0)) <0 when eye closed.
Adaptive algorithm controls V(kx, ky) to maximize F(g(nT; t0)).
desiredbit interval
undesiredbit intervals
trthtptg
nTtt
tgtnTg 0
0;
0
000 ;;0;n
tnTgtTgtnTgF
ReceivedBits 1,0ˆ na
Transmitted Bits 1,0na
Receiver ImpulseResponse
r(t)
Transmitted Pulse Shape
p(t)
MMF ImpulseResponse
h(t)+
Noisen(t)
nTtt 0
ISIObjective Function
F(g(nT; t0))AdaptiveAlgorithm
ISI Estimation
ControlSLM
V(kx, ky)
Iin(t) Iout(t) ReceivedBits 1,0ˆ na
ReceivedBits 1,0ˆ na
Transmitted Bits 1,0na
Receiver ImpulseResponse
r(t)
Receiver ImpulseResponse
r(t)
Transmitted Pulse Shape
p(t)
Transmitted Pulse Shape
p(t)
MMF ImpulseResponse
h(t)++
Noisen(t)
nTtt 0
ISIObjective Function
F(g(nT; t0))AdaptiveAlgorithmAdaptive
AlgorithmISI
EstimationISI
Estimation
ControlSLM
V(kx, ky)
Iin(t) Iout(t)
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Experimental SetupExperimental Setup
X. Shen, J. M. Kahn and M. A. Horowitz, Optics Letters, November 15, 2005.
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10 Gb/s 10 Gb/s 1030 m, Good and Bad SOPs, Binary SLM 1030 m, Good and Bad SOPs, Binary SLM
Launched SOP Condition SLM
Pattern System Impulse
Response Eye Pattern Result
Good SOP Before
adaptation
g(t)
0 500 1000t (ps)
g(t)
0 500 1000t (ps)
-1
0
1 -1
0
1-1
-0.5
0
0.5
1
S2/S
0S1/S
0
S3/S
0
S3/S0
1
S1/S0
1 1S2/S0-1
0
1 -1
0
1-1
-0.5
0
0.5
1
S2/S
0S1/S
0
S3/S
0
S3/S0
1
S1/S0
1 1S2/S0
Good SOP After 1
iteration
g(t)
0 500 1000t (ps)
g(t)
0 500 1000t (ps)
Error-free to 231 – 1 PRBS
Bad SOP Before
adaptation
g(t)
0 500 1000t (ps)
g(t)
0 500 1000t (ps)
-1
0
1
-1
0
1-1
-0.5
0
0.5
1
S1/S
0S
2/S
0
S3/S
0
S3/S0
1
S2/S0
1 1S1/S0
-1
0
1
-1
0
1-1
-0.5
0
0.5
1
S1/S
0S
2/S
0
S3/S
0
S3/S0
1
S2/S0
1 1S1/S0
Bad SOP After 3
iterations
g(t)
0 500 1000t (ps)
g(t)
0 500 1000t (ps)
Error-free to 231 – 1 PRBS
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10 Gb/s 10 Gb/s 11081 m, Good SOP, Vertical Misaligment of Launch, 11081 m, Good SOP, Vertical Misaligment of Launch,Binary vs. Quaternary SLMBinary vs. Quaternary SLM
Launched SOP Condition SLM
Pattern Eye Pattern Result
Good SOP Vertical misalignment
Before adaptation
Good SOP Vertical misalignment
After 3 iterations Binary SLM
Error-free to 27 – 1 PRBS
-1
-0.5
0
0.5
1 -1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
S2/S
0S1/S
0
S3/S
0
S3/S0
1
S1/S0
11S2/S0
-1
-0.5
0
0.5
1 -1
-0.5
0
0.5
1
-1
-0.5
0
0.5
1
S2/S
0S1/S
0
S3/S
0
S3/S0
1
S1/S0
11S2/S0
Good SOP
Vertical misalignment After 3 iterations Quaternary SLM
Error-free to 231 – 1 PRBS
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10 Gb/s 10 Gb/s 11081 m, Good SOP, Binary SLM, 11081 m, Good SOP, Binary SLM,Tune Laser Over 600 GHzTune Laser Over 600 GHz
Condition SLM Pattern Eye Pattern Result
Good SOP Before adaptation
Channel 58 (193.40 THz)
Good SOP After 2 iterations
Channel 58 (193.40 THz)
Error-free to 231 – 1 PRBS
Good SOP Keep SLM fixed
Channel 51 (193.75 THz)
Error-free to 231 – 1 PRBS
Good SOP Keep SLM fixed
Channel 62 (193.20 THz)
Error-free to 231 – 1 PRBS
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Adaptive Spatial Optical Signal ProcessingAdaptive Spatial Optical Signal Processing
Key to exploiting principal modes.
Can be implemented using spatial light modulators.
One SLM can serve multiple WDM channels.
SLM requirements are at least somewhat independent ofbit rate and fiber length.
Contrast with electrical equalizers: Must be implemented separately for each WDM channel.
FIR filter-based equalizers: number of taps proportional tobit rate fiber length.
Maximum-likelihood sequence detectors: number of states exponential in bit rate fiber length.
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Ongoing and Future WorkOngoing and Future Work
Ongoing Modeling propagation and principal modes
Optimal one-shot and adaptive algorithms
Robustness to perturbations of fiber
Future What can we learn from adaptive systems to improve design
of lower-complexity systems?
Extension to other multimode media, e.g., polymer waveguidesfor board-level interconnects
Electronics and optics for faster adaptation
Multi-input, multi-output transmission
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Optical Networks: Meter to Megameter ScaleOptical Networks: Meter to Megameter Scale
Sensors
Local- and campus-area
Access Metropolitan Long-haul Submarine
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Segments of Telecom NetworksSegments of Telecom Networks
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Access NetworksAccess Networks
Technologies (Heterogeneity Rules) Wireless (radio and microwave) Free-space optical DSL over copper twisted pair QAM over hybrid fiber-coax (single-mode fiber) TDM / WDM over passive optical networks (single-mode fiber)
Some Issues Performance vs. cost of installation and maintenance Initial cost vs. upgradeability
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Passive Optical Networks for AccessPassive Optical Networks for Access
Downstream / upstream: 1.55 m / 1.31 m (coarse WDM) Downstream: broadcast and select (TDM or TDM / WDM) Upstream: TDMA
single downstreamwavelength (TDM)
multiple downstreamwavelengths (TDM / WDM)
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Dense Wavelength-Division-MultiplexingDense Wavelength-Division-Multiplexing
ff1
ff2
ffN
…
ff1 f2 fN…
Mux Demux
Erbium-Doped Fiber Amps
Tx1
Tx2
TxN
…
Rx1
Rx2
RxN
…
ff1
ff2
ffN
…
EDFA Bands4.4 THz 5.4 THz
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Metropolitan and Long-Haul NetworksMetropolitan and Long-Haul Networks
Use single-mode fiber Use electrical TDM (e.g., SONET) on WDM Transmission
Maturing technology, challenging business 10 Gb/s transceivers approaching commodity status Unused (dark) fibers exist on many routes Many underutilized systems exist, e.g., 10 Gb/s 80 wavelengths
with only 10 wavelengths in use
Switching Circuit switching: becoming more dynamic and flexible, for
reprovisioning, survivability, etc. Packet switching: infeasible for several reasons, especially
lack of scalable optical buffers
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Metropolitan and Long-Haul NetworksMetropolitan and Long-Haul Networkswith Wavelength Routingwith Wavelength Routing
Long-haul core
Router
RouterMetro ring Metro ring
Opticaladd-drop
demultiplexer
Opticalcross-
connect
Chicago Cleveland New York
Phila-delphia
NashvilleKansas
City
Router
Router
Router
1
2
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Long-Haul Transmission: TrendsLong-Haul Transmission: Trends
Increasing per-channel bit rates For a given capacity, reduces number of ports on routers and
optical switches. Last generation: 10 Gb/s New generation: 40 Gb/s Goal (of some): 160 Gb/s
Increasing capacity Traffic continues to increase exponentially; capacity must increase,
cost per bit must decrease. EDFA bandwidth is limited, Raman amps are expensive. Best solution is to increase spectral efficiency:
Last generation: 0.2 - 0.4 b/s/Hz New generation: 0.8 b/s/Hz Binary limit: 1 b/s/Hz Non-binary limit: perhaps 3-5 b/s/Hz
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Long-Haul Transmission: ChallengesLong-Haul Transmission: Challenges
Increasing per-channel bit rates Chromatic dispersion: Polarization-mode dispersion: Electronic circuits
Increasing spectral efficiency Crosstalk and distortion in muxes, demuxes, OADMs, OXCs Requires higher signal-to-noise ratio, but transmitted power is
limited by nonlinearities in fiber.
LB 2penalty
LB penalty
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Approaches for 40 Gb/s Systems at 0.8 b/s/HzApproaches for 40 Gb/s Systems at 0.8 b/s/Hz
Mainstream Goal: maximize unrepeatered transmission distance (à la Qtera) Use OOK or DPSK with RZ pulses (broader spectrum) Achieves: > 2000 km, 3.2 Tb/s (C band), 6.4 Tb/s (C + L bands) Higher cost:
Two-stage modulator, complex receiver (for DPSK) Careful control of chromatic dispersion in transmission system
Requires specially designed transmission system
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RZ DPSK with Interferometric DetectionRZ DPSK with Interferometric Detection
0
0
1 0
ESI
ESQ
1 1i
1
1
OpticalBPF
fS
Elect.LPF
i0
T
Differ-ential
EncoderBits
fS ffS
…
Laser
Clock
t
ES
t
ES
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Approaches for 40 Gb/s Systems at 0.8 b/s/HzApproaches for 40 Gb/s Systems at 0.8 b/s/Hz
Mainstream Goal: maximize unrepeatered transmission distance (à la Qtera) Use OOK or DPSK with RZ pulses (broader spectrum) Achieves: > 2000 km, 3.2 Tb/s (C band), 6.4 Tb/s (C + L bands) Higher cost:
Two-stage modulator, complex receiver (for DPSK) Careful control of chromatic dispersion in transmission system
Requires specially designed transmission system
StrataLight Communications (founded in June 2000) Goal: minimize cost per Gb/s•km with sufficient unrepeatered distance Use OOK with NRZ pulses and line coding (narrower spectrum) Achieves: > 1200 km, 3.2 Tb/s (C band), 6.4 Tb/s (C + L bands) Lower cost:
Simple modulator and receiver Less careful control of chromatic dispersion in transmission system
Can retrofit to some underutilized 10 Gb/s transmission systems
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Line-Coded OOK with Direct DetectionLine-Coded OOK with Direct Detection
i0 1
0 1
|ES|2
0 1
ES0 11
LaserLine
CoderBits
fS
…
fS
ft
ES
OpticalBPF
fS
ElectricalLPF
i
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Spectral Efficiency vs. SNR EfficiencySpectral Efficiency vs. SNR Efficiency
SNR/bit Required Relative to 2-PAM (dB)
-3 0 3 6 9 12 151
2
3
4R
elat
ive
Spe
ctra
l Effi
cien
cy l
og2(
M)
(b/
sym
bol)
Num
ber
of C
onst
ella
tion
Poi
nts
M
2
4
8
16
QAM / Coherent
PSK / Coherent
DPSK / Interferometricor Diff. Coherent
PAM / Director Non-Coherent
nb/neq Required for Pb = 109 (photons/bit)
20 50 100 200 500 1000 2000
2 DF / pol.1 DF / pol.
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4-PSK with Coherent Detection4-PSK with Coherent Detection
Example: synchronous homodyne
detection (optical phase-locked
loop not shown)
ffS
fS
…
LaserEncoderBits
90
EI
EQ
Elect.LPF
iI
Elect.LPF
iQLOLaser
Pol.Contr.
fL = fS
f0
f0
90
0
10
01
11
00
ESI
ESQ
Signal Local Oscillator
ELI
ELQ
Photocurrent
iI
iQ
0001
1011
A. Porter and J. M. Kahn, 1992
S. Norimatsu et al, 1992
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16-QAM with Coherent Detection16-QAM with Coherent Detection
…
LaserEncoderBits
90
EI
EQ
Elect.LPF
iI
Elect.LPF
iQLOLaser
Pol.Contr.
90
0
Local Oscillator
ELI
ELQ
ESI
ESQ
Signal Photocurrent
iI
iQ
0000 0001 0011 0010
0100 0101 0111 0110
1100 1101 1111 1110
1000 1001 1011 1010
0000 0001 0011 0010
0100 0101 0111 0110
1100 1101 1111 1110
1000 1001 1011 1010
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Coherent Optical Detection: Pros and ConsCoherent Optical Detection: Pros and Cons
Advantages Yields 2 degrees of freedom: higher spectral efficiency Receiver detects all information in signal electric field
enables digital signal processing to compensate impairments Chromatic dispersion Polarization-mode dispersion Nonlinear phase noise
Can use tunable local oscillator with electrical filtering to select channel enables fast-tunable receiver for wavelength switching (or FHSS)
Drawbacks Requires local oscillator laser at receiver Requires polarization tracking or diversity at receiver
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Nonlinear Phase NoiseNonlinear Phase Noise
EI
EQ
Linear Regime
EI
EQ
Nonlinear Regime
37
Optical Communication Research IssuesOptical Communication Research Issues
Transmission Higher spectral efficiency vs. wider utilized bandwidth Spectral efficiency vs. robustness vs. implementation complexity Signal processing: optical vs. analog vs. digital
Switching Circuit switching: faster and more flexible Packet switching?
Component evolution New fiber types New amplifier types Optical buffers?
Analysis Spectral efficiency limits Nonlinear phase noise
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DARPA MTO TACOTA ProgramDARPA MTO TACOTA Program
Coherent links for tactical air-to-air communications
Major team: CeLight, Stanford (Fejer, Kahn), Boeing, HRL
Transmit at 3.8 m to minimize atmospheric effects 1.55 m transmitters and receivers Transmitter: 1.55 3.8 m downconverter Receiver: 3.8 1.55 m upconverter
Use frequency hopping for LPI/LPD
Receiver architecture Homodyne (direct conversion to baseband) Use sampling and DSP algorithms to compensate carrier phase,
Doppler shifts, atmospheric turbulence, etc.