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

2

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

3

Optical Networks: Meter to Megameter ScaleOptical Networks: Meter to Megameter Scale

Sensors

Local- and campus-area

Access Metropolitan Long-haul Submarine

4


Sensors



5

Local- and Campus-Area NetworksLocal- and Campus-Area Networks

6

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

7

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.

8

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?

9

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.

10

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)

11

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

12

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


r(t)


r(t)


p(t)


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)

13

Experimental SetupExperimental Setup

X. Shen, J. M. Kahn and M. A. Horowitz, Optics Letters, November 15, 2005.

14

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)


15

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


-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


16

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



Good SOP Keep SLM fixed



Good SOP Keep SLM fixed



17

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.

18

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

19


Sensors



20

Segments of Telecom NetworksSegments of Telecom Networks

21

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

22

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)

23

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

24

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

25

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

26

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

27

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

28

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

29

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

30

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

31

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

32

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.

33

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

34

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

35

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

36

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

38

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.

optical fiber communications: even more fun in the post-bubble era

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