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High Speed Optical NetworksHigh Speed Optical Networks----ExperimentsExperiments

Professor Gary M. Carter Dr. Anthony Lenihan

and Professor Curtis Menyuk

University of Maryland Baltimore County

UMIACS/LTS Review 6/18/04

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Collaborators

Robert RunserTelcordia, LTS

L. MercerNaval Research Laboratory

T. Gibbons, P. SchulzMIT Lincoln Laboratory

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Goals—Past year(s)

• Carry out experiments on installed optical communication systems

• To identify and investigate issues of interest in helping establish high speed operation of optical networks

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This year’s effort

• We have shown in laboratory experiments that dispersion distribution is important for communications performance

• We moved these experiments from the lab to ATDNET/BossNet

• Implications for network management • Investigated novel formats that can increase

performance and relax constraints

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Nonlinear Interaction

• Dispersion causes adjacent pulses to spread in time and overlap for part of the transmission distance

• The nonlinear interaction due to the optical fiber between adjacent pulses leads to timing jitter

• The dispersion distribution can alter the amount of timing jitter

• Large amounts of timing jitter causes errors

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Nonlinear Interaction—Frequency Shift

TL uuu +=

tu

zf TL

dd

dd

2

γ−=

FWHM = 100 ps

0

0.01

0.02

600 800 1000

Time (ps)

Pow

er (m

W)

Lu

LuTuIntensity variation of shifts frequency of

Tu

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Residual compensation all at the end

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Nearly closed eye and large error rate due to timing jitter

10 Gb/s

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Split compensation between beginning and end

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Open eye end low bit-error rate

10 Gb/s

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Laboratory Results

• Intra-channel cross-phase modulation → timing jitter

• Dependent on dispersion map configuration– Most work to date on undersea transmission systems– Mitigated by symmetric pre- and post-compensation

• Laboratory investigation of terrestrial system– Xu, et al., IEEE Phot. Tech. Lett. 16, 314 (2004)

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Experiments on ATDNET/BossNet

• Measure timing jitter at 10 Gb/s on an installed terrestrial fiber path

• Obtain excellent qualitative agreement between measured values and numerical simulations

• Arrangement of dispersion compensation important for obtaining best transmission

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Installed fiber path

Segment 1 (ATDNet):

Segment 3 (BOSSNet):

Washington, D.C.

• TrueWave RS (+4.5 ps/nm-km), 41 km• Uncompensated

• TrueWave Classic (+2.8 ps/nm-km)– To Wilmington: 186 km– To New York City: 389 km

• Compensated at 1550 nm• Uneven EDFA spacing

Segment 2• AllWave (+17 ps/nm-km), 16 km• Uncompensated

One way distances:

=Tx/Rx

Wilmington, DE

New York, NY

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Experimental setup

InstallednetworkCW Laser

@1558.98nm EAM EOM

DC

NYC

BERT

Oscilloscope

Clock/DataRecovery

OBF

10 GHz Clock

10 Gb/sPRBS

Pre-Compensation

Post-Compensation

OSA

Rx:

Tx:

• Tx/Rx co-located in D.C.

EDFA

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Dispersion mapsLoopback at Wilmington, DE Loopback at New York, NY

• Not including pre- and post-compensation• Total accumulated dispersion for round trip at 1558.98 nm

= EDFA

Accu

mul

ated

Dis

pers

ion

(ps/

nm)

Distance (km) Distance (km)

0

800

1600

0 6000

900

1800

0 1000

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Numerical model

• Full nonlinear Schrödinger equation• 200 Monte Carlo simulations• Utilizing best estimate of system parameters:

– Dispersion map– Amplifier noise figures– Power map

Timing jitter results compared with numerical simulation:

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Eye diagramsW

ilmin

gton

, DE

New

Yor

k, N

Y

50 % Pre-compensation 90 % Pre-compensation

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Wilmington loopback results

• Total transmission distance ~520 km• Symmetric dispersion compensation minimizes timing jitter• Excellent qualitative agreement with theory

0.5

1.5

2.5

3.5

0 25 50 75 100

Pre-Compensation (%)

Tim

ing

Jitte

r (ps

)

ExperimentTheory

7.0

7.5

8.0

8.5

9.0

0 25 50 75 100

Pre-Compensation (%)Q

Fact

or

Experiment

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New York loopback results

• Total transmission distance ~930 km• Jitter increased, but similar trend

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 25 50 75 100

Pre-Compensation (%)

Tim

ing

Jitte

r (ps

)

ExperimentTheory

6.0

6.5

7.0

7.5

8.0

8.5

9.0

0 25 50 75 100

Pre-Compensation (%)Q

Fact

or

Experiment

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Summary

• Timing jitter measured on installed terrestrial link

• Qualitative agreement with simulations

• Ability to control dispersion compensation layout will be important in typical terrestrial systems may influence network management when paths change

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Novel Formats

• Explore Novel formats • DPSK• QPSK• QPSK and PolMux• Others

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DPSK

• Phase modulated signal• 3 dB advantage over ASK• Constant amplitude pulses• Less “patterning”• Security implications

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DPSK Transmitted Intensity Pattern

10 Gb/s

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Receiver

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Demodulated Eye Pattern—10Gb/s

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DPSK Loop Back from LPS to ISIError Rate < 1 10−13

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DPSK

• 3 dB performance advantage over ASK• Less Pattern Dependence in Transmission—

maybe smaller timing jitter• More difficult to detect—need specialized

receiver• Provides different information to network

management

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New Directions for Upcoming Year

• We are working with LTS to adapt measurements and theory to interactions with system and optical control plane

• LTS Personnel: Mark Ciccarello , Dave Hardesty, and Walter Kaechele