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New functionalities for advanced optical interfaces (Dispersion compensation)

Kazuo YamanePhotonic systems development dept.

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Outline

Chromatic dispersion effect Dispersion compensating techniques Optimization of residual dispersion or its map PMD compensation Conclusions

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Signal distortion due to chromatic dispersion

Spectrum broadening

Difference in group velocity

Wavelength

Gro

up

vel

oci

ty

Δλ

1

Time

1 0Original signal

Time

Transmitter output

Time

Receiver input

Time

111Regenerated signal

Wavelength

Optical spectrum

Δλ

Pulse broadening(Waveform distortion)

Optical fiber

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Waveform distortion due to fiber non-linearity

Transmitter out Received waveform

Low optical power High optical power

High power intensity

Frequencychirp

Refractive index change

Waveform distortion due to chromatic dispersion

Optical fiber

Spectrumbroadening

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After fiber transmission

40 Gb/s optical signal

Transmitter output

25 ps

Transmission fiber

Positive dispersion(Negative dispersion)

+Dispersion compensating fiber (DCF)

After dispersion comp.

Negative dispersion (Positive dispersion)

Longer wavelength

Slow (Fast)

Shorter wavelength

Fast (Slow)

Longer wavelength

Fast (Slow)

Shorter wavelength

Slow (Fast)

Dispersion compensation example

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Fiber#1

DC allocations and dispersion maps

DCDC

Fiber#2

Distance [km]

R.D

. [p

s/n

m]

Fiber#1

DC DC

Fiber#2

Fiber#1

DCDC

Fiber#2

DC

Distance [km]

R.D

. [p

s/n

m]

Distance [km]

R.D

. [p

s/n

m]

Post-comp.

Pre-comp.

Post- & Pre-

comp.

+

-

-

-

+

+

0

0

0

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Residual dispersion and tolerance of receiver

Distance [km]

R.D

. [p

s/n

m]

Dispersiontolerance

of receiver

-

+

Need to consider the variation of tolerance due to characteristics of transmitter, fibre non-linear effects and dispersion map.Even if residual dispersion values are same, the received waveforms are different, affected by these parameters.

Parameters affecting to the tolerance - Signal bit rate - Channel counts and spacing - Distance or number of spans - Fibre type - Fibre input power - Pre-chirping of transmitter - Modulation scheme of transmitter - DC allocation / value

-

R.D

. [p

s/n

m]

+

0

Penalty [dB]

Longer wavelength

Shorter wavelength

Center wavelength

Allowable penalty

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CS-RZ Optical duobinaryNRZ RZ

Opt

ical

pow

er (

dBm

)

Wavelength (nm)

0

-20

-40

1542 1545 1548

0

-20

-40

1542 1545 1548

0

-20

-40

1542 1545 1548 1542 1545 1548

0

-20

-40

Wavelength (nm) Wavelength (nm) Wavelength (nm)

Chromatic dispersion toleranceFibre non-linear tolerance (Maximum input power)Spectral tolerance (Degradation due to filter narrowing)

108 GHz 180 GHz 165 GHz 70 GHz

Now evaluating transmission performance

Comparison of 40Gbit/s modulation schemes

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A past field experiment example

Berlin DarmstadtLink for field trial

-400 ps/nm +900 ps/nm

E/OO/E

10Gbit/s 750km WDM field trial between Berlin and Darmstadt (Ref.: OFC/IOOC’99, Technical Digest TuQ2, A. Ehrhardt, et.al.)

Post-amplifier

Post-amplifier

Pre-amplifier

Pre-amplifier

E/OO/E

After optimization

Before Optimization

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Dispersion maps and waveforms in the trial

-2000

-1500

-1000

-500

0

500

1000

1500

2000

Dis

per

sio

n (

ps/

nm

)

Channel 1

Channel 2

Channel 3

Channel 4

-2000

-1500

-1000

-500

0

500

1000

1500

2000

0Distance (km)

Dis

per

sio

n (

ps/

nm

)

Before optimization

After optimization

800600400200

0Distance (km)

800600400200

Channel 1Channel 1

(Before)(After)

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i

Provisioning1

40 Tx #40

Tx #1

2 Tx #2

Rx #1

Rx #2

Rx #40

Automatic dispersion compensation example

VDC VDC

DispersionMonitor

DispersionMonitor

DC

Dispersion compensator (fixed or variable)

Provisioning&

Tracking

Collimating lens

Line-focusing lens

Glass plate

Focusing lens

3-Dimensional Mirror

Optical circulator

Variablex-axis

VIPA : Virtually Imaged Phased Array

VIPA variable dispersion compensator

DC

DC > 0

DC < 0

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Dispersion compensation trend

Photonic network

Manage dispersion or residual dispersion (dispersion map) !!

NE

NE

NE

NE

NE

Transmitter / Receiver

Adjust parameters including residual

dispersion to optimum!!

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Polarization Mode Dispersion (PMD)

- Well defined, frequency independent eigenstates

- Deterministic, frequency independent Differential Group Delay (DGD)

- DGD scales linearity with fiber length

1st-order PMD

Ideal Practical

Core

Cladding

Cross-section of optical fiber

Fast axis

Slow axis

Fast

Slow

Differential Group Delay (DGD)

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1 42

3n

Mode-coupling at random locations with random strength

Higher-order PMD

…

Fre

qu

enc

y o

f o

ccu

rren

ce

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

-Frequency dependence of DGD

-Statistically varying due to environmental fluctuations

-Fiber PMD unit: ps/ km

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Automatic PMD compensation

PMD characteristic changes slowly due to “normal” environmental fluctuations (e.g. temperature)

But, fast change due to e.g. fiber touching

High-speed PMD compensation device & Intelligent control algorithm

PMD compensation scheme in receiver

Before PMD comp.

After PMD comp.

40Gb/s waveforms

Distortionanalyzer

Controlalgorithm

PMDcomp.

device #3

PMDcomp.

device #2

PMDcomp.

device #1

O/Emodule

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Conclusions

In fibre optical high bit rate (such as 10G or 40G bit/s) long-haul transmission systems, dispersion compensation is one of the most important items to be considered for design.

Management or optimization of residual dispersion are required for photonic networks, i.e., for fibres, repeaters and optical interfaces.

PMD compensation is also required especially for 40Gbit/s or higher bit rate long-haul systems.