1. Kazuo Yamane Photonic systems development dept.
New functionalities for advanced optical interfaces (Dispersion compensation)
Kazuo Yamane
Photonic systems development dept.
Fujitsu

2. Outline Chromatic dispersion effect Dispersion compensating techniques
Optimization of residual dispersion or its map
PMD compensation
Conclusions
Fujitsu

3. Signal distortion due to chromatic dispersion (Waveform distortion)
Optical spectrum
Spectrum broadening Δλ
Difference in group velocity
Wavelength
Pulse broadening
(Waveform distortion)
Transmitter output
Receiver input
Optical fiber
Time
Time
Group velocity
Original signal
Regenerated signal 1 1 1 1 1
Wavelength Δλ
Time
Time
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4. Waveform distortion due to fiber non-linearity
High power
intensity
Refractive index change
Frequency
chirp
Spectrum
broadening
Waveform distortion due to chromatic dispersion
Optical fiber
Low optical power
High optical power
Transmitter out
Received waveform
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5. Dispersion compensation example
Transmission fiber
Dispersion compensating fiber (DCF) +
Positive dispersion
(Negative dispersion)
Negative dispersion
(Positive dispersion)
Longer wavelength
Slow (Fast)
Longer wavelength
Fast (Slow)
Shorter wavelength
Fast (Slow)
Shorter wavelength
Slow (Fast)
40 Gb/s optical signal
25 ps
Transmitter output
After fiber transmission
After dispersion comp.
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6. DC allocations and dispersion maps
Post-comp. +
Fiber#1
Fiber#2
R.D. [ps/nm] DC DC
Distance [km] -
Pre-comp. +
Fiber#1
Fiber#2
R.D. [ps/nm] DC DC
Distance [km] -
Post- & Pre- comp. +
Fiber#1
Fiber#2
R.D. [ps/nm] DC DC DC
Distance [km] -
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7. Residual dispersion and tolerance of receiver
Allowable penalty + +
Longer wavelength
Dispersion
tolerance
of receiver
Center wavelength
R.D. [ps/nm]
R.D. [ps/nm]
Shorter wavelength - -
Distance [km]
Penalty [dB]
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
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.
Fujitsu

8. Comparison of 40Gbit/s modulation schemes
NRZ RZ
CS-RZ
Optical duobinary
Optical power (dBm)
Wavelength (nm)
-20
-40
1542
1545
1548
108 GHz
180 GHz
165 GHz
70 GHz
This shows the optical waveforms and the optical spectra for four 40-Gbit/s optical modulation schemes.
Their respective spectral bandwidth at a 20-dB reduction was located here.
RZ and CS-RZ has a larger spectral bandwidth than that of NRZ.
Optical duobinary has a narrower spectral bandwidth than that of NRZ.
CS-RZ has a narrower spectral bandwidth than that of RZ.
We compared experimentally the chromatic dispersion tolerance, optical power limit, and the waveform degradation caused by optical MUX/DEMUX filters for these modulation schemes.
Next, I will talk about the experiment results.
Now evaluating transmission performance
Chromatic dispersion tolerance
Fibre non-linear tolerance (Maximum input power)
Spectral tolerance (Degradation due to filter narrowing)
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9. A past field experiment example
10Gbit/s 750km WDM field trial between Berlin and Darmstadt (Ref.: OFC/IOOC’99, Technical Digest TuQ2, A. Ehrhardt, et.al.)
Link for field trial
Berlin
Darmstadt
Before Optimization
O/E
E/O
Post-amplifier
Pre-amplifier
After optimization
-400 ps/nm
+900 ps/nm
O/E
E/O
Post-amplifier
Pre-amplifier
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10. Dispersion maps and waveforms in the trial
Before optimization
-2000
-1500
-1000
-500
500
1000
1500
2000
Dispersion (ps/nm)
Channel 1
Channel 3
Channel 2
Channel 4
200
400
600
800
Distance (km)
After optimization
-2000
-1500
-1000
-500
500
1000
1500
2000
Dispersion (ps/nm)
Channel 1
(Before)
Channel 1
(After)
200
400
600
800
Distance (km)
Fujitsu

11. Automatic dispersion compensation example
Provisioning
&
Tracking
Provisioning l1
Tx #1
Rx #1 l2
Tx #2
VDC
VDC
Rx #2
l40
Tx #40
Rx #40 DC DC li
Dispersion compensator
(fixed or variable)
Dispersion
Monitor
VIPA variable dispersion compensator
DC > 0
Line-focusing lens
Variable
x-axis
Optical circulator
DC < 0
Focusing lens
Collimating lens
Glass plate
3-Dimensional Mirror
VIPA : Virtually Imaged Phased Array
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12. Dispersion compensation trendNE NE
Photonic network
Manage dispersion or residual dispersion (dispersion map) !! NE NE NE
Transmitter / Receiver
Adjust parameters including residual dispersion to optimum!!
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13. Polarization Mode Dispersion (PMD)
Cross-section of optical fiber
Ideal
Practical
Cladding
Fast axis
Core
Slow axis
1st-order PMD
Fast Dt Dt
Slow
D t : Differential Group Delay (DGD)
- Well defined, frequency independent eigenstates
- Deterministic, frequency independent Differential Group Delay (DGD)
- DGD scales linearity with fiber length
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14. Higher-order PMD … D t1 D t2 D t3 D t4 D tn
Mode-coupling at random locations with random strength
-Frequency dependence of DGD
Maxwellian distribution
of the instantaneous DGD
-Statistically varying due to
environmental fluctuations
Frequency of occurrence
Prob.(DGD>3xPMD)
= 4x10-5 = 21 min/year
-Fiber PMD unit: ps/ km
Prob.(DGD>3.5xPMD)
=10-6 = 32 sec/year
PMD
3.5PMD
Instantaneous DGD (ps)
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15. Automatic PMD compensation
PMD compensation scheme in receiver
40Gb/s waveforms
Before PMD comp.
PMD
comp.
device #1
PMD
comp.
device #2
PMD
comp.
device #3
O/E
module
Control
algorithm
Distortion
analyzer
After PMD comp.
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
Fujitsu

16. 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.
Fujitsu