New functionalities for advanced optical interfaces (Dispersion compensation)

1
New functionalities for advanced optical
interfaces (Dispersion compensation)

• Kazuo Yamane
• Photonic systems development dept.

2
Outline

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

3
Signal distortion due to chromatic dispersion
Spectrum broadening
Optical spectrum
??
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
0
1
Wavelength
??
Time
Time
4
Waveform distortion due to fiber
non-linearity
High power intensity
Frequency chirp
Refractive index change
Waveform distortion due to chromatic dispersion
Spectrum broadening
Optical fiber
Low optical power
High optical power
Received waveform
Transmitter out
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)
Slow (Fast)
Shorter wavelength
40 Gb/s optical signal
After fiber transmission
Transmitter output
After dispersion comp.
6
DC allocations and dispersion maps
Post-comp.

Fiber1
Fiber2
0
R.D. ps/nm
Distance km
-
Pre-comp.

Fiber1
Fiber2
0
R.D. ps/nm
Distance km
-
Post- Pre- comp.

Fiber1
Fiber2
0
R.D. ps/nm
Distance km
-
7
Residual dispersion and tolerance of
receiver
Allowable penalty


Longer wavelength
Dispersion tolerance of receiver
Center wavelength
0
R.D. ps/nm
R.D. ps/nm
Shorter wavelength
-
-
Distance km
Penalty dB
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
8
Comparison of 40Gbit/s modulation
schemes
CS-RZ
Optical duobinary
NRZ
RZ
108 GHz
180 GHz
165 GHz
70 GHz
Now evaluating transmission performance
Chromatic dispersion tolerance Fibre non-linear
tolerance (Maximum input power) Spectral
tolerance (Degradation due to filter narrowing)
9
A past field experiment example

• 10Gbit/s 750km WDM field trial between Berlin and
  Darmstadt (Ref. OFC/IOOC99, Technical Digest
  TuQ2, A. Ehrhardt, et.al.)

Link for field trial
Berlin
Darmstadt
Before Optimization
E/O
O/E
Post-amplifier
Pre-amplifier
After optimization
900 ps/nm
-400 ps/nm
E/O
O/E
Post-amplifier
Pre-amplifier
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Dispersion maps and waveforms in the trial
Before optimization
Dispersion (ps/nm)
Channel 1
Channel 3
Channel 2
Channel 4
0
800
600
400
200
Distance (km)
After optimization
Dispersion (ps/nm)
Channel 1
(Before)
Channel 1
(After)
0
800
600
400
200
Distance (km)
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Automatic dispersion compensation example
Provisioning Tracking
Provisioning
Rx 1
l1
Tx 1
Tx 2
l2
Rx 2
VDC
VDC
l40
Tx 40
Rx 40
DC
DC
li
Dispersion compensator (fixed or variable)
Dispersion Monitor
VIPA variable dispersion compensator
DC gt 0
Line-focusing lens
Variable
x-axis
Optical circulator
DC lt 0
Focusing lens
Collimating lens
Glass plate
3-Dimensional Mirror
VIPA Virtually Imaged Phased Array
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Dispersion compensation trend
Photonic network Manage dispersion or residual
dispersion (dispersion map) !!
Transmitter / Receiver Adjust parameters
including residual dispersion to optimum!!
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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Higher-order PMD

D tn
D t4
D t1
D t2
D t3
Mode-coupling at random locations with random
strength
Maxwellian distribution of the instantaneous DGD
-Frequency dependence of DGD
-Statistically varying due to environmental
fluctuations
Frequency of occurrence
Prob.(DGDgt3xPMD) 4x10-5 21 min/year
-Fiber PMD unit ps/ km
Prob.(DGDgt3.5xPMD) 10-6 32 sec/year
PMD
3.5PMD
Instantaneous DGD (ps)
15
Automatic PMD compensation
PMD compensation scheme in receiver
40Gb/s waveforms
Before PMD comp.
PMD comp. device 3
PMD comp. device 2
PMD comp. device 1
O/E module
Distortion analyzer
Control algorithm
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
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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.