New functionalities for advanced optical
interfaces (Dispersion compensation)
Kazuo Yamane
Photonic systems development dept.
1
Fujitsu
Outline





Chromatic dispersion effect
Dispersion compensating techniques
Optimization of residual dispersion or its map
PMD compensation
Conclusions
2
Fujitsu
Signal distortion due to chromatic dispersion
Spectrum broadening
Optical spectrum
Δλ
Difference in group velocity
Wavelength
Pulse broadening
(Waveform distortion)
Transmitter output
Original signal
1
0
1
3
Time
Group velocity
Time
Receiver input
Optical fiber
Time
Regenerated signal
1
Δλ
Fujitsu
1
1
Wavelength
Time
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
Received waveform
Transmitter out
4
High optical power
Fujitsu
Dispersion compensation example
Dispersion compensating fiber (DCF)
Transmission fiber
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
5
After fiber transmission
Fujitsu
After dispersion comp.
DC allocations and dispersion maps
Fiber#1
+
Fiber#2
DC
DC
R.D. [ps/nm]
Postcomp.
Fiber#1
DC
6
R.D. [ps/nm]
Distance
[km]
+
Fiber#2
DC
0
-
DC
Fujitsu
R.D. [ps/nm]
Post- &
Precomp.
+
Fiber#2
DC
DC
Distance
[km]
-
Pre-comp.
Fiber#1
0
0
-
Distance
[km]
Residual dispersion and tolerance of receiver
Allowable
penalty
R.D. [ps/nm]
Longer wavelength
Center wavelength
0
Shorter wavelength
Dispersion
tolerance
of receiver
R.D. [ps/nm]
+
+
-
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
7
Fujitsu
Comparison of 40Gbit/s modulation schemes
Optical power (dBm)
NRZ
RZ
0
CS-RZ
0
Optical duobinary
0
0
108 GHz
180 GHz
165 GHz
-20
-20
-20
-20
-40
-40
-40
-40
1542
1545
1548
Wavelength (nm)
1542
1545
1548
Wavelength (nm)
1542
1545
Wavelength (nm)
1548
70 GHz
1542
Wavelength (nm)
Now evaluating transmission performance
Chromatic dispersion tolerance
Fibre non-linear tolerance (Maximum input power)
Spectral tolerance (Degradation due to filter narrowing)
8
Fujitsu
1545
1548
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.)
Berlin
Link for field trial
Darmstadt
Before Optimization
E/O
O/E
Post-amplifier
Pre-amplifier
After optimization
+900 ps/nm
-400 ps/nm
O/E
E/O
Post-amplifier
9
Pre-amplifier
Fujitsu
Dispersion (ps/nm)
Dispersion maps and waveforms in the trial
Before optimization
2000
1500
1000
500
0
-500
Channel 1
Channel 2
-1000
-1500
Channel 3
Channel 4
-2000
0
200
400
600
800
Dispersion (ps/nm)
Distance (km)
After optimization
2000
1500
1000
500
0
-500
-1000
Channel 1
Channel 1
-1500
(Before)
(After)
-2000
0
200
400
600
800
Distance (km)
10
Fujitsu
Automatic dispersion compensation example
l1 Tx #1
l2
Tx #2
Provisioning
&
Tracking
Provisioning
Rx #2
VDC
VDC
l40 Tx #40
Rx #1
Rx #40
DC
DC
li
Dispersion compensator
(fixed or variable)
Dispersion
Monitor
VIPA variable dispersion compensator
DC > 0
Line-focusing
lens
Optical circulator
Variable
x-axis
DC < 0
Collimating lens
Glass
plate
Focusing
lens
3-Dimensional Mirror
VIPA : Virtually Imaged Phased Array
11
Fujitsu
Dispersion compensation trend
NE
NE
Photonic network
Manage dispersion or
residual dispersion
(dispersion map) !!
NE
NE
Transmitter / Receiver
Adjust parameters
including residual
dispersion to optimum!!
12
Fujitsu
NE
Polarization Mode Dispersion (PMD)
Cross-section of optical fiber
Cladding
Practical
Ideal
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
13
Fujitsu
Higher-order PMD
D t1
D t2
D t3
D t4
…
D tn
-Frequency dependence of DGD
-Statistically varying due to
environmental fluctuations
-Fiber PMD unit: ps/
km
Frequency of occurrence
Mode-coupling at random locations with random strength
Maxwellian distribution
of the instantaneous DGD
Prob.(DGD>3xPMD)
= 4x10-5 = 21 min/year
Prob.(DGD>3.5xPMD)
=10-6 = 32 sec/year
PMD
3.5PMD
Instantaneous DGD (ps)
14
Fujitsu
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
Control
algorithm
O/E
module
Distortion
analyzer
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
15
Fujitsu
After PMD comp.
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.
16
Fujitsu