Vadim Winebrand
Faculty of Exact Sciences
School of Physics and Astronomy
Tel-Aviv University
Research was performed under a supervision of Prof. Mark Shtaif
Outline
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Design of long haul fiber optic communication systems
Signal propagation in the optical fiber
Introduction to polarization effects in the systems
Emulation with help of optical recirculating loop
Simulations vs. Experiments
Measurements performed to show
Polarizations/Nonliniarities interactions
Fiber optic DPSK systems
Introduction to WDM long haul fiber optic
communication systems
Loss
Dispersion
Polarization
Non-liniarities
Noise
TX
RX
TX
RX
TX
..
.
TX
MUX
MUX
DCM
DCM
DCM
DCM
RX
..
.
RX
Degrees of freedom
•
Transmitted waveform (modulation format)
•
Optical power
•
Dispersion management
Loss management
The Q factor grows linearly with input power
But non-linear effects become significant
Q factor dB
1
 Q 
BER  erfc 

2
 2
QdB  20log10(Q)
Input power dBm
5
System design – Loss management
OSNR dB
For given average optical power
Number of amplifiers
6
Acc dispersion (ps/nm)
Acc dispersion (ps/nm)
Dispersion management
Length (km)
Exact-compensation
Acc dispersion (ps/nm)
Length (km)
Over-compensation
Length (km)
Under-compensation
Propagation in optical fibers
Non linear Schrödinger equation NLSE
A
i
 A

2
  2
 i A A  A
2
z
2 T
2
2
A is envelope of
the signal
Dispersion of the
signal
non-linear
interaction
Loss of the signal
NLSE Dynamics
Characteristic length-scales
Nonlinear length
Dispersion length
LNL
1

 P0
LD 
T02
2
Non-linear effect self phase
modulation (SPM)
With negligible dispersion
LNL  LD
A(T , L)  exp(i A T  Leff ) A(T , 0)
2
SPM
• SPM induces chirp on the signal
d
 (t )  
dt
Group velocity dispersion(GVD)
When neglecting non-linearities
LD  LNL
i
A( , L)  exp(  2 2 L) A( , 0)
2
Dispersion
• GVD induces chirp as the pulse propagates
Combined Effect of SPM and GVD
• When both Non-liniarities and Dispersion are present things cannot be
described analytically.
• They get complicated….
WDM system considerations – Four
wave mixing
Each 3 frequencies generate 4th ijk  i   j  k
Power
FWM noise
1
2
3
Spectrum
4
5
WDM system considerations – cross
phase modulation(XPM)
Phase of the signal depends on neighboring channels

Aj  Pj exp i Leff

SPM
M


 Pj   Pm  
m j

 
XPM
14
WDM system considerations – cross
phase modulation (XPM)
XPM causes timing jitter and power fluctuations
15
WDM system considerations –
Raman crosstalk
Power
It depletes higher frequencies
Amplifies lower ones
Spectrum
16
WDM system considerations –
Raman crosstalk
It depletes higher frequencies
Amplifies lower ones
It causes power fluctuations
17
Brillouin scattering
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The power is scattered back once the Brillouin
threshold is passed
Negligible in communication systems
Brillouin threshold
Power
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CW case
Modulated signal
case
Spectrum
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Polarization and Nonlinearity
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In most of the existing literature – these two
phenomena are separated.
•
In the new generation of high-data-rate terrestrial
systems this neglect is no longer possible.
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One of the goals of this work was to demonstrate
and characterize polarization effects in long
nonlinear systems.
Polarization effects
Lack of cylindrical symmetry in fibers
The outcome:
Polarization Mode dispersion (PMD)
Polarization dependent loss (PDL)
Position dependent birefringence - PMD
To 1st order in bandwidth
=
NLSE with PMD
In each segment the Coupled Nonlinear
Schrödinger Equations (CNLSE) are solved:
u
u 1  u
2
2
2
i
 i      2   u u   v u
z
t 2
t
3
2
v
v 1  v
2
2
2
i  i      2   v v   u v
z
t 2 t
3
where:
u,v - signals along the two PSPs
  - group velocity difference between PSPs
2
Penalties of PMD/Non linear
interactions
•
Penalties are shown with cumulative Q distribution
Optical recirculating loop scheme
Amp
Bias
Amp
PC
CW
Bias
PC
Pulse
carver
Modulator
Filter
80/20
50/50
50% RZ pulse
Amp
PreFilter
Eigen
Eigen
10%
1x2
switch
OSA
Amp
4
Eigen
80%
Amp
Post
modul
ator
90%
90/10
Eigen
Wide
band
filter
20%
PC
75km
SMF
DCM
Amp
2
CDR
Data I/P
Error
detector
DCM
75km
SMF
CK I/P
75km
SMF
Scope
DCM
Amp
3
Amp
1
Measurement methods – Bit error
rate
PDF
BER = p(1)p(0/1)+p(0)p(1/0)
V0
V1
Voltage
1
 Q 
BER  erfc 

2
 2
QdB  20log10(Q)
Measurement methods – eye
diagram
Eye-diagram is a bit chain that is folded to a single bit slot
Measurement methods-optical
spectrum
Power spectral density provides significant information
Power dB
Signal power
OSNR
Bandwidth
Spectrum
Noise level
Simulations vs. Experiments
Criterions for comparisons
• Bandwidth evolution
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Optical spectrum
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Eye-diagram - difficult.
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Q factor – difficult.
Comparisons results
x 10
bandwidth vs length
9
x 10
8.5
7.4
8
7.2
Bandwidth(Hz)
Bandwidth
Bandwidth vs length
9
7.5
7
6.5
7
6.8
6.6
6
Simulation
Expirimental
5.5
0
1000
2000
3000
Length(km)
4000
5000
2dBm power and no precompensations
x 10
Simulation
Expirimental
6.4
0
1000
2000
3000
Length(km)
4000
5000
2dBm power and -precompensator of 290ps/nm
bandwidth vs length
9
numerical
Expirimental
7.6
Bandwidth (Hz)
7.4
7.2
7
6.8
6.6
1000
2000
3000
length(km)
4000
5000
3dBm power and -precompensator of 290ps/nm
Comparison between theoretical and experimental
spectrums
PMD/Non linear measurements –
Idea
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Changes in dispersion map will worsen effects of PMD
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But will not affect average Q factor
30
PMD/Non linear interactions–
experimental setup to measure penalties
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The Q statistics was gathered
The Idea is to find that small change in dispersion map
increases penalties
TX
Amp
PreFilter
RX
Pre
Compe (HOM DMD)
nsator
Polarization
Scrambler
10%
1x2
switch
90%
Amp
4
Filter
75km
SMF
PC
Amp
2
DCM
PM
fiber
75km
SMF
DCM
Amp
3
75km
SMF
DCM
Amp
1
Difficulties measuring Q penalty of
non-linear PMD
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Periodic PDL & EDFA amplifiers causes BER fluctuations
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Periodicity does not allow true PMD measurement
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Requires high accuracy in measuring BER
32
PMD&PDL states in the recirculating
loop are constant
PMD states in the real system are random, but in the
recirculating loop they are periodic
Real system case
Recirculating loop case
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Periodic PDL in the recirculating loop
Different states of polarizations lead to different OSNR levels
Orthogonal noise is attenuated – increasing OSNR
PDL
element
Orthogonal signal is attenuated – decreasing OSNR
PDL
element
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Periodic amplifiers in the recirculating
loop
Amplifiers are calibrated for the first cycle only
Amplifiers experience polarization dependent gain
PDL causes gain fluctuations
PDL
element
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Solution (?) - Polarization scrambler - at
the transmitter
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Polarization scrambler makes polarized light to unpolarized
Effects of PDL are averaged out –but effects of PMD are
unchanged
• Gain and noise levels of the amplifiers are more stable
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OSNR variations transformed to amplitude jitter
Eye diagram at 1e-8
Eye diagram at 1e-5
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Solution (?) - Loop synchronous
polarization controller
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Changes input polarization to a random state
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Break periodicity of the PMD and PDL states
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Does not break periodicity of the amplifiers and PDG
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Problems with LSPC
DPSK - introduction
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The data is stored in the phase of adjacent bits.
Reception is performed with delay interferometer
DPSK
OOK
Re{E}
Im{E}
Re{E}
Im{E}
Modulation scheme of the signal
MZDI
Balanced receiver
Scheme of the reception system
DPSK – transmitter
Transmitter experimental setup
Laser
Scheme of the DPSK modulator
Re{E}
modulat
or
Carver
Bit
stream
Sinusoidal
signal
DCA
Im{E}
Requires additional bandwidth
Eye diagram at the output
DPSK reception system
MZDI
I out 
1
I in 1  cos  2 f  T  
2 
Problems
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Exact one bit delay
Phase mismatch
Polarization match
Controllable environment
Frequency response of the
interferometer
DPSK – combining all the system
together
Laser
modulat
or
Carver
Bit
stream
Sinusoidal
signal
MZDI
DCA
Output
OOK vs. DPSK
Many thanks to Prof. Mark Shtaif
Many thanks for Prof. Moshe Tur
Many thanks to Chen Rabiner and Efi Shahmon
Many thanks to all members of the laboratory