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Vadim Winebrand Faculty of Exact Sciences School of Physics and Astronomy Tel-Aviv University. Emulation of penalties in fiber-optic communications systems with the help of a recirculating loop. Research was performed under a supervision of Prof. Mark S htaif. Outline. - PowerPoint PPT Presentation
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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
• 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
DCM DCM DCM DCM
Introduction to WDM long haul fiber optic communication systems
TX
TX
TX
TX
...
MUX
RX
RX
RX
RX
...
MUX
LossDispersionPolarizationNon-liniarities
Noise
Degrees of freedom
• Transmitted waveform (modulation format)
• Optical power
• Dispersion management
Loss management
5
Q f
acto
r dB
Input power dBm
The Q factor grows linearly with input power
ASE domina
ted
But non-linear effects become significant
Non-linear dominated
1
2 2
20log10( )dB
QBER erfc
Q Q
System design – Loss management
6
For given average optical power
OS
NR
dB
Number of amplifiers
Acc
dis
pers
ion
(ps/
nm)
Length (km)
Dispersion management
Acc
dis
pers
ion
(ps/
nm)
Length (km)
Under-compensation
Over-compensation
Acc
dis
pers
ion
(ps/
nm)
Length (km)
Exact-compensation
Propagation in optical fibers
22
2 22 2
A i Ai A A A
z T
A is envelope of the signal
Dispersion of the signal
non-linear interaction
Loss of the signal
Non linear Schrödinger equation NLSE
NLSE Dynamics
Characteristic length-scales
0
20
2
1NL
D
LP
TL
Nonlinear length
Dispersion length
Non-linear effect self phase modulation (SPM)
With negligible dispersion
2( , ) exp( ) ( ,0)effA T L i A T L A T
SPM
• SPM induces chirp on the signal ( )d
tdt
NL DL L
Group velocity dispersion(GVD)
When neglecting non-linearities
22( , ) exp( ) ( ,0)
2
iA L L A
• GVD induces chirp as the pulse propagates
D NLL L
Dispersion
• When both Non-liniarities and Dispersion are present things cannot be described analytically.
• They get complicated….
Combined Effect of SPM and GVD
WDM system considerations – Four wave mixing
Each 3 frequencies generate 4thijk i j k
Pow
er
Spectrum
1
FWM noise
14
WDM system considerations – cross phase modulation(XPM)
Phase of the signal depends on neighboring channels
expM
j j eff j mm j
A P i L P P
SPM XPM
15
WDM system considerations – cross phase modulation (XPM)
XPM causes timing jitter and power fluctuations
16
WDM system considerations – Raman crosstalk
Pow
er
Spectrum
It depletes higher frequenciesAmplifies lower ones
17
WDM system considerations – Raman crosstalk
It depletes higher frequenciesAmplifies lower ones
It causes power fluctuations
Brillouin scattering
• The power is scattered back once the Brillouin threshold is passed
• Negligible in communication systems
18
Pow
er
Spectrum
Brillouin threshold
CW case
Modulated signalcase
Polarization and Nonlinearity• 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.
• One of the goals of this work was to demonstrate and characterize polarization effects in long nonlinear systems.
Lack of cylindrical symmetry in fibers
Polarization Mode dispersion (PMD)Polarization dependent loss (PDL)
The outcome:
Polarization effects
=
To 1st order in bandwidth
Position dependent birefringence - PMD
NLSE with PMDIn each segment the Coupled Nonlinear
Schrödinger Equations (CNLSE) are solved:
22 2
2
22 2
2
1 2
2 3
1 2
2 3where:
- signals along the two PSPs
- group velocity difference between PSPs
u u ui i u u v uz t t
v v vi i v v u vz t t
u,v
Penalties of PMD/Non linear interactions
• Penalties are shown with cumulative Q distribution
Optical recirculating loop scheme
Eigen
75km SMF
75km SMF
75km SMF
50/50
Eigen
CW
Scope
CK I/PData I/PError
detector
FilterAmpPre-Filter
AmpPost
modulator
Amp3
DCM
Eigen
DCM
Amp2
Amp4
Wide bandfilter
80/20
OSA
80% 20%
CDR
Amp1
Modulator
PC
Amp Bias
Pulse carver
PC
Amp Bias50% RZ pulse
DCM
Eigen
PC
1x2switch
90/10
10%
90%
Measurement methods – Bit error rate
BER = p(1)p(0/1)+p(0)p(1/0)
1
2 2
20log10( )dB
QBER erfc
Q Q
PD
F
Voltage
V0 V1
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
Pow
er d
B
Spectrum
Signal power
Noise level
OSNR
Bandwidth
Simulations vs. Experiments
Criterions for comparisons• Bandwidth evolution
• Optical spectrum
• Eye-diagram - difficult.
• Q factor – difficult.
Comparisons results
0 1000 2000 3000 4000 5000
5.5
6
6.5
7
7.5
8
8.5
x 109 bandwidth vs length
Length(km)
Ba
nd
wid
th
SimulationExpirimental
0 1000 2000 3000 4000 5000
6.4
6.6
6.8
7
7.2
7.4
x 109
Length(km)
Ba
nd
wid
th(H
z)
Bandwidth vs length
SimulationExpirimental
1000 2000 3000 4000 5000
6.6
6.8
7
7.2
7.4
7.6
x 109
length(km)
Ba
nd
wid
th (
Hz)
bandwidth vs length
numericalExpirimental
Comparison between theoretical and experimental spectrums
2dBm power and no precompensations 2dBm power and -precompensator of 290ps/nm
3dBm power and -precompensator of 290ps/nm
30
PMD/Non linear measurements – Idea
• Changes in dispersion map will worsen effects of PMD
• But will not affect average Q factor
PMD/Non linear interactions–experimental setup to measure penalties
• The Q statistics was gathered• The Idea is to find that small change in dispersion map increases penalties
Difficulties measuring Q penalty of non-linear PMD
• Periodic PDL & EDFA amplifiers causes BER fluctuations
32
• Periodicity does not allow true PMD measurement
• Requires high accuracy in measuring BER
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
33
Real system case
Recirculating loop case
Periodic PDL in the recirculating loop
34
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
Periodic amplifiers in the recirculating loop
35
PDL causes gain fluctuations
PDL element
Amplifiers experience polarization dependent gain
Amplifiers are calibrated for the first cycle only
36
Solution (?) - Polarization scrambler - at the transmitter
• Polarization scrambler makes polarized light to un-polarized
• Effects of PDL are averaged out –but effects of PMD are unchanged
• OSNR variations transformed to amplitude jitter
• Gain and noise levels of the amplifiers are more stable
Eye diagram at 1e-8 Eye diagram at 1e-5
Solution (?) - Loop synchronous polarization controller
• Changes input polarization to a random state
• Break periodicity of the PMD and PDL states
• Problems with LSPC
• Does not break periodicity of the amplifiers and PDG
DPSK - introduction• The data is stored in the phase of adjacent bits.• Reception is performed with delay interferometer
Modulation scheme of the signal Scheme of the reception system
OOKDPSK
MZDI Balanced receiver
Re{E}
Im{E}
Re{E}
Im{E}
DPSK – transmitter Transmitter experimental setup
Eye diagram at the output
Scheme of the DPSK modulator Lasermodulat
orCarver DCA
Bit stream
Sinusoidal signalRe{E}
Im{E}
Requires additional bandwidth
DPSK reception system
MZDI 11 cos 2
2out inI I f T
Frequency response of the interferometer
Problems
• Exact one bit delay• Phase mismatch• Polarization match• Controllable environment
DPSK – combining all the system together
Output
OOK vs. DPSK
Lasermodulat
orCarver MZDI
Bit stream
Sinusoidal signal
DCA
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