optical transmission system

“Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra
“Recent Trends in Optical
Transmission Systems”
Thomas Sphicopoulos (thomas@di.uoa.gr)
Optical Communications Laboratory National and Kapodistrian
University of Athens, Greece
Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece

Advantages of Optical Technology
Optical Technology Provides:
• Ultra Low Transmission Losses
• Ultra Wide Band
• Very High Bitrates
• (Mostly) Linear Behavior
• Very Low Crosstalk
But:
• Optics are not smart
• InP / Si / Polymer platforms do not yet
provided increased scale of integration
• No means of storage

The Optical Value Chain
MATERIAL AND PROCESSES
Silicon, GaAs, InP
Polymers / Organic Materials,
Etchers, MEMS
PHOTONIC COMPONENTS
Lasers, Optical Amplifiers
Transceivers, Optical Filters
Fiber cables, Wavelength
Converters, Regenerators
Dispersion Compensators
EQUIPMENT MAKERS
Routers, Switches, Hubs
Base Stations, Satellites
Servers
NETWORK OWNERS
Wireless, Backbone, Metro
Access, Satellites, Spread
Spectrum Communications
SERVICE PROVIDERS
Long Distance, Cellular
ISP, Broadcast, Cable TV
VPN
CONTENTS AND APPLICATIONS
Music, Movies, E-mail
VoIP, Shopping, Surveillance
eBusiness
APPLIANCES
Computers, Phones
Media Players,
Cameras, PDAs
END USERS
Consumer, Government
Education, Medical
Business

Evolution of Transmission Rates/Channel
2.5Gb/s
10Gb/s
40Gb/s
160Gb/s(?)
1990 1995 2005 2010
Year

Wavelength Division Multiplexing (WDM)
λ1
λ2
λ3
λ4
λ1
λ2
λ3
λ4
The aggregate bit rate can be drastically increased by using
Wavelength Division Multiplexing (>1Tb/s exhibited )
In optical transmission systems, the available bandwidth can exceed
40nm
To efficiently utilize this enormous bandwidth one can assign each
channel a different wavelength and lead all the wavelengths inside
the fiber
Channel spacing as narrow as 10GHz(!) can be achieved!

Transmission Impairments
Linear Impairments:
• Optical Losses
• Chromatic Dispersion
• Polarization Mode Dispersion
Non-linear Impairments:
• Self Phase Modulation
• Cross Phase Modulation
• Four Wave Mixing
• Stimulated Raman Scattering
• Stimulated Brillouin Scattering

The Fiber: A Nearly Lossless Channel
Typical Losses can be as low as 0.2dB/Km
Poses no problem if optical amplification is used

Linear Impairments: Dispersion
As in most types of waveguides the different
spectral parts of the pulse travel with slightly
different phase velocities (chromatic dispersion)
This causes pulse broadening!
Types of Cables according to dispersion:
•G652: D~15-20ps/nm/Km (λ=1.55μm)
•G653: D~0ps/nm/Km (λ=1.55μm)
•G655: D~2-6ps/nm/Km (λ=1.55μm)

Linear Impairments: Polarization Mode
Dispersion (PMD)
The fiber is not completely circular and hence supports
two degenerate modes with slightly different group
velocities (birefrigence)
The principal polarization axes of the fiber may change
randomly along the cable due to temperature / size
variations. This causes Polarization Mode Dispersion
PMD can also cause pulse broadening at high bit rates

Nonlinear Impairments due to the non-linearity
of the refractive index
2
linear 2 n  n  n E Intensity of the
Electric Field
Self Phase Modulation: Phase modulation due to the
intensity modulation of the Signal (introduces chirp)
Cross Phase Modulation: Phase modulation due to the
intensity modulation of other interfering wavelength
channels (pulse broadening and time jitter)
Four Wave Mixing: Crosstalk with other nearby channels
due to frequency mixing (three photon interaction)

Nonlinear Impairments: Stimulated
Scatterings
Brillouin Scattering: Energy is transferred from a photon to an
acoustic phonon (molecular vibration) and to a photon of smaller
frequency (≈-10GHz) (unwanted reflections at the source).
Raman Scattering: Energy is transferred from a photon to an
optical phonon (molecular vibration) and to a photon of smaller
frequency (optical crosstalk from higher to lower frequency
channels)
Current WDM systems avoid problems with both type of
scatterings by limiting the optical power and increasing the
channel spacing

Technological Landmarks: Optical Sources
Distributed Feed Back Lasers (DFB) are ideal Optical
Sources for ~40Gb/s providing:
High Launch Power (>20mW)
Wavelength Stability (~0.001nm/0C)
Very Low RIN (>-145dB/Hz)
High Side Mode Suppression Ratio (<-45dB when
MQW is used)
Narrow Linewidths (~2MHz)
At ~40Gb/s only external modulation can be used:
LiNbO3 Mach Zehnder Modulator (electroptical effect)
Electroabsorption Modulator (electroabsorption effect)

Technological Landmarks: Amplifiers
Two types of Amplification is used:
Erbium Doped Fiber Amplifier (EDFA):
High Gain (~40dB)
High Output Power (~400mW)
Very Low Noise
Very Linear
Wide Band (~40nm)
Raman Amplifier
Higher Power than EDFA (~700mW)
Can offer distributed and/or lumped amplification
Ultra wide band (~100nm)

Technological Landmarks: MUX/DEMUX
Arrayed Waveguide Gratings (AWGs):
Can multiplex up to 1000 channels!
Channel spacing can be as small as 10GHz!
Commercial systems multiplex 64 channels x
50GHz
Can be integrated with SOAs and provide an
integrated ADD/DROP MUX
Have small polarization sensitivity
Have small insertion loss
Can be designed with “flat-top” transfer function

System Design: Dispersion Management (1)
First Generation Dispersion Management System
SMF
DSF
DSF
(D= -0.2ps/nm/Km)
Distance
SMF
(D=+18ps/nm/Km)
Accumulated
Dispersion (ps/nm)
150ps/nm
Repeater
DSF=Dispersion Shifted
Fiber
SMF=Single Mode Fiber
This scheme was used in
the past for single
channel ~5Gb/s
systems but is
unsuitable for WDM:
high nonlinearity
Compensates
dispersion for one
wavelength

Dispersion Management for multi-channel 10Gb/s
Repeater
LCF
NZDSF
LCF / NZDSF
(D= -2ps/nm/Km)
SMF
(D=+18ps/nm/Km)
Parameters NZ-DSF LCF
D (ps/nm/Km) -2~-3 -2~-3
Ds (ps/nm2/Km) 0.05~0.06 0.1~0.14
Aeff(μm2) 50~55 70~80
Loss(dB/Km) 0.2 0.22
Accumulated
Dispersion (ps/nm)
SMF
~500Km
+6000ps/nm
-6000ps/nm
~500Km
λ1
λΝ
LCF = Large Core Fiber
NZDSF= Non-Zero DSF
LCF is used first to reduce non-linearity
SMF is placed in the middle of the
period and the accumulated
dispersion alternates sign

Dispersion Management for multi-channel 10Gb/s
To further residual dispersion at
edge channels we use
pre/post-compensantion
(50:50) on a channel by
channel basis:
Less Maximum Dispersion
Less Waveform Distortion
Overall:
Less Nonlinearity
Ideal for 16x10Gb/s (~20nm)
Repeater
LCF
NZDSF
LCF / NZDSF
(D=-0.2ps/nm/Km)
SMF
(D=+18ps/nm/Km)
Accumulated
Dispersion (ps/nm)
SMF
~500Km
+6000ps/nm
-6000ps/nm
~500Km
λ1
λΝ

Expanding the Bandwidth from ~20nm to ~40nm
Repeater
~50Km
ΕΕ-PDF
SPCDF
EE-PDF
(D=+20~22ps/nm/Km)
SMF
Accumulated
Dispersion (ps/nm)
SMF
~500Km
+600ps/nm
-600ps/nm
λ1
λΝ
SCDCF
(D=-40~-60ps/nm/Km)
EE-PDF: Aeff Enlarged Positive
Dispersion Fiber
SC-DCF: Slope Compensating
Dispersion-Compensation Fiber
Parameters EE-PDF SCDCF
D (ps/nm/Km) +20~22 -40~-60
Ds (ps/nm2/Km) +0.06 -0.12~-0.18
Aeff(μm2) >100 30~22
Loss(dB/Km) 0.15~0.19 0.23~0.27

Moving to 40Gb/s…
It is preferable to lower accumulated dispersion
+600ps/nm
SMF+SCDCF
Cumulative
Dispersion (ps/nm)
EE-PDF+SCDCF+EE-PDF
~40Km
-200ps/nm

System Design: Integrated Optics
Dispersion Compensation
Modifying the Geometry of
an Arrayed Waveguide
Grating by a Variable
Reflecting Membrane
introduces Second Order
Dispersion that can be used
to compensate the
accumulated dispersion of a
multiwavelength 40Gb/s
signal
Tunable: Applying Voltage
1000ps/nm Tuning Range

System Design: Electronic Dispersion
Compensation
One idea is to predistort the signal for each channel
Processor D/A
Amp
Laser
Processor D/A
d1(t)
Ein ETX
Amp
MZM
d2(t)
   
V E t
   
V E t
1
1
( )
( ) ( ) cos TX
in
d t t
E
 


         
   
1
2
( )
( ) ( ) cos TX
in
d t t
E
 


         
   
   
2
 
2 exp
2 TX RX
L
E E j
 
 
  
 
Use a MZM interferometer to predistort the signal in order
to counteract the effects of dispersion
Works very well in theory but you need fast electronics
and D/A (even if you parallelize!)

System Design: Mitigation of
Nonlinearities
Methods for Reducing Nonlinearities: FWM
Use unequal channel spacings
Use optical prechirped pulses
Transmitter Receiver
DEMUX
MUX
NZD fiber DCF fiber
Tx
Tx
Tx
ASK MOD
ASK MOD
...
ASK MOD
Rx
Rx
Rx
DCF fiber
used for
prechirping

System Design: Mitigation of
Nonlinearities
Methods for Reducing Nonlinearities: XPM
Dispersion Compensation at each span
High channel spacing
Pre-chirped optical pulses
Advanced modulation schemes

How to Model and Design? (1)
Use Numerical Tools:
Numerically Solve the Propagation Equation
2
2
A j A a z
 
z A j z A A
  
 ( ) 
( )
2 2
( )
z t
 
2 2
A=A(z,t): Envelope of the Electric Field
β2(z): Second order dispersion
γ(z): Nonlinear Kerr Coefficient
You can add amplifier gain and noise in each
span

Use Numerical Tools:
Calculate Q-factor from Receiver Eye-Diagram
m 
m
1 0
 

1 0
Q

For Gaussian Noise:
Q
1
2 2
P erfc
e
 
  
 

Example: Estimate Performance of Modulation
Formats in G655 fibers:
30
25
20
15
10
5
NRZ Duobinary
DPSK NRZ single channel
-8 -6 -4 -2 0 2 4
Q factor
P
in
(dBm)
(a)
FWM limits the
quality of
multichannel
systems and
hence DPSK has
superior
performance
Δfch=100GHz, Lspan=80Km, Nspan=4

But:
The Gaussian Assumption is usually not valid!
The Q-factor provides a crude estimate for the
error probability
Use Saddle-Point approximation to compute the
error probability from the MGF (if it is known!)
Use Monte Carlo methods to estimate the error
probability numerically

Example: Estimation of FWM probability density
function using MCMC simulations
Gaussian PDF is
inadequate!
MCMC requires very
few iterations (~106)
for probabilities of the
order of 10-14
Single span system, Nch=8, Δfch=100GHz

Small Size Components: Photonic Crystals as a
Possible Candidate for Nanophotonics
Photonic Crystals: Artificial Periodic Structures
Exhibit Bandgaps (no guided modes exist)
“Defects” introduce highly localized modes
Confine light (can implement sharp bends)
Are highly non-linear (signal processing)

Slow Light: Towards Integrated All-Optical
Buffers?
OPTICAL RESONATORS
Certain waveguiding structures can support pulse
propagation with very low group velocities
Coupled Resonator Optical Waveguides (CROWs)
Integrated Optical Delay Lines
Photonic Memories
Signal Processing (Linear + Nonlinear)

In conclusion…
Optical Transmission Systems have
made significant advances and are
operational.
But much can be gained by improving
optical integration and exploring optical
buffering!

THANK YOU FOR
YOUR ATTENTION!

optical transmission system

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